Emerging Trends in Geotechnical Engineering - IIT Guwahati

Emerging Trends in Geotechnical Engineering Edited by

A. Murali Krishna

Proceedings of National workshop on Emerging Trends in Geotechnical Engineering (ETGE 2012) 8th June 2012, IIT Guwahati, Guwahati

Organised by Department of Civil Engineering Indian Institute of Technology Guwahati and Indian Geotechnical Society Guwahati Chapter (NE) Guwahati

Proceedings of National Workshop on Emerging Trends in Geotechnical Engineering (ETGE 2012), Guwahati, 08th June 2012

Emerging Trends in Geotechnical Engineering Edited by A. Murali Krishna Department of Civil Engineering Indian Institute of Technology Guwahati Guwahati.

Workshop Organisers:

Department of Civil Engineering Indian Institute of Technology Guwahati Indian Geotechnical Society Guwahati Chapter (NE) Guwahati

National Workshop on Emerging Trends in Geotechnical Engineering (ETGE 2012), Guwahati, 08th June 2012

Organising Committee: Chairmen: Prof. AK Sarma, Department of Civil Engineering, IIT Guwahati Prof. UC Kalita, President, IGS Guwahati Chapter (NE)

Organising Secretaries: Dr. A. Murali Krishna, IIT Guwahati Dr. Diganta Goswami, Assam Engineering College, Guwahati Dr. Utpal Barua, Assam Engineering Institute, Guwahati

Members: Shri. KL Das, Guwahati Dr. Baleshwar Singh, IIT Guwahati Dr. S. Sreedeep, IIT Guwahati Dr. Arindam Dey, IIT Guwahati Dr. Binu Sharma, Assam Engineering College, Guwahati Mr. Sashanka Bora, Assam Engineering College, Guwahati Mr. Abinash Mahanta, Assam Engineering College, Guwahati Dr. Kumar Pallav, IIT Guwahati

Sponsors: Purbanchal Cements Ltd. Indian Geotechnical Society, New Delhi.

Workshop on Emerging Trends in Geotechnical Engineering (ETGE 2012) 8th June 2012, Guwahati

Preface India has emerged as one of the fastest growing economies in the world. Various new infrastructure projects are under progress across the country especially in Northeast India. Geotechnical engineering is the first and most important aspect of any infra project and urban development. Over the years many new trends were being developed and practiced in terms of analysis, design and construction of geotechnical structures and associated site investigations. In this vein, a National workshop on “Emerging trends in Geotechnical Engineering” (ETGE 2012) is organised under the auspices of the Department of Civil Engineering, Indian Institute of Technology Guwahati in association with Indian Geotechnical Society Guwahati Chapter (NE). I express sincere gratitude to both the organisations for their support. “Emerging trends in Geotechnical Engineering”, encompass many different subdivisions of the field of Geotechnical Engineering. To name: Trends in Site investigation; Trends in laboratory testing, analysis; New trends in design methods and designs; Trends in ground improvement methods; Trends in geoenvironmental applications; Trends in utilisation of waste by products; Trends in predicting soil behaviour; Trends in dealing with underground structures offshore geotechnical structures; Trends in earthquake related problems and designs; Trends in construction methods under challenging situations etc. North eastern states being the most rapid developing parts of India, in terms of various infrastructure projects and hydro projects, are the places where the application of these new trends with amalgamation of classical soil mechanics principles is vital. There is a need for students, academicians and practicing professionals for updating their knowledge about the emerging trends in various geotechnical areas. This workshop presents some of the emerging trends in the geotechnical engineering practice and research activities. I am very grateful to the speakers for accepting my request to delivering the lectures and preparing the paper contributions. It gives me great pleasure to bring out the workshop proceedings. In this edited volume, about a dozen contributions are included from the eminent geotechnical researchers in India and other academicians, practicing

engineers. The chapters of this book covers and presents some of the ongoing trends in analyses of ground improvement methods, utilisation of waste products, ground stabilisations, ground response analyses of rock structures, foundation treatments, designs and analysis, geoenvironmental challenges and applications, along with construction trends for roads under challenging conditions. I am very thankful to all the authors for their time and effort in developing the workshop lectures and sharing their technical experiences. I hope the proceedings will enrich knowledge of the workshop participants and helps for the future growth of geotechnical engineering with new trends. I would to express my sincere thanks to the organising committee for their support in organising this workshop. On behalf of organising committee, I extend our sincere thanks to the sponsors Purbanchal Cements Ltd., Guwahati and Indian Geotechnical Society, New Delhi. I also thank all the student volunteers for their support and assistance in all respects of smooth conduct of the workshop. Last, but not the least I express thanks for all the participants to making this workshop ETGE 2012, a grand success.

IIT Guwahati June 2012

Murali Krishna

Workshop on Emerging Trends in Geotechnical Engineering (ETGE 2012) 8th June 2012, Guwahati.

Contents Sl. No.

Title

Page No.

1. Soft Ground Improvement with PVDs --- M. R. Madhav and Ayub Khan

1

2. Coal Ashes in Geotechnical Engineering Practice: Beneficial Aspects --- A. Sridharan

11

3. Ground Response and Support Measures for a Railway Tunnel in the Himalayas --- K. S. Rao 4. Treatment of Foundations and Geological Faults of Almatti Dam on Krishna River: A Case Study --- M. Bidasaria

27

5. Recent Experiences of Ground Stabilization Techniques --- Satyendra Mittal

45

6. Performance Based Earthquake Resistant Design of Geotechnical Structures – A New Trend --- S. K. Prasad and P. Nanjundaswamy 7. A Decision Support System for Risk Assessment and Remediation Option Selection for Contaminated Soils and Groundwater --- R. K. Srivastava

61

8. Risk Analysis of Bearing Capacity of Shallow Foundations --- Dasaka S Murty

89

9. Prediction of Soil Behavior – A Reappraisal

99

37

75

--- Binu sharma 10. Road Embankments in Water Logged and Frost Affected Areas – Problems and Solutions --- Jai Bhagwan and Kanwar Singh

113

11. The Principles and Application of Geo-Environmental Engineering --- Anil Kumar Mishra

119


Soft Ground Improvement with PVDs M.R. Madhav1 and Ayub Khan, P.2 1 Professor Emeritus, JNTUH CE and Visiting Professor, IIT, Hyderabad, email: [email protected] 2 ACE Engineering College, Ankushapur, Ghatkesar, email: [email protected]

ABSTRACT: Treating soft ground with PVDs is one of the most popular techniques for improving soft soil deposits. PVDs improve the ground by accelerating consolidation arising from preload. This paper presents an analysis of non-linear theory of radial consolidation due to PVDs in thick soft clay deposits. Keywords: PVDs, Ground improvement, soft ground, consolidation, non-linear theory

1. INTRODUCTION India has a long coast line of nearly 5,000 km length. Soft, weak, highly compressible soft soils are prevalent along this stretch. Considerable infrastructure development is taking place along the coast for obvious reasons. Construction of facilities on these deposits is very challenging because of their low strength, high compressibility, etc. Several alternatives, e.g. preloading without or with PVDs, granular piles, heavy tamping, etc., are available to engineer the ground. Amongst these, preloading with PVDs and reinforcing with granular piles/stone columns are the most preferred alternatives. 2. PREFABRICATED VERTICAL DRAINS (PVDs) These are band or strip shaped plastic drains about 100 mm wide and 4 mm thick usually installed in square or triangular arrays (Fig. 1a). Consequently, the zone of influence of each drain is either square or hexagonal area. The strip drain and the zone of influence of each drain are replaced by a unit cell of equivalent circular shapes. The flow pattern around the drain is considered to be axi-symmetric (Fig. 1b). The equivalent diameter of the drain, dw =2(a+b)/p, where ‘a’ and ‘b’ are the width and thickness of the PVD respectively and the equivalent diameter of the zone of influence, de = 1.13S & 1.05S for square and triangular patterns respectively, where S is the spacing of drains. Barron (1947) presented analytical solutions for the radial consolidation due to sand drains for both free and equal strain conditions. This classical theory is based on the assumptions of small strains, linear void ratio-effective stress relationship and constant coefficients of volume compressibility, mv, and horizontal permeability kh. Hansbo (1981) presented a simple solution for radial consolidation with band shaped vertical drains. Lekha et al. (1998) presented a non-linear theory of consolidation with sand drains under time dependent loading for equal strain case. Full-scale test conducted by Bergado et al. (2002) on soft Bangkok clay with PVDs revealed that the degree of consolidation obtained from pore pressure measurements is lower than the corresponding values obtained from settlement measurements.

1

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ETGE 2012

S

S

de 1 1

H

Drain

dw (a)

(b)

de

Figure 1(a) Triangular Arrangement of PVDs and (b) Flow in Unit Cell The various modeling aspects of PVDs are comprehensively discussed by Indraratna et al. (2003) along with the evaluation of their effectiveness in practice. Indraratna et al. (2005a) developed a theory for consolidation with radial flow using e - logs ¢ (Cc and Cr) & e – log kh (Ck) relationships and for different loading increment ratios (Ds ¢ / s i¢ ) . Indraratna et al. (2005b) developed a modified consolidation theory for vertical drains incorporating vacuum preloading for both axi-symmetric and plane strain conditions. Two- and three-dimensional multidrain finite-element analyses of a case study of combined vacuum and surcharge preloading with vertical drains is presented by Rujikiatkamjorn et al. (2008). The numerical predictions compared well with observed data. Indraratna et al. (2008) developed a new technique to model consolidation by vertical drains beneath a circular loaded area by transforming system of vertical drains into a series of concentric cylindrical drain wall. Walker et al. (2009) presented the spectral method for analysis of vertical and radial consolidation in multilayered soil with PVDs by assuming constant soil properties within each layer. For relatively large applied stress range the void ratio is not proportional to effective stress and the coefficients of compressibility and permeability decrease during consolidation. A nonlinear theory of consolidation was developed by Davis & Raymond (1965) considering e – log σ΄ relationship but it is valid only for vertical flow and thin layer of clay. A theory of nonlinear consolidation for radial flow around a vertical drain is developed by Ayub Khan et al. (2010a) based on linear void ratio-log effective stress relationship but assuming constant coefficient of consolidation for thin clay layers. This theory is further extended by Ayub Khan et al. (2010b) for thick clay deposits as well. The general equation of non-linear consolidation with radial flow (Ayub Khan et al. 2010b) in terms of a parameter, w, is æ ¶ 2 w 1 ¶w ö ¶w ÷ = cr ç + ç ¶r 2 r ¶r ÷ ¶t è ø

with

w = log 10

s¢ s ¢f

or

(1) w = log10

(s ¢f - u )

s ¢f

, where σ΄ and u are the effective vertical stress and the

excess pore pressures respectively at time, t and radial distance, r, cr - coefficient of consolidation for radial flow and σ΄f the final effective vertical stress. The parameter ‘w’ varies with the depth in thick deposits of clay as the initial effective in-situ stress, σ΄o and the final effective stress, σ΄f (=σ΄o +qo) vary with depth due to overburden stresses, and qo is the applied load intensity. The thick clay layer of thickness, H is divided into m layers of

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Soft Ground Improvement with PVDs

thickness, ΔH=H/m (Fig. 2). The equation governing the consolidation process of each layer is æ ¶ 2 w j 1 ¶w j ö ÷ = c r çç + ¶t r ¶r ÷÷ ç ¶r 2 è ø

¶w j

(2)

where the subscript j refers to the layer number and wj = wj (r, t) of the jth layer. Eqs. (1 and 2) are of the same form as that given by Barron (1947). Even though the initial and final effective stresses are different in each layer, but the flow in each layer is assumed to be purely radial and independent of the flows in the adjacent layers. Degree of Settlement of the layer j is, re æ ö ç ò w j 2p r dr ÷ ç ÷ r r U S, j = w = ç1 - w ÷ re re ç ÷ ò wo, j 2p r dr ÷ ò (e o - e f ) 2p r dr ç rw è rw ø s ¢j s o¢ , j where w j = log 10 , the initial , wo, j = log 10 s ¢f , j s ¢f , j re

ò (e o - e) 2p r dr

effective in-situ stress in the jth layer, σ΄o,j

Drain

(3)

.zj.

Surcharge, qo Pervious Flow Path o

Layer-1 z H/m

f

Layer- m Impervious

rw

Centre Line of Drain

re

Figure 2 Radial Flow in Clay Layers in a Thick Deposit Average degree of consolidation for the entire thickness, m

å U s, j .(DH ) j

Us =

j =1

H

(4)

Normalized average excess pore pressure for the layer j is: re

ò u j 2p r dr

U * avg , j =

rw re

ò u o 2p r dr

rw

(5)

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Degree of dissipation of average excess pore pressure at the layer, j is: U P, j = (1 - U * avg , j )

(6)

Average degree of dissipation of excess pore pressure for the entire thickness is: m

å U P, j .(DH ) j

UP =

j =1

(7)

H

Initial and Boundary Conditions For t = 0 and rw £ r £ re ; u j (r ,0) = (s ¢f , j - s o¢ , j ) or w j (r ,0) = wo, j (r ,0) = log 10

s o¢ , j s ¢f , j

(8)

where σ΄f,j= (σ΄o,j+Δσ΄) and Δσ΄=qo For t > 0 and r =r w ; u j (r = rw , t ) = 0 or w j (r = rw , t ) = 0 For t > 0 and r = re ;

¶u j ¶r

= 0 or r = re

¶w j ¶r

(9)

=0 r = re

(j=1, 2, 3…m)

(10)

The radius of influence zone, re = 0.5de and radius of equivalent drain, rw =0.5dw. Eq. (2) is rewritten in non-dimensional form as æ ¶ 2 w j 1 ¶w j ö ÷ =ç + ¶T ç ¶R 2 R ¶R ÷ ø è

¶w j

( j=1, 2, 3……m)

(11)

where R = r/de and time factor, Th = cr.t./de2.

3. RESULTS AND DISCUSSION A thick clay layer of thickness, H, is divided into ‘m’ (=20) layers of equal thickness, ΔH= H/m, as shown in Figure 2. The numerical analysis is carried out for each layer independently using the corresponding ratio σ΄f/σ΄o and the results obtained in terms of degree of settlement, Us, and excess pore pressures, u. Variations of these results along the depth and radial distance are studied. As non-linear consolidation is mainly influenced by the stress ratio, σ΄f /σ΄o, the variation of σ΄f/σ΄o with depth is shown in Fig. 3 for different values of normalized applied load intensity, q*o=qo/(γ΄.H). The decrease of σ΄f /σ΄o with depth is very sharp as the initial effective stress is very small near the top. The decrease of σ΄f /σ΄o with depth is significant for depths in the range 0.1H to 0.4H and relatively very small for z>0.4H for all q*o. Hence, the effect of non-linearity in the void ratio - effective stress on consolidation in a thick clay layer can be pronounced at shallow depths compared to that at greater depths. Increases in q*o can be either due to increase of load intensity, qo for a given thickness, H, or due to decrease of thickness of clay deposit for a given load intensity. In either case, only the non-dimensional stress ratio, σ΄f/σ΄o, influences the rate of consolidation and not the thickness of the deposit or the magnitude of loading individually. The degree of settlement, Us, is determined for different layers along the depth for various qo* values for different values of n ranging from 5 to 40 and shown in Fig.4. The degree of


5

settlement is identical at all the layers in given thick clay for all values of n and q*o. The degree of settlement obtained from the present non-linear theory is identical to that from the linear theory (Barron 1947) for free strain case. The degree of settlement, Us, is independent of σ΄f /σ΄o and varies only with n as is the case for vertical flow (Davis and Raymond 1965). The degree of settlement decreases with increase of n since it takes relatively long time for dissipation of pore pressure for larger radial distances. Us decreases from 58% to 24.8% for n increasing from 5 to 40 at a time factor of 0.10.

Figure 3 Variation of σf΄/σo΄ with Depth

Figure 4 Us versus Th for both Linear and Non-Linear Theories – Effect of ‘n’

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ETGE 2012

The degree of dissipation of average excess pore pressure for the entire thickness, Up is presented in Fig. 5 for n=15 along with the degree of settlement, Us. While the degree of settlement is independent of q*o, the degree of dissipation of pore pressure, Up is sensitive to q*o values. Thus for a given thickness of clay deposit, the increase of load intensity, qo results in decrease of degree of dissipation of average excess pore pressure or for a given load intensity the decrease of thickness of clay deposit results in decrease of degree of dissipation of average excess pore pressure.

Figure 5 Variations of Us and Up with Th The normalized average excess pore pressure, U*avg(z) = (uavg(z)/uo).100 at different depths determined and its variation with time shown in Fig. 6 for n=15 and q*o=1 along with the degree of settlement. The excess pore pressure is relatively high at shallow depths where the σ΄f /σ΄o ratio is relatively high compared to the values at greater depths. The importance of the above finding is that when large load is applied on soft ground, the possibility of shallow seated rotational bearing failure is to be examined in view of the large residual pore pressures at these depths over longer periods of time. The normalized average excess pore pressures, U*avg (z) at different depths of the thick clay are presented in Fig. 7 for n=15 at time factor, Th=0.20 along with the results of linear theory. The remarkable phenomenon observed is that average pore pressure values from the nonlinear radial consolidation theory vary with depth in contrast to the depth-independent U*avg values of linear theory. The differences between the pore pressures of non-linear and linear theories are relatively large at shallow depths due to large values of σ΄f /σ΄o compared to those at greater depths. This difference increases with increase of q*o. Moreover, at shallow depths the variation of pore pressure with depth is relatively very large compared to that at greater depths due to sharp variation of σ΄f /σ΄o at shallow depths. While the excess pore pressures in the linear theory are independent of, q*o, the pore pressures according to non-linear theory are dependent on q*o as the variation of q*o influences the ratio σ΄f /σ΄o..


7

The residual average excess pore pressures thus are underestimated in the conventional linear theory. In view of the above, instead of applying the entire preload instantaneously on the soft ground, it may be applied in increments with proper time lag to allow quicker dissipation of pore pressures and gain of shear strength.

Figure 6 Variation of Us & U*avg(z) with Th –- Effect of Depth

Figure 7 Variation of U*avg(z) with Depth—Effect of Load Intensity Fig. 8 shows the pore pressure variation with radial distance at different depths for q*o=1, n=15 and Th=0.20. The excess pore pressures are relatively large at shallow depths wherein the stress ratio is extremely large but decrease with depth since the ratio of final to initial stress decreases with increase of depth. The pore pressure variation with radial distance, r, is relatively more significant in the upper half of the deposit compared to that in the lower half.

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Figure 8 Variations of Pore Pressures with Radial Distance Effect of Depth

4. CONCLUSIONS Analysis of consolidation of soft ground treated by PVDs is presented. A simple non-linear theory of radial consolidation developed for thick deposit of clay treated with PVDs considering linear void ratio-log effective stress relationship predicts that while the degree of settlement is independent of the final to initial stress ratio, the degree of dissipation of pore pressure is very much dependent on the stress ratio. The residual excess pore pressures are under-estimated in the conventional linear theory. The proposed nonlinear theory substantiates the actual in-situ slower rate of degree of dissipation of excess pore pressures compared to that of the degree of settlement. The non-linear consolidation effect is pronounced at shallow depths compared to the effect at greater depths. The excess pore pressures due to radial drainage vary not only with time and radial distance but also with depth in contrast to depth-independent pore pressures from the conventional theory for radial flow. The significance of the proposed theory is that it can explain failure of high embankments constructed rapidly on thick deposits of fine grained soils. REFERENCES Ayub Khan, P., Madhav, M.R. and Saibaba Reddy, E. (2010a). Effect of non-linear consolidation for radial flow on pore pressure dissipation. Indian Geotechnical Journal, 40(1), 47-54. Ayub Khan, P., Madhav, M.R. and Saibaba Reddy, E. (2010b). Consolidation of thick clay layer by radial flow-nonlinear theory. Geomechanics and Engineering, 2 (2), 157-160. Barron, R.A. (1947). Consolidation of fine grained soils by drain wells. Trans. ASCE, 113(2346), 718-754. Bergado, D.T., Balasubramaniam, A.S., Fannin, R.J. and Holtz, R.D. (2002). Prefabricated vertical drains (PVDs) in soft Bangkok clay: a case study of the new Bangkok International Airport project. Can. Geotech. J., 39, 304-315. Davis, E.H. and Raymond, G.P. (1965). A non–linear theory of consolidation. Geotechnique, 15(2), 161–173. Hansbo, S. (1981). Consolidation of fine grained soils by prefabricated drains. Proc. of the


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10th ICSMFE, Stockholm, Sweden, June, 3, 667-682. Indraratna, B., Aljorany, A. and Rujikiatkamjorn, C. (2008). Analytical and numerical modeling of consolidation by vertical drains beneath a circular embankment. Intl. Jl. of Geomechanics, 8 (3), 119–206. Indraratna, B., Bamunawita, C., Redana, I.W. and McIntosh, G. (2003). Modeling of prefabricated vertical drains in soft clay and evaluation of their effectiveness in practice. Ground Improvement, 7(3), 127–137. Indraratna, B., Rujikiatkamjorn, C. and Sathananthan, I. (2005a). Radial consolidation of clay using compressibility indices and varying horizontal permeability. Can. Geotech. J., 42(5), 1330–1341. Indraratna, B., Sathananthan, I., Rujikiatkamjorn, C. and Balasubramaniam, A.S. (2005b), Analytical and numerical modeling of soft soil stabilized by prefabricated vertical drains incorporating vacuum preloading. Intl. Jl. of Geomechanics, 5(2)114–124. Lekha, K.R., Krishnaswamy, N.R. and Basak, P. (1998). Consolidation of clay by sand drains under time–dependent loading. J of Geotech. and Geoenv., 124(1), 91-94. Rujikiatkamjorn, C., Indraratna, B. and Chu, J. (2008). 2D and 3D numerical modeling of combined surcharge and vacuum preloading with vertical drains. Intl. Jl. of Geomechanics, 8(2), 114–156. Walker, R., Indraratna, B. and Sivakugan, N. (2009). Vertical and radial consolidation analysis of multilayered soil using the spectral method, J. of Geotech. And Geoenv.Engrg, ASCE, 135(5), 657– 663.

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Coal Ashes in Geotechnical Engineering Practice: Beneficial Aspects A. Sridharan INSA Honorary Scientist, Formerly Professor of Civil Engineering, Indian Institute of Science, Bangalore, email: [email protected]

ABSTRACT: Coal ashes have been shown to have advantageous properties such as low specific gravity, lower compressibility, higher rate of consolidation, high strength, high CBR, high volume stability, water insensitiveness to compaction and pozzolanic reactivity. The use of coal ashes having these beneficial properties, which are being considered as industrial wastes, serves as a very use full material in the field of geotechnical engineering. Their use in bulk in the field of geotechnical engineering is an eco-friendly way of their safe disposal. Keywords: coal ash, compressibility, consolidation, compaction, CBR

1. INTRODUCTION Since, the economic development of any country is directly related with the energy production and consumption of that country, more thrust is being applied of late on the electrical power generation sector. Due to their inherent limitations, establishing either the large scale hydroelectric power plants or nuclear power plants is receiving lesser priority. Instead, installing the coal based thermal power plants is being encouraged worldwide. The burning of pulverised coal in thermal power plants results in the production of huge quantum of coal ashes namely fly ash and bottom ash. The wet disposal of these ashes separately or in combination in storage ponds results in pond ashes. With the depletion of high quality coal resources, low quality coals are also being used, which enhance the quantum of coal ashes generated. The current worldwide production of coal ashes is more than 700 million tonnes of which about 70% is fly ash (Malhotra and Mehta, 2002). Huge quantum of coal ashes thus generated coupled with their very low specific gravity has made the ash handling and disposal problems very acute. A proper planning, sensible execution and good management of coal based thermal power generation projects will help not only in minimising the coal ash storage / disposal problems, but also in achieving many ‘positives’. This requires a better understanding of physical, chemical and engineering properties of coal ashes. This paper intends to critically evaluate these properties in general, the beneficial properties in particular, which have been hitherto considered as unwanted wastes and to suggest where exactly they can be effectively used in the field of geotechnical engineering. Most of the experimental data discussed in this paper are the outcome of the experimental investigation carried out at the Indian Institute of Science, Bangalore, on coal ashes procured from various thermal power plants distributed all over India. supported by Fly Ash utilization scheme of DST, Govt of India. Some of the data documented in the geotechnical engineering literature have also been made use of in the discussion.

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ETGE 2012

1.1 Applications of coal ashes High Value utilizations: Mineral extraction; Ceramic industry; Floor and wall tiles; Acid refractory bricks; Fly ash distempers & paints; and Extraction of ceno spheres to name some of them. Medium value utilizations: Pozzolana cement; Cellular cement; Fly ash concrete; Fly as bricks & blocks; Prefabricated building blocks; Light weight aggregates; Grouting; Soil amendment agents/Fertilizers; and Soil stabilization, to name some of them. Low value but large scale utilizations: Mine filling; Back filling; Structural fills; Road construction; Mass concreting; and Embankment and dam construction. Considering the huge quantum of coal ashes being produced, the quantity of coal ashes being used in the non-geotechnical applications is negligible. Coal ashes appear to have attracted limited applications as construction materials except in some developed countries. Limited applications of coal ashes in the field of geotechnical engineering field hitherto can be attributed to lack of better understanding of the beneficial physical, chemical and engineering properties of coal ashes and the advantages they possess over fine-grained soils.

2.

SOIL AND COAL ASHES

Chemical compositions of coal ashes and soils are essentially similar (Table 1) except for the fact that the type of the trace elements present in coal ashes and soils can be quite different. However, coal ashes differ from soils on certain counts which favour their use as alternate / substitute materials to soil in the field of geotechnical engineering (Table 2). Table 1. Range of chemical composition of Indian coal ashes and soils Compounds SiO2 Al2O3 TiO2 Fe2O3 MnO MgO CaO K2O Na2O L.O.I

Fly ash: % 38 -65 16 -44 0.4 -1.8 3 – 20 0 – 0.5 0.01 - 1.53 0.2 – 8 0.04 – 0.9 0.09 – 0.43 0.2 - 3.4

Pond ash: % 37 – 75 11 – 54 0.2 - 1.4 3 – 35 * - 0.6 0.1 – 0.8 0.2 – 0.6 0.1 – 0.7 0.05 – 0.31 0.1 – 7.91

LOI: Loss on Ignition at 9500C (includes H2O (+))

Bottom ash: % 23 – 73 13 – 27 0.2 - 1.8 3 – 11 * - 0.3 0.1 - 0.7 0.1 - 0.8 * - 0.56 * - 0.3 0.61 - 12.8

Soils: % 43 – 61 12 – 39 0.2 – 2 1 – 14 0 - 0.2 0.5 – 4 0–7 0.3 – 2 0.2 – 3 5 – 17

*: trace

Table 2. Differentiating factors between fine-grained soils and coal ashes • • • •

Factors Nature of particles Reactive silica content Free lime content Pozzolanic nature

Natural fine-grained soils Mostly charged platelets Almost nil Nil Mostly non-pozzolanic

Coal ashes No surface charges Could be appreciable Could be appreciable Could be appreciable

Majority of the fine-grained soils are physico-chemically active, which can be attributed to their unbalanced surface charges. Some of the undesirable properties of these soils such as high swell-shrink potential, high compressibility, low permeability, low strength and low CBR are essentially due to the surface charges of fine-grained soils. These surface charges

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Coal Ashes in Geotechnical Engineering Practice

favour the development of diffuse double layer, which in turn can be taken to contribute to the water holding capacity of soils. As a consequence of this the volume stability of the soils gets adversely affected; effective voids volume gets reduced which is responsible for low permeability of such soil; actual void ratio becomes more than the theoretical which is responsible for higher compressibility; double layer repulsion increases which is responsible for the lower effective stress and hence, for lesser strength (Sridharan and Jayadeva, 1982; Sridharan and Venkatappa Rao, 1973; Sridharan and Venkatappa Rao, 1979; Sridharan and Prakash, 1999 Prakash and Sridharan, 2009).

3.

BENEFICIAL PROPERTIES OF COAL ASHES

3.1 Specific Gravity While the values of specific gravity of soils vary over a narrow range of 2.55 – 2.8, those of coal ashes are found to vary over a wide range (i.e. 1.47 – 2.78). In addition, it has been found that the specific gravity of coal ashes is a function of their grain size (Pandian et al. 1998; Trivedi and Sud, 2004). Table 3 presents the representative values of specific gravity of coal ashes from different countries. Low values of specific gravity of coal ashes can be attributed to: spongy / porous nature of ash particles; presence of cenospheres; and unburned carbon content. Low to very low specific gravity of coal ashes makes them suitable for the use as the backfill materials in retaining wall construction, as construction fill materials on weak compressible soils, as fill materials for low-lying areas and as embankment materials. The advantages realised as a consequence of lower specific gravity of coal ashes in these applications are less lateral pressures on retaining structures • less over burden pressures on foundation soils • reduction in the settlement of foundation soils • reduction in the tendency of weak sub soil to undergo failure • realisation of relatively steep side slopes of embankments Table 3: Specific gravity of coal ashes from different countries Type of coal ash Fly ash

Pond Ash

Bottom Ash

Country India USA Canada Thailand India UK Poland India USA

Specific Gravity 1.66 – 2.55 2.03 – 2.49 1.90 – 2.90 2.27 – 2.45 1.64 – 2.66 2.10 – 2.24 1.90 – 2.31 1.47 – 2.19 2.28 – 2.78

Reference Author’s files Matin et. al. 1990 Indraratna et. al. 1991 Indraratna et. al. 1991 Author’s files Skarzynska et. al. 1989 Skarzynska et. al. 1989 Author’s files Seals et. al. 1972

3.2 Pozzolanic Reactivity Based on chemical composition, fly ashes have been classified into two groups namely class F and Class C fly ashes (ASTM C 618 – 94a). While class F fly ashes are pozzolanic, class C fly ashes have both pozzolanic and cementations properties. A pozzolanic reaction is one in which siliceous material reacts in the presence of moisture and calcium to form compounds exhibiting cementations properties. This property of coal ashes, particularly of fly ashes, makes them drastically different from fine-grained soils. The fly ashes are known for their

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pozzolanic value as they are sources of reactive silica available in them in the amorphous form and / or as aluminate in the crystalline form. The pozzolanic reactivity or lime reactivity is normally expressed as the compressive strength of standard mortar cubes prepared using coal ashes and tested under specified conditions (IS: 1727, 1967). The lime reactivity of some typical Indian coal ashes are presented in Table 4. Fly ashes exhibit greater lime reactivity than the pond and bottom ashes due to their high reactive silica content. Table 4 also indicates that the lime reactivity reduces with aging. Table 4. Lime reactivity values of typical Indian coal ashes* Sl. No.

Source of coal ashes

Vijayawada

Ramagundam

Farakka FA: Fly ash

Type of coal ashes FA PA BA FA PA BA FA PA BA PA: Pond ash

Lime reactivity: kPa Fresh samples

Aged samples

3640 2186 220 34 4240 4026 85 200 5237 1211 119 55 BA: Bottom ash * Data from author’s files

The engineering performance of fly ashes gets improved with time, by virtue of the pozzolanic reactions. This property is responsible for them to exert lower lateral pressure on retaining structures, lower over burden pressures on foundation soils; to experience reduced secondary settlements and to have an increased shear strength and CBR with time. 3.3 Compaction Characteristics Compaction is an important process to which soil is subjected in the field to achieve the required dry unit weight at specified water content. The reference data for the field compaction is obtained through either standard Proctor or modified Proctor compaction tests in the laboratory. The test data is normally expressed through compaction curves (i.e. dry unit weight vs water content plot) along with zero air voids line. The range of specific gravity variation of coal ashes is more when compared with that of soils, in spite of identical chemical composition and grain size distribution. Hence, Sridharan et. al. (2001) felt that it would not be appropriate to compare the compaction characteristics of coal ashes with those of soils obtained through conventional compaction curves and that such a comparison would not be realistic (Fig. 1). They suggested the plotting of dry unit weights and corresponding water contents of coal ashes after normalising with a standard specific gravity. They suggested that 2.65 be taken as the standard value (Gstd) as it represented most of the soils. If γdn and wm are the dry unit weight and corresponding compaction water content of a coal ash of specific gravity Gm obtained from the compaction test, then the corresponding normalised dry unit weight and normalised water content can be calculated using equations 1 and 2. G  γ dn = γ dm  std  (1) G  m 

15

Coal Ashes in Geotechnical Engineering Practice 25

Dry unit weight, kN/m3

20

Source of Fly Ash Rihand (last field) Ramagundam Raebareli Vijayawada BSES Badarpur Neyveli

G 2.29 2.23 2.06 2.11 2.09 2.12 2.55

A-Z: Soils (Joslin, 1958)

2.70

A D H L P

15

T

X Z

ZAV (G

10

= 2.7 0)

ZAV (G =

5

0

20

40

2.06)

60

80

Water content, % Figure 1. Compaction curves of typical fly ashes (data source: Sridharan et. al. 2001)

Normalised dry unit weight, kN/m3

25

20

A D H L P

15

T

Source of Fly Ash Rihand (last field) Ramagundam Raebareli Vijayawada BSES Badarpur Neyveli

G 2.29 2.23 2.06 2.11 2.09 2.12 2.55

A-Z: Soils (Joslin, 1958)

2.70

X Z ZAV (G

10

5

0

20

40

= 2.6 5)

60

80

Normalised Water content, % Figure 2. Normalised compaction curves for fly ashes (data source: Sridharan et. al. 2001)

G w n = w m  m  G std

  

(2)

Fig. 2 represents typical compaction curves of Indian fly ashes in the normalised mode. Table 5 presents the compaction characteristics of coal ashes published in the literature. Fig. 2 suggests that the compaction curves and compacted dry unit weights are insensitive to the water content variation during compaction. These observations are of primary importance in that the field compaction does not require much of compaction control. This facilitates the

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coal ashes to be effectively used in the construction of pavements and embankments. However, if the fly ash is of pozzolanic type (i.e. class C), then care should be exercised to avoid delay between mixing and compacting the fly ash in the field, as the delayed compaction results in lower dry unit weights and higher OMC (Sivapullaiah et. al. 1998). Table 5. Compaction characteristics of coal ashes from literature Standard Proctor compaction OMC: % γdmax kN/m3 Fly ash India 8.0 – 15.5 15.5 – 59.9 9.2 – 17.1 Canada – 12.91 – 37.21 10.23 – 20.22 Thailand – 12.42 – 15.16 16.01 – 17.98 Pond ash India 7.8 – 15.8 14.6 – 36.8 12.2 – 17.1 Bottom India 7.6 – 12.6 21.3 – 58.1 7.5 – 13.7 ash USA 8.89 – 13.29 14.2 – 23.2 12.98 – 17.10 Note: The values of dry unit weight and OMC are normalised values. Type of coal ash

Country

γdmin kN/m3

Reference Author’s files Indraratna et. al. (1991) Indraratna et. al. (1991) Author’s files Author’s files Seals et. al. (1972)

3.4 Shear Strength Shear strength parameters depend upon the type of test, placement condition of the specimen and drainage conditions during testing. Table 6 gives the typical values of shear strength parameters of typical Indian coal ashes tested in shear box apparatus in the loose as well as compacted states. The study of shear strength behavior of coal ashes in the shear box apparatus reveals the following. • Being cohesionless, non-plastic materials, coal ashes owe all their shear strength to frictional component except in the compacted, unsaturated state where apparent cohesion is also present which reduces to zero upon saturation. • Coal ashes exhibit higher angle of shearing resistance, at both peak and residual stress levels, even in the soaked conditions. • Coal ashes have angle of shearing resistance varying in the range 25o – 34o, even under loose conditions. The strength loss upon saturation is very small. • In spite of their low unit weights, coal ashes exhibit high shear strengths when compared with natural soils. Some typical results obtained from tri axial shear tests on Indian coal ashes are summerised in Table 6. The study of the shear strength behaviour of Indian coal ashes in triaxial testing apparatus both at the peak and at the residual test levels have indicated the following (Gray and Lin, 1972; Sridharan et al. 1998; Pandian et al. 2001b; Pandian et al. 2001c; Sridharan et al. 2002, Prakash and Sridharan, 2009). • Variation of effective friction angle is negligibly small, irrespective of whether it is observed from consolidated undrained test or consolidated drained tests. • Variation of effective friction angle of fly ash with initial dry density is not appreciable. • For pozzolanic fly ashes, shear strength increases with curing period. • Peak and residual shear strength parameters are comparable. • Over consolidation increases the angle of shearing resistance appreciably. High to very high shear strength parameters of coal ashes both at peak and residual stress levels, both in the loose condition and compacted / compacted – saturated condition favour their use in the field as all the problems concerned in the field with bearing capacity, slope stability of embankments, design of pavements and retaining structures are dependent on shear strength characteristics.

17


3.5 California Bearing Ratio (CBR) The CBR is an useful parameter in judging the suitability of the material for its intended use in the road construction and in the design of pavements. The CBR test can be carried out on Table 6. Typical values of Shear Strength Parameters of Indian coal ashes at different states (Sridharan et al. 1998) e at 95% γdmax

Compacted state c', φ', kPa degrees

Compacted, saturated state c', φ', kPa degrees

30 30 31

0.66 0.96 1.51

23 16 17

34 31 32

00 00 00

33 30 31

32 31 32

31 32 33

0.58 1.03 1.12

16 13 10

37 33 34

00 00 00

35 32 33

31 32 33

30 31 33

1.01 1.25 1.38

26 14 19

32 30 31

00 00 00

32 29 30

Loose state

Source of coal ash

Type

eloose

φ'dry, degrees

φ'res, degrees

φ'sat, degrees

Raebareli

FA PA BA

2.36 1.38 2.60

31 32 32

29 30 31

Vijayawada

FA PA BA

1.53 1.39 1.98

33 33 34

Badarpur

FA PA BA

1.52 2.59 2.09

32 33 34

FA: Fly ash PA: Pond ash Note: The values indicated are from box shear tests

BA: Bottom ash

Table 7. Shear Strength Parameters of compacted and saturated coal ashes from triaxial shear tests* Placement condition Compacted to 95% γdmax. on dry side

Type of coal ash FA PA BA

Consolidated drained tests

Consolidated undrained tests

φcd: degrees 33-43 -

φcu: degrees 20-41 25-34 24-35

ccd: kPa 00 -

ccu: kPa 00 0-56 0-27

φ': degrees 26-39 28-36 24-35

c': kPa 16-96 28-101 28-55

* Data from author’s files.

compacted specimen either in the un soaked condition or after 96 hours of soaking. Both class F and class C fly ashes exhibit higher CBR in the un soaked condition. These higher CBR values are due to capillary forces, which exist in the partly saturated state. The CBR of class F fly ash tends to reduce drastically as the capillary forces reduce to zero on submergence. However, class C fly ashes retain very high CBR values even when tested after soaking, which can be attributed to pozzolanic reactions Normally, the design practice is to prefer soaked CBR values. However, it is justifiable to use soaked CBR values for those areas which are low-lying with poor drainage facilities, resulting in the submergence of roads. However, for areas that have good drainage facilities such that the roads will not get submerged even in the worst rains, it is justifiable to use the un soaked CBR in the design of pavements. The CBR of soils belonging to groups OH, CH, and MH have been observed to vary in the range 0-7% (Bowles, 1988). It is also observed that the CBR of coal ashes are much more than those of many fine-grained soils (Table 8). This characteristic makes them suitable for use as sub-base materials in the construction of pavements.

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ETGE 2012

3.6 Compressibility and Consolidation Characteristics Compressibility characteristics namely compression index (Cc) and coefficient of volume change (mv) are important from the view point of calculation of settlement of structures. Table 8. Values of CBR of compacted coal ashes and soils ( Prakash and Sridharan, 2009) Sl. No.

Source of Material

Type of Material

Thailand

FA (class C)

Raichur

FA

Vijayawada Raichur Badarpur Kahalgoan Davanagere – FA: Fly ash

FA PA BA PA PA BA PA BA Black cotton soil Heavy clay PA: Pond ash

CBR: % Testing condition Compacted at OMC Compacted at 0.95% γdmax, on the dry side Compacted at 0.95% γdmax, on the dry side - do - do - do - do -

Unsoaked condition

Soaked condition

325

280

6.9

3.5

20.6 10.5 6.8 10.1 11.1 11.3 8.9 9.7

0.2 0.9 6.0 4.4 8.5 4.6 5.9

4.15

1.83

Compacted at 7.8 OMC BA: Bottom ash

–

Compressibility characteristics of fly ashes depend upon their initial dry unit weight, degree of saturation, self hardening characteristic, pozzolanic reactivity and mixing time (Gray and Lin, 1972; Yudhbir and Honjo, 1991). Their values along with the placement conditions are more meaningful while judging their suitability in the field than just their numerical values as depicted in Table 9. Fully saturated fly ashes are more compressible than the partly saturated fly ashes. Self hardening fly ashes compacted at OMC and saturated are less compressible than those compacted at OMC. If the fly ashes are of pozzolanic type, then the curing period also has appreciable influence on their compressibility (Yudhbir and Honjo, 1991). Table 9. Cc as a function of placement void ratio (Yudhbir and Honjo, 1991) Placement void ratio

Placement condition

0.3 – 1.0

Conditioned compacted and conditioned grab placed

1.0 – 2.0

Hydraulically placed in lagoons

2.0 – 3.0

Lagoon fly ashes to loose dumps

Cc (0.1 ± 0.7) to (0.225 ± 0.15) (0.225 ± 0.16) to (0.4 ± 0.21) (0.4 ± 0.21) to (0.625 ± 0.26)

Comments Very dense to medium dense Loose to medium dense

Very loose

Normally, 75% - 80% of total settlement of structures founded on fly ashes is due to primary consolidation, which depends upon their coefficient of consolidation (cv). The coefficient of consolidation of fly ashes is so high that it is extremely difficult to record time-compression readings in the laboratory consolidation testing to determine cv using curve fitting procedures.

19


In addition, it observed that the values of cv calculated from the curve fitting procedures from the laboratory very much underestimate the actual field behaviour. Hence, it is preferable to calculate the value of cv from the measured value of coefficient of permeability (k) and coefficient of volume change from equation 3. cv = k / [mvγw]

(3)

Table 10 lists the values of cv of coal ashes compacted at OMC from different countries. It has been observed that the values of cv of fly ashes from Hong Kong calculated using the field permeability with the help of eq. 3 are about 3 – 10 times higher than those listed in Table 10 (Yudhbir and Honjo, 1991). This lends support to the suggestion that the realistic values of cv can be obtained from eq. 3. In addition, the values of cv of fly ashes, which are silt sized particles, calculated from eq. 3 are in the range of values that corresponds to silts. Pond ashes and bottom ashes exhibit much higher cv values owing to their coarser size. Table 10. cv of Coal ashes compacted at OMC Country *India

Type of coal ash FA FA PA BA

cv: cm2/s 0.08-2

0.14-3.25 0.96-10 1.43-10.15 9.51 x 10-3 Hong Kong FA - 19.03 x10-3 9.5 x10-4 U.K FA - 6.34 x10-3 3.2 x 10-4 Thailand FA -7.61 x 10-3 *cv calculated from measured coefficient of permeability using eq. 3.

Reference Kaniraj and Gaythri, 2004 Author’s file Author’s file Author’s file Yudhbir and Hanjo, 1991 -do-do-

The higher values of cv of coal ashes signify an important fact that the primary consolidation of structures founded on coal ashes will be practically over during the period of construction itself. This feature makes the coal ashes superior for use as foundation base materials, as reclamation fills and as materials of construction for embankments and dams. 3.7 Permeability Coefficient of permeability of coal ashes depends upon their grain size distribution, testing conditions of coal ashes and the pozzolanic reactivity of coal ashes. Being coarser in size, bottom ashes are relatively more permeable than pond and fly ashes. Table 11 presents the values of k of coal ashes from different countries. The values of k of most of the fly ashes are in the range of k of silts. The permeability of coal ashes remains almost constant over a wide range of over burden pressure. These observations indicate that the coal ashes are normally freely draining materials, and are best suited for use as backfill materials behind the retaining structures, as sub-base materials in pavements and as embankment shell materials. The self cementing and pozzolanic fly ashes (i.e., class C type) exhibit lower permeability than class F fly ashes, and their permeability tends to reduce appreciably with time in the filed Such fly ashes can be more effectively used as liner materials in waste containment structures and as additives in the construction of effective seepage cutoffs like impervious blankets and cores in water retaining earth structures.

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ETGE 2012

3.8 Swell and Shrink Potential Coal ashes exhibits high to very high volume stability (i.e., low swell and shrink potential), which can be attributed to their non-plastic nature and uniform gradation. Table 11. Values of k for coal ashes from different countries Country India

Thailand UK Japan USA

Type of Coal Ash

Testing condition of the coal ash

k: cm/s

Reference

FA PA BA

Compacted at γdmax and saturated

8 × 10-6 – 1.87 x 10-4 5 × 10-5 – 9.63 × 10-4 9.9 × 10-5 – 7.07 × 10-4

Author’s files

FA

Compacted to 0.95 γd max and Saturated

1.4 ×10-5 – 4.23 × 10-4

FA

γd max

4.6 ×10-6 – 6 × 10-6

Compacted at OMC γd max Slurry (ei = 0.85 to 1.02) Relative density = 50 %

< 10-7

FA (class C) FA FA BA

Low carbon, high calcium FA γd max (class-C) High carbon, low calcium FA γd max (class-F) FA γd max BA γd max Poland PA Canada FA In situ BA Silt FA: Fly ash PA: Pond ash

5 × 10-7 – 8 × 10-5

Pandian and Balasubramonian, 1999 Kaniraj and Gayathri, 2004 Indrarathna et.al., 1991 Gray and Lin, 1972

10-5 –10-4

Porbaha et.al., 2000

5 x 10-3 – 0.094

Seals et.al.,1972

1 × 10-7 -2 × 10-7 Yudhbir and Honjo, 1991 6 × 10-5 – 2 × 10-6 1.8 ×10-5 -1.2×10-4 1.2 × 10-3 1.5× 10-5 – 5 × 10-5 10-7 – 10-4 3.4 x 10-3 – 4.8 x 10-3 1 × 10-7 – 1 x 10-3 BA: Bottom ash

Martin et.al., 1990 Skarzynska et.al.,1989 Toth et al., 1988 -

In the field of geotechnical engineering, the degree of expansively of soils can be judged based on Free Swell Ratio (FSR). It is defined as V FSR = d (4) Vk where Vd is the equilibrium sediment volume of 10 g of oven dried soil passing 425 µm sieve placed in a 100 ml jar containing distilled water with an initial volume of soil – water mixture equal to 100 ml after 24 hours of equilibration, and Vk is the equilibrium sediment volume of an identical soil sample in carbon tetra chloride or kerosene (Sridharan and Prakash, 2000b). Fine-grained soils can be classified as per the criteria given in Table 12. The Indian fly ashes, pond ashes and bottom ashes have been observed to have free swell ratios in the ranges 0.513 – 0.95, 0.647 – 1.1 and 0.8 – 1.16 respectively, indicating negligible degree of expansivity or swell potential. The shrinkability of soils is normally judged by their shrinkage limit. The shrinkage limit of soil is primarily controlled by the relative grain size packing of different sized particles

21


composing the soils (Sridharan and Prakash, 1998; Sridharan and Prakash, 2000a Prakash and Sridharan, 2009). Well-graded soils have lower shrinkage limits, and uniformly graded soils exhibit higher shrinkage limits. The non-plastic nature of coal ashes does not allow their shrinkage limit to be determined in the laboratory. However, it can be inferred that they exhibit high shrinkage limit owing to their uniform gradation. Table 12. Soil classification based on FSR (Sridharan and Prakash 2000b) Free swell ratio ≤ 1.0 1.0 – 1.5 1.5 – 2.0 2.0 – 4.0 > 4.0

Clay type Non-swelling Mixture of swelling and non-swelling Swelling Swelling Swelling

Soil expansivity Negligible Low Moderate High Very high

Low to very low swell and shrink potential of coal ashes can be taken the best advantage of in the construction of pavements, embankments, dams and as foundation base materials. 4. SOIL STABILISATION The poor gradation with silt / sand sized particles of fly ashes and their high to very high frictional strength even in the loose condition make them good mechanical admixtures in the filed of soil stabilisation. Addition of fly ash to cohesive soil will increase the strength of the resulting mix by virtue of the enhanced frictional strength and pozzolanic reactions. Fig. 3 shows the variation of CBR of a black cotton soil (wL = 56%; wP = 23%; wS = 10.3%; clay size fraction = 45.9%), which is an expansive soil, with the addition of different fly ash contents to the soil. Fig. 2 represents typical compaction curves of Indian fly ashes in the normalised mode. Table 5 represents the compaction characteristics of coal ashes represented in the literature. The study of Fig. 2 suggests that the compaction curves and compacted dry unit weights are insensitive to the water content variation during compaction. These observations are of primary importance in that the field compaction does not require much of compaction control. This facilitates the coal ashes to be effectively used in the construction of pavements and embankments. However, if the fly ash is of pozzolanic type (i.e. class C), then care should be exercised to avoid delay between mixing and compacting the fly ash in the field, as the delayed compaction results in lower dry unit weights and higher OMC (Sivapullaiah et. al. 1998). The addition of class F type of fly ash to the soil has resulted in a mix having more CBR than those of soil and fly ash alone. Both the curves, corresponding to soaked and unsoaked conditions of testing, exhibit two peaks. The first peak (represented by A and A') corresponds to soil stabilisation. This peak is a consequence of the following mechanisms. 1)Addition of fly ash provides coarser particles to improve the gradation of fine-grained soil. This will help in achieving better compacted density and hence, more strength. 2) Fly ash provides frictional component of shear strength to cohesive soil which has cohesive shear strength component already. This is responsible for the improved strength and CBR of the mix. These two mechanisms together help the stabilised cohesive soils in the field to exhibit a

22

ETGE 2012

better performance from the strength point of view. The second peak (represented by B and B') corresponds to fly ash stabilisation. This peak is due to the following mechanisms. The deficiency of finer particles in the fly ash is made up by the addition of fine-grained soil particles. This results is a better compacted density and hence, more strength Cohesive shear strength imparted by the addition of cohesive soil to fly ash has helped the resulting mix in achieving These two mechanisms together help the stabilised fly ash to exhibit higher strength and higher CBR. The CBR of class F fly ash can also be improved by the addition of coarser soil to it which results in a better grain size packing. 12

Unsoaked condition Soaked condition

A' 10

B'

CBR: %

8

6

B

A 4

2

0

0

20

40

60

80

100

Raichur fly ash content in the mix: % Figure 3. Variation of CBR of BC soil – Raichur fly ash (class F fly ash) mixtures (data source: Pandian et al. 2001a) The addition of class C type fly ash to a fine-grained clayey soil will continue to increase the CBR of the resulting mix with time due to the pozzolanic reactions (Fig. 4). In addition, the fly ashes when used as mechanical admixtures to stabilise expansive soil reduce the swell – shrink potential of expansive soils, thus providing them an improved volume stability. Apart from knowing these beneficial characteristics of coal ashes, one has to be aware of their certain undesirable properties also. • Class F fly ashes are highly dispersive. With the result, they are easily erodible. • At very low compacted densities, they exhibit high collapse potential. • Their frost susceptibility is high. However, these undesirable properties can be improved by treating them with chemical admixtures such as lime or cement or lime – gypsum and / or with mechanical admixtures such as soils.

23

Coal Ashes in Geotechnical Engineering Practice 200

Unsoaked condition Soaked condition

160

CBR: %

120

80

40

0

0

20

40

60

80

100

Neyveli fly ash content in the mix: %

.

Figure 4. Variation of CBR of soil – Neyveli fly ash (class C fly ash) mixtures (data source: Krishna, 2001)

5.

CONCLUSIONS

The common understanding among the people is that the coal ashes, which are by-products of thermal power generation industry, are waste materials which are harmful to the environment and to the people of the region as well. However, the study of the physical, chemical and engineering properties of coal ashes shows that the coal ashes are potential resourceful materials from the geotechnical engineering applications view point. The present paper has discussed many properties of coal ashes which can be used with the advantage in various geotechnical engineering applications. They are – low specific gravity, lower compressibility, higher rate of consolidation, higher frictional strength, higher CBR, negligible swell – shrink potential, water insensitiveness of compaction characteristics and pozzolanic reactivity. The beneficial properties of coal ashes discussed in this paper encourage their use as • fill materials for low-lying areas • construction fill materials on weak compressible soils • The ever increasing scarcity for good materials in various geotechnical engineering projects can also be overcome by the use of large scale use of coal ashes as • back fill materials in retaining structures • good foundation base materials • sub-base materials for pavements • construction of earth embankments and dams • mechanical admixtures in stabilising expansive and cohesive fine-grained soils. ACKNOWLEDGEMENT The author thanks Prof. K Prakash, Professor of Civil Engineering, SJCE, Mysore for his help in preparing this paper and his contribution as co worker in many of his investigations. He is

24

ETGE 2012

grateful to the Indian Academy of Sciences for providing the Honorary Scientist position to him. The author wishes to thank Dr. Murali Krishna, IIT Guwahati for his help in formatting this paper. REFERENCES American Society for Testing Materials (1995),ASTM Designation C 618 – 94a, Standard Specifications for coal ash and raw or calcined natural pozzolan for use as a mineral admixture in portland cement concrete, Annual book of ASTM standards, Vol. 104.02, ASTM, Philadelphia. Bowles, J.E. (1988), Engineering properties of soils and their measurement, McGraw Hill Book Company, New York. Gray, D.H. and Lin, Y.K. (1972), “Engineering properties of compacted fly ash”, J. Soil Mech. Found. Div. ASCE, Vol.98, No. SM 4, pp.361-380. H.M.S.O. (1957), Soil mechanics for road engineers, Her Majesty’s Stationery Office, London. Indian Standard Institution (1967), IS: 1727, Method of test for pozzolanic materials, BIS, New Delhi. Kaniraj, S.R. and Gayathri, V. (2004), “Permeability and consolidation characteristics of compacted fly ash”, J. Energy Engineering, ASCE, Vol. 130, No. 1, pp. 18-43. Krishna, K.C. (2001), CBR Behavoiur of Fly Ash - Soil - Cement Mixes, Ph.D thesis submitted to IISc, Bangalore, India. Malhotra, V.M. and Mehta, P.K. (2002), High-Performance, High-Volume Fly Ash Concrete, Supplementary Cementing Materials for Sustainable Development Inc., Ottawa, Canada. Martin, J.P., Collins, R.A., Browning, J.S. and Biehl, F.J. (1990), “Properties and use of fly ashes for embankments”, J. Energy Engineering, ASCE, Vol. 116, No. 2, pp. 71-86. Pandian, N.S., and Balasubramonian, S. (1999), “Permeability and consolidation behaviour of fly ashes”, J. Testing and Evaluation, ASTM, Vol. 27, No. 5, pp. 337-342. Pandian, N.S., Krishna, K.C. and Sridharan, A. (2001a), “California bearing ratio behaviour of soil / fly ash mixture”, J. Testing and Evaluation, ASTM, Vol. 28. No. 2, pp. 220-226. Pandian, N.S., Rajasekhar, C. and Sridharan, A. (1998), “Studies of the specific gravity of some Indian coal ashes”, J. Testing and Evaluation, ASTM, Vol. 26. No. 3, pp. 177-186. Pandian, N.S., Sridharan, A. and Chittibabu, G. (2001b), “Shear strength of coal ashes for geotechnical applications”, Proc. of Ind. Geotech. Conf. Indore, Vol. 1, pp. 466-469. Pandian, N.S., Sridharan, A. and Chittibabu, G. (2001c), “Strength behaviour of compacted coal ashes for geotechnical applications”, Proc. of the International Symposium on Geotechnical and Environmental Challenges in Mountainous Terrain, Kathmandu, Nepal. pp. ?. Porbaha, A., Pradhan, T.B.S. and Yamane, N. (2000), “Time effect on shear strength and permeability of fly ash”, J. Energy Engineering, ASCE, Vol. 126, No. 1, pp. 15-31. Prakash, K. and Sridharan, A. (2006), “A geotechnical classification system for coal ashes”, Geotechnical Engineering, Proc. Inst. Civil Engg. (London), Vol. 159. No. GE2, pp. 9198. Prakash, K., and Sridharan, A. (2009), “Beneficial Properties of Coal Ashes and Effective Solid Waste Management”, Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, ASCE, Vol. 13, No. 4, pp. 239-248 Seals, R.K., Moulton, L.K. and Ruth, B.E. (1972), “Bottom ash: An engineering material”, J. Soil Mech. Found Div. ASCE, Vol. 98, No. 4, pp. 311-325. Sivapulliaih, P.V., Prashanth, J.P. and Sridharan. A. (1998), “Effect of delay between mixing and compaction on strength and compaction properties of fly ash”, Geotech. Engg.


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Bulletin, Vol. 7, No. 4, pp. 277-285. Skarzynska, K.M., Rainbow, A.K.M. and Zawiska, E. (1989), “Characteristics of ash in storage ponds”, Proc. 12th Int. Conf. on S.M.&F. Engg., Rio de Janerio, Vol. 3, pp. 19151918. Sridharan, A., Chittibabu, G. and. Pandian, N.S. (2002), “Strength behaviour of over consolidated fly ashes”, Proc. of Ind. Geotech. Conf. Allahabad, Vol. 1, pp. 3-6. Sridharan, A., and. Jayadeva, M.S. (1982), “Double layer theory and compressibility of clays”, Geotechnique, Vol. 32, No. 2, pp. 133-144. Sridharan, A., Pandian, N.S. and Srinivas, S. (2001d), “Compaction behaviour of Indian coal ashes”, Ground Improvement, Vol. 5, No.1, pp. 13-22. Sridharan, A., Pandian, N.S. and Srinivasa Rao, P. (1998b), “Shear strength characteristics of some Indian fly ashes”, Ground Improvement, Vol. 2, No. 3, pp. 141-146. Sridharan, A. and Prakash, K. (1998), “Mechanism controlling the shrinkage limit of soils”, Geotechnical Testing Journal, ASTM, Vol.21, No. 3, pp.240-250. Sridharan, A. and Prakash, K. (1999), “Influence of clay mineralogy and pore medium chemistry on clay sediment formation”, Canadian Geotechnical Journal, Vol.36, pp.961966. Sridharan, A. and Prakash, K. (2000a), “Shrinkage limit of soil mixtures”, Geotechnical Testing Journal, ASTM, Vol. 23, No. 1, pp.3-8. Sridharan, A. and Prakash, K. (2000b), “Classification procedures for expansive soils”, Geotechnical engineering, Proc. Inst. Civil Engg. (London), Vol.143, pp.235-240. Sridharan, A. and Venkatappa Rao, G. (1973), “Mechanism controlling volume change of saturated clays and the role of the effective stress concept”, Geotechnique, Vol. 23, No. 3, pp. 359-382. Sridharan, A. and Venkatappa Rao, G. (1979), “Shear strength behaviour of saturated clays and the role of the effective stress concept”, Geotechnique, Vol. 29, No. 2, pp. 177-193. Toth, P.S., Chan, H.T. and Cragg, C.B. (1988), “Coal ash as structural fill with specific reference to Ontario experience”, Canadian Geotechnical Journal, Vol. 25, pp. 694-704. Trivedi, A. and Sud, V.K. (2004), “Collapse behaviour of coal ash”, J. Geotech. and Geoenv. Engg. ASCE, Vol.130, No. 4, pp.403-415. Yudhbir and Honjo, Y. (1991), “Applications of geotechnical engineering to environmental control”, Proc. 9th Asian Reg. Conf. on S.M.&F.E., Bangkok, Thailand, Vol. 2, pp. 431469.

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Ground Response and Support Measures for a Railway Tunnel in the Himalayas K. S. Rao Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, email: [email protected]

ABSTRACT: The Pir Panjal tunnel linking between Banihal and Qazigund stations is the important tunnel in the railway line from Udhampur to Baramula in the Himalayas. The Pir Panjal ranges are having complex geological set up with major folds and faults. More than six major lithological units are traced along the 11 km length of the tunnel with very high overburden at many sections. The phenomenon of squeezing is studied using the limit equilibrium and FLAC methods for this tunnel. A detailed stress and displacement assessment has been attempted in this study, in order to stabilise the tunnel sections with suitable support measures. Keywords: ground response, support measures, railway tunnels

1. INTRODUCTION A large number of power and transport tunnel projects are being constructed in the tectonically active and young Himalayan Mountains. The main areas of concern regarding tunnel stability are the existence of weak, highly deformable and anisotropic rock mass and high degree of weathering and fracturing. Tunnel squeezing is common in the Himalayas in weak rock such as shale, slate, phyllite, schist and in weakness/fault zones and represents one of the major areas of concern regarding stability. Also in some areas due to very high overburden and brittle rock mass, explosive conditions develop resulting in rock bursts. Rock burst is the explosive failure in rock which occurs when very high stress concentrations are induced around underground openings. Though ravelling, swelling, running and flowing are occasional but rock squeezing is common in the Himalayas, leading to tunnel collapses. Several tunnels and bridges are being constructed by Ircon Int. Ltd and Konkan Rly Corp. Ltd for ambitious railway link between Katra to Qazigund in the Himalayas for the Northern Railways. The Pir Panjal rail tunnel is a part of the new railway line from Udhampur to Baramulla. The tunnel crosses the different formation of Pir Panjal range and runs around 11km length. In this study an attempt is made to evaluate ground response of Pir Panjal tunnel through limit equilibrium and Finite element approaches. Especially efforts were made to assess the squeezing and rock bursting conditions through out the length of the tunnel and stability measures are suggested for the affected sections. 1.1. Rockmass Response and Collapses Changes in stress around tunnel excavations can result in the behaviour of the rock mass which in turn may lead to damage, failure and consequent collapse of the rock mass. Sequence of rock mass behaviour leading to regional failure is explained schematically by Szwedzicki (2003) as shown in Fig.1. Accordingly, there will be several indicators and precursors which will lead to local damage and subsequently regional failure. An indicator is defined as a sign, 27

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a state or a contributing factor that points out or suggest that the rock mass may be prone to damage or failure. In general potential failure is indicated by geotechnical and operational factors. A geotechnical precursor is a state or behaviour that suggests that the structure of the rock mass has been damaged prior to possible failure. Precursors, including results from instrumentation, warn of the development of excess ground deformations or high stress. Local damage is manifested by the following precursors e.g. spalling, squeezing, bursting, roof sagging, local falls, slabbing, joint dilation, creep, floor heaving, support damage etc.

Figure 1 .Sequence of Rockmass Behaviour 1.2 Squeezing Time dependent large displacement occurring around tunnels and other openings essentially associated with creeping is known as “squeezing rock conditions”. Several authors tried to explain the phenomena in the past (Dube et al. 1986, Ayden et al. 1993, Singh et al. 1999, Goel et al. 1995 and Panthi and Nilsen, 2007). Tunnels in weak rocks such as schists, shales, slates and phyllites when subjected to high ground stresses, experience squeezing conditions. This behaviour implies that yielding will occur around the tunnel resulting in convergence and face displacements. Singh et al. (1992) differentiated squeezing and non-squeezing overburden pressure and Q-ratting. Similarly several authors defined squeezing based on other criteria as well. Hoek and Marinos (2000) defined squeezing based on strain % (which is tunnel closure/tunnel diameter X 100) and ratio of rock mass strength and in situ stress. Based on this they proposed a classification for squeezing level as shown in Fig. 2. When σcm/po is low, the strain is very high (10%) indicating extreme squeezing conditions where as when the stress ratio is high (0 – 6), the strain % is < 1 % implying few support problems. The above classification is used in this study to define the squeezing problems.

2. THE PIR PANJAL TUNNEL The proposed Pir Panjal tunnel is part of the railway line from Udhampur to Baramula in the Himalayas. The tunnel across Pir Panjal range is located between the future railway stations Banihal in the south and Qazigund in the north. The total length of the horse shoe shape with flat floor tunnel will be 11 km (10960m) length with 8.0m height and 8.94m width. It is

Ground Response and Support Measures for a Railway Tunnel in the Himalayas

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completely straight and runs almost parallel to North-South direction. The overburden at both portals is about 10m above tunnel crown, while the maximum overburden is approximately 1150m. About 4 km of the tunnel length has an overburden of more than 500m and about 650m of the length has more than 1000m. The excavation method is NATM with drill-blast. The tunnel layout is shown in Fig. 3.

Figure 2. Classification of Squeezing

Figure 3. 2D View of Pir Panjal Tunnel 2.1 Geological Setup The tunnel alignment traverses through steeply slopping highly undulating hill slopes of the Pir Panjal range which is part of the young lower Himalayas. Formation levels at tunnel portals are at elevation 1713.63 (South portal) and 1956. 70 (North portal). The highly folded and faulted mountain ranges have a strike of bedding is NW-SE. Distinct folding is visible in the central regions. Bedding of the southern slopes dip with 60 – 90⁰ towards NE while on northern slopes dip with 36 – 45 towards SW. Contact between rock units are often faulted. The lithological units are Zewan beds, Gangamopteris bed, Panjal traps, conglomerate bed, agglomerate slates, fenestella shale and syringothyeis limestone. The main rock units are limestones, quartzites, shales, sandstone, conglomerate and fluvoglacial materials. Table 1 details the anticipated rock units at different chainage of the tunnel. The table also presents maximum and minimum overburden at different sections of the tunnel.

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Table 1. Anticipated Rock Types Over the Length

2.2 Geotechnical Parameters Extensive geotechnical investigations were carried out through drilling number of boreholes as well as several shafts and drifts. Because of high overburden, there was a limitation of drilling depth. However, based on surface mapping, drilling and mapping in the drifts and adits, the rock type classes were determined. Extensive laboratory tests were carried out for obtaining bulk density, cohesion, c, friction φ, uniaxial compressive strength, σc , tensile strength, σt, Modulus, Et and Poisson’s ratios for all rock varieties. Rock mass properties and joint parameters were also established through relevant field and laboratory tests. Adopted geotechnical parameters for the ground response analysis are presented in Table 2.

3. GROUND RESPONSE ANALYSIS: ANALYTICAL APPROACHES Predictions of stresses and displacements around a circular opening in rock mass at great depth are an important problem in geotechnical, petroleum and mining engineering. The main analytical approaches adopted are: i. Limit Equilibrium Method ii. Numerical Method


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The closed form solutions are based on simplified assumptions e.g. shape of the opening is regular (mostly circular, elliptical, or spherical), the media is homogeneous and isotropic. They are easy and provide insight into how the mechanical variables influence the deformation behaviour (Hoek and Brown, 1994). To identify the magnitude of stresses and deformations in Pir Panjal tunnel, calculations were carried out based on closed form solution for circular shape of equivalent opening in elasto-plastic medium with primary stress field of Ko=1. Table 2. Geotechnical Design Parameters

Numerical methods include such techniques as finite element, finite difference and boundary element. Depending upon geological media two approaches to numerical modelling is identified. A continuum approach treats the rock mass as continuum intersected by a number of discontinuities, while a discontinuum approach views the rock mass as an assemblage of independent blocks or particles (Goodman and John, 1977). Further, continuum models are of two types: differential and integral. Differential models characterise the entire region of interest and include the finite difference and the finite element methods. Where as integral or boundary element models feature discretisation only along interior or exterior boundaries. Ground response in the study was obtained using the powerful FLAC method. The Fast Langrangian Analysis of Continua (FLAC) is a two dimensional explicit finite difference program. In order to setup a model to run a simulation with FLAC, in the fundamental components of the problem shall be specified: a finite difference grid, constitutive behaviour and material properties and boundary and initial conditions. The general solution procedure as indicated in FLAC manual version 5.0 is adopted for the study. The Mohr- Coulomb model which is convenient is used in the study. The descretised model of the Pir Panjal tunnel obtained by FLAC is shown in Fig. 4. Because of axisymmetric half of the tunnel is descretised into 15876 square and rectangular zones. The boundary conditions are applied in terms of both stresses and displacements. The bottom boundary is fixed in Y-direction where as the left vertical boundary is restrained in Xdirection. A vertical stress Syy and the horizontal stress, Sxx are applied on top boundary and vertical right side boundary. Stresses and deformations before and after support system are calculated using both the methods.

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Figure 4. Descretized Model of Tunnel

4. RESULTS AND DISCUSSION Analysis based on closed form solutions for the six varieties of rock masses available along the Pir Panjal tunnel has been carried out and stresses and displacement are obtained. Variation of radial and tangential stresses with distance from the centre of the tunnel and also variation of displacements (radial) with distance are plotted for all rock types. Typical such variations for shale are shown in Fig. 5(a) and (b). As per the figures, it is clear that the σθ = 19.56 MPa occurs at 10.5 m from the centre of tunnel, where as radial stress is zero at the boundary. Maximum deformation of 59.6 m was observed at the tunnel boundary and also radius of plastic zone is 10.5 m. Results for the rocks at different sections are summarised in Table. 3. For prediction of squeezing behaviour, strains were calculated from the displacements and presented in Table 3 for all six types of rocks. As shown in table agglomeratic shale and shale show 1.03 and 1.33% of strain respectively indicating moderate squeezing where as all other rock types show no squeezing. Table 3. Comparison of Analysis


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Stresses and displacements are also obtained from FLAC and corresponding strains were obtained for all rocks. The results are shown in Fig. 5 (a), (b), (c) and (d) for shale and similar graphs were plotted for other rocks as well. The results are compared in Table 4 along with the results obtained by closed form solutions. It is clear that the deformation values obtained from FLAC are higher than the closed form solutions. The strains for both rocks fall under moderate squeezing category. Rock mass response behaviour from closed form solutions and FLAC obtained for all rock masses. FLAC software is used to stabilise the tunnel at all sections with shotcrete lining and rock bolts of appropriate input parameters. After installation of support shortcrete lining is checked in bending and direct stresses and rock bolts are checked in tensile stresses. SHALE 1100

(a)

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Stress (MPa)

20 15 10

Radial stress

5

Tangential t

0 0

10

20

30

40

50

Distance from centre (m)

SHALE 1100 Radial displacement (mm)

70 60 50

(b)

40 30 20 Radial displacement

10 0 0

10

20

30

40

50

Distance from centre (m)

Figure 4. (a) Stress Variation (b) Deformation in Shale Table 4. Details of Stresses

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Bending movements, axial force and structural displacements in supports for all rock types are plotted and typical results obtained for shale are presented in Fig. 6 (a) and (b). Comparison of results without and with support is given in Table 4 and 5 for all sections. Table 5. Comparison of Results

(a)

(c)

(b)

(d)

Figure 5. (a) Maximum Principal Stress (b) Minimum Principal Stress (c) Y Displacement (d) Plasticity Indicator Contours for Shale

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Table 5 shows the shotcrete lining of 300 mm thickness is not safe in bending stresses in shale at section 1100/22. Therefore, the thickness of shotcrete needs to increase. After installation of supports all sections except shale are stabilised. Shale is suffering from moderate squeezing. For such condition, forepoling and advance face stabilisation are required. Yielding support system may be required in extreme cases to prevent squeezing conditions. JOB TITLE : Pir Panjal Tunnel in Shale VI H=1100 m

(*10^1)

FLAC (Version 5.00) 8.750

LEGEND 19-May-08 18:01 step 1124 6.000E+01