ISRM Congress 2015 Proceedings - Int'l Symposium ...

ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

ROCK MASS GROUTING FOR DAMS, AN OBSERVATIONAL DESIGN APPROACH

*J. A. Lopez-Molina, J. A. Valencia-Quintanar, and J. A. Espinosa-Guillen Rock Mechanics and Grouting Division Comisión Federal de Electricidad, México (* [email protected]) S. García Instituto de Ingeniería UNAM Ciudad Universitaria Coyoacán, México, DF


ROCK MASS GROUTING FOR DAMS, AN OBSERVATIONAL DESIGN APPROACH ABSTRACT The design and implementation of grouting treatments in rock masses are procedures that require continuous adjustment of parameters and criteria to optimize the results. In this proposal we describe a set of tools that enhance decision-making for this type of jobs particularly in dam projects. The methodology is focused on hydrogeological zoning of the site and its constant update combining engineer’s experience with artificial intelligence techniques to integrate the site knowledge; as well as the evaluation of grouting results for different scrutiny scales, with special attention on the relationship between water absorption and grout consumption.

KEYWORDS Observational methodology, Grouting, Hydrogeology, Dams, Permeability, Artificial Intelligence INTRODUCTION Several methodologies for design, implementation and evaluation for grouting rock masses have been developed for the solution of specific problems and tested in diverse geological conditions. Each of these methodologies is focused on results evaluation for different scrutiny scales (Table 1) and is employed to assist decision-making before or during the grouting process. Some methods define specifications for grouting each stage of the borehole, making theoretical, semi empirical or experience-based considerations to define basic parameters as maximum pressure, maximum volume, grouting time, flow rate, etc. Ultimately each approach aim to optimize the injection process, adjusting volumes or the grouting time required to obtain satisfactory results, reducing risk of creating new flow paths or unnecessary grout travels, and enhancing the mixture properties to achieve adequate penetrability and long-term behaviour. Some of these criteria have been widely used in dam projects, for example the concept of Grouting Based on Facts to define the maximum pressure and volume (Ewert, 1997), the GIN method (Lombardi & Deere, 1993; El Tani, 2012), or the North American standards based on Apparent Lugeon measures (Naudts, 1995; Bruce 2011); on the other hand, there are relatively new criteria that have shown good results for dam grouting as the Aperture Control Method (Bonin et al., 2012; Carter et al., 2012) or the Real Time Grouting Control Method (Stille et al., 2012). The results verification for a larger scale, taking into account the behavior of a group of holes located in areas with similar geological characteristics, has been presented by several authors as Deere (1982) and Ewert (1985, 1997), which require a statistical evaluation of the data and where generally a reduction in consumption is expected by decreasing the distance between boreholes. This evaluation allows the identification and correction of some results irregularities attributable to the geology or the grout curtain characteristics (e.g., karstic areas or inadequate drilling direction). For an evaluation scale that includes the global vision of the dam project, some recommendations are based on the hydrogeological assessment in order to identify preferential flow paths given the structural geology (Ewert, 1985, 1997), or the quantitative design of the grout curtain (Wilson & Dreese, 2003). For all scrutiny scales the application of observational methodologies during the development of the works are needed for a comprehensive setting of the grouting parameters and the geometrical project of the grout curtain.


Table 1 – Scrutiny scale for different design, execution and evaluation grouting criteria Overall Stage Borehole Zone project Maximum pressurevolume definition Closure criteria Statistical evaluation Quantitative design Hydrogeological evaluation Observational methodologies

The work we present here integrates diverse design and assessment tools for a wide range of scrutiny scales to finally obtain an optimal global solution for rock mass waterproofing in dam projects. The approaches described have been implemented mainly in sedimentary and igneous rocks with diverse structural conditions during studies and construction stages, and verified in operation stages by piezometric instrumentation and seepage measurements. One of the fundamental principles of the methodology is based on the hydrogeological knowledge update and the identification of susceptible areas to seepage (Figure 1). Its implementation considers that in addition to the general understanding of the water behaviour within the rock mass, the permeability evolution in each step of the process and its relation to grout consumption for different observation scales are essentials to define opportune strategies to design optimization.

Investigation during Construction

Original Design

Grouting

Grouting Consumption by Stage

Geology Survey Evaluation of Results by scale

Drilling Information Hydrogeological Zonation

(-)

Stage Borehole

Water Pressure Tests

Residual Permeability Evolution

Zone (+)

Piezometric Information

Grouting Consumption Evolution

Identification of Vulnerable Areas to Seepage

Subsequent Investigation

Overall Grout material monitoring

Design Adjustment

Subsequent Evaluation

Figure 1 – Design methodology diagram for grouting works in dam projects INVESTIGATION THROUGHOUT CONSTRUCTION The research needed for proper design of grouting treatments in dams have been presented for several authors (Ewert 1985, 1997; Weaber & Bruce 2007; Bruce 2011), among these, some are considered specially relevant for its impact on the results and are outlined below.


The geological model of the site is probably the most important reference point for the waterproofing system design, it includes the preliminary identification of potentially conductive structures and the preferential water flow paths in the rock mass, information that needs to be updated from surface and underground survey and verified with permeability tests. Information obtained from exploration and grouting boreholes is sometimes underestimated, however, it is a tool that has proved to be very useful for decision-making (Bruce 2011; Carter et al., 2012; Høien, 2014). The information can be derived from recovered cores, or effortlessly through optical and acoustical televiewer, alternatively Measurement While Drilling technologies have presented advantages for identification of main geological conditions. Acquiring as much information as possible from boreholes (mainly characteristics and spatial distribution of geological structures) becomes relevant when these data are associated with permeability and groutability results, since this evaluation can provide knowledge to extrapolate conditions to other zones and for early design optimization (Garcia et al. 2015a, 2015b). Water pressure tests (WPT) are the most appropriate method to assess the injection influence in rock mass permeability (Bruce, 2011), therefore its execution before, during and at the end of the works, using test pressures similar to those that will be exercised by the reservoir during operation stage, is indispensable to optimize treatments. HIDROGEOLOGICAL ZONATION A suitable hydrogeological model for decision-making throughout a grouting process must contain at least the spatial distribution of hydraulic conductivity parameters in the rock mass and its relationship to the structural geology, the identification of preferential water flow paths and groundwater level position. Distribution of hydraulic properties in rock masses can be performed with several approaches, from its definition using structural geology data and the engineer’s experience, to the distribution with geostatistical tools, however, due to the large number of variables that influence the permeability value (aperture, spacing, filling and other discontinuities features, stress state, anisotropy, etc.), it is recommended combining empirical criteria with tools that exploit the information obtained in investigation and construction stages. For this purpose Garcia et al. (2015a) have successfully implemented soft computing tools and artificial intelligence for defining water absorption distribution in rock masses from diverse data. In this methodology some rock mass descriptors (RQD, core recovery, number of fractures, and material type) are modeled into a 3D–recurrent neural network that defines their spatial variation, and subsequently a neuro/fuzzy structure for indirect estimation of water absorption employs the spatial relations obtained from the 3D descriptors variation. In Figure 2 the synthetic process of this method of characterization is presented, from which artificial absorption tests can be obtained at any point of the studied space. This process helps identifying areas with higher permeability, which eventually will be of interest for waterproofing systems design; one of the advantages of this methodology is that implicitly takes into account the structural geology of the rock mass since information comes from conventional water pressure tests and the respective description of discontinuities in the stage. Figure 3 shows an example of the threedimensional model of absorption distribution, within the abutments of a dam project founded on sedimentary rocks with slightly developed karst (see details in Garcia et al. 2015a). IDENTIFICATION OF SEEPAGE VULNERABLE AREAS In order to make a simple estimation of the seepage susceptibility throughout the dam grout curtain axis, some simplifications can be made from the basic principle of water flow, that seepage is proportional to the ground permeability (k) and the hydraulic gradient (i).


Measured

Absorption (l/min/m)

Estimated

ABSORPTION

Borehole: MD-28 X=482442 UTM Y=1878112 UTM Z = 70 to 75 m

Pressure (MPa)

Figure 2 – Synthetic example of water absorption distribution from several rock mass descriptors. Artificial absorption test and its comparison with field results are presented.

Figure 3 – Absorption distribution example in dam abutments The permeability and its distribution are defined from field tests in combination with procedures outlined in preceding sections. In Figure 4a water absorption distribution is presented in terms of Lugeon Units (LU) throughout the axis of the grout curtain on the right abutment of a 220 m high concrete face rock fill dam which was built on igneous rocks. In this case the absorption distribution was obtained from a


20x20 m grid, which was sufficient to capture the main features associated to the hydraulic conductivity of the rock mass.

a)

b)

c)

Figure 4 – Section through grout curtain axis in dam right abutment, distribution of: a) Water absorption in terms of LU, b) Hydraulic gradient, c) Susceptible areas to seepage

Preferential water flow paths From reservoir to grout curtain From grout curtain to nearest discharge downstream Grout curtain axis

Figure 5 – Definition of preferential water flow paths for a specific elevation (+515)


Hydraulic gradient is defined from the hydrogeological model review, identifying preferential water flow paths and reservoir operation levels. The hydraulic head differential (∆H) can be obtained for each point of the proposed grout curtain, considering the maximum water head at that point and the minimum level established downstream. On the other hand, the most likely water flow path from upstream to downstream (L) is determined taking in to account the structural geology of the rock mass. An example of this determination for a given elevation and for various points on the grout curtain is presented in Figure 5, in this case the water paths in areas close to the concrete face foundation (plinth) are very short therefore higher hydraulic gradients are generated. The hydraulic gradient determination is also affected by the presence of drainage curtains or underground excavations which must be considered in defining the preferential flow paths. The ∆H/L ratio for each point on the grout curtain provides an approximate map of equal hydraulic gradients as Figure 4b shows. The distribution of permeability and hydraulic gradient over the grout curtain surface will allow generating a waterproofing requirements map (Figure 4c). According to the experience from several projects and particularly from the instrumentation data obtained during the operation stage, it is possible to define the minimum number of grouting lines necessary to obtain adequate results for every condition identified and the implementation of similar criteria for future projects. GROUTING ASSESSMENT AND DESIGN AJUSTEMENT In practice it is common to identify the lack of correlation between permeability and groutability of a rock mass (Ewert, 1985; Houlsby, 1990), however, the evaluation of these differences can also be employed to determine any necessary adjustment to the grouting parameters and the design of the waterproofing treatments. The assessment of water absorption and grout consumption relationship can be done particularly in each grouted stage or from results evolution for a group of boreholes located in areas with similar hydrogeological features, for the methodology suggested here some principles proposed originally by Ewert (1985) were applied. Stage evaluation Each stage grouted in a borehole can be evaluated comparing the results of water pressure test and grout takes in order to optimize the grouting parameters at other stages located in similar hydrogeological zones or in subsequent holes. In Figure 6 four possible combinations absorption-consumption originally proposed by Ewert (1985) are included (I, II, III and IV) with two additional variations (IIIa and IVa). Taking in to account the predefined parameters for grouting (pressure-volume or GIN curve), represented with continuous lines in Figure 6, and the absorption-consumption combination in the evaluated stage, it is possible to define some recommendations to optimize these parameters for other stages located in the same hydrogeological region (dashed lines in Figure 6). The absorption and consumption limits suggested in Figure 6 to classify the results are only an average value of studied cases; however, these must be fixed according to the results obtained in each project or waterproofing requirements defined. For each case shown in Figure 6, some general suggestions defined originally by Ewert (1997) and complemented according to results obtained in several projects are: I. Low Absorption and Low Consumption. Usually only selective treatment is required to obtain residual permeability requirements. II. Low Absorption and High Consumption. Hydraulic fracturing or hydraulic jacking may be causing unnecessary consumptions (which can be verified for each grouted stage); it is recommendable to reduce pressure and volume parameters. Generally only selective treatment is required.


III. High Absorption and High Consumption. In the first instance it may be appropriate to reduce the maximum pressure or even apply gradual thickening of grout mix to optimize the procedure; usually it requires more than one row of holes to obtain an acceptable residual permeability. On the other hand, to set the optimal volume and the pattern of holes is indispensable to evaluate the sequential consumption of contiguous boreholes. IIIa. Moderate-High Absorption and Moderate-High Consumption. The ground can be treated adequately with the methodology and parameters selected, however, it is recommendable to optimize the process in accordance with the sequential grout consumption. IV. High Absorption and Low Consumption. Selected grout mix is inappropriate and the use of other materials with smaller particle sizes must be tested. IVa. Moderate-High Absorption and Low Consumption. Grout mix may have limited penetrability, therefore, the injection pressure can be increased with proper monitoring to prevent hydraulic fracturing; reduce grout mix cohesion and viscosity and/or minimize the distance among holes must be investigated.

Moderate-High

• Hydrofracturing? • Decrease parameters • Selective grouting

• Multiple grout curtain • Grout thickening

II

III V

V

P

P

IIIa

P

Low-Moderate (< 50 kg/m)

Grout Consumption

P

P

More than 200 kg/m

Is grouting necessary?

• Increase parameters • Minimize distance between boreholes

Selective grouting

I

IVa V

Low (< 5 LU)

Change grout material

IV V

V Moderate (5-40 LU)

High (> 40 UL)

Water Absorption

Figure 6 - Suggestions to optimize grouting parameters from stage absorption-consumption evaluation Zone evaluation With sufficient sequential evolution data of water absorption and grout takes, it is feasible to establish different behaviors according to nomenclature shown in Figure 7. To employ this tool the first step is to identify the most suitable absorption-consumption combination in primary holes taking into account the considerations mentioned in previous section (behavior I, II, III or IV in Figure 6). The same evaluation must be performed for secondary, tertiary, and subsequent holes; in this way behavioral vectors can be established to describe the evolution of absorption-consumption (An, Bn, Cn, and Dn vectors in Figure 7).


According to the experience obtained in different projects, the sequential evolution of consumption and absorption provides information about actions we can take to optimize the grouting process, considering that the ideal behavior is to get a reduction of both values. In this way with at least one full grouted panel (group of holes between two primaries grouted in split spacing method) within a given hydrogeological zone and defining its more accurate behavioral vector, we can adapt the grouting parameters in accordance with the suggestions indicated in Table 2 for other boreholes in the same region. 120

High

III

II

80 60 40

Low

Consumption (kg/m)

100

IV

20

I

0 0

10 Low

20

30 Moderate

40

50 High

60

Absorption (LU)

Figure 7 – Absorption –Consumption evolution vectors Table 2 – Suggestions to optimize grouting parameters from evolution absorption-consumption A

B

C

D

ABSORPTION Low-Moderate (˂ 20 LU)

Moderate-High (˃ 50 kg/m) Increases or Remains Decreases

Low-Moderate (˂ 50 kg/m) Increases or Decreases Remains

2 |3

CONSUMPTION

1

Decreases

Increases or Remains

• Sings of HF* or HJ* • Reduce max pressure and/or max volume • Evaluate selective grouting from WPTs and residual permeability requirements

• Local effects** • Reduce max pressure and/or max volume • Optimize hole direction according geology

• Evaluate selective grouting from WPTs and residual permeability requirements

• Local effects*** • Decrease distance among holes • Reduce injection flow rate.

• Sings of HF* or HJ* • Evaluate real need for treatment from WPTs and residual permeability requirements

• Evaluate selective grouting from WPTs • Local effects** • Reduce max pressure

Moderate-High (˃20 LU) Decreases • Sings of HF* or HJ* • Reduce max pressure and or max volume • Implement grout mix thickening

• Adequate behavior

• Sings of HF* or HJ* • Reduce max pressure • HJ* may be adequate if sequential reduction of permeability is observed.

Increases or Remains • Local effects** • Sings of HF* or HJ* • Reduce max pressure and/or max volume • Implement grout mix thickening • Optimize hole direction according geology • Local effects*** • Decrease grout mix viscosity and cohesion. • Decrease distance among holes • Optimize hole direction according geology • Reduce injection flow rate.

• Sings of HF* or HJ* • Reduce max pressure • Decrease grout mix viscosity and cohesion.

4

• Inadequate grout mix • Evaluate the use of grout materials with smaller • Adequate behavior particle sizes • Reduce injection flow rate. *HF: Hydraulic Fracturing; HJ: Hydraulic Jacking. Details in Bruce (2005). Local effects (HF, HJ, faults, karst, etc.): ** Better solved thickening grout mix, *** Better solved decreasing cohesion and viscosity of grout mix.

• Evaluate real need for treatment from WPTs and residual permeability requirements

• Evaluate selective grouting from WPTs • Local effects*** • Reduce injection flow rate.


To optimize the opportunity of decision making using this proposal, it is desirable to have at least one full grouted panel and random permeability test in primary holes in each hydrogeological zone; the main parameters than can be adjusted in holes within the same region include: maximum pressure, maximum volume, injection flow rate, borehole pattern and grout mix rheology. Some application examples of this methodology can be revised in Lopez-Molina & Espinosa-Guillén (2013). Grout mix monitoring As discussed in previous sections, the grout mix properties should be adjusted according to the results obtained during the injection process in order to optimize results. The most appropriate materials for each condition and the procedures to make adjustments on the rheological properties of the mix have been discussed for several authors (De Paoli et al., 1992; Bremen, 1997; Bruce 2011). Starting from any selected grouting criterion and once defined the most suitable mix or mixes for the works execution, assurance and control of the grout material properties becoming essential, particularly is required the identification of variations due to environmental changes, modifications in proportions or characteristics of the employed materials and adjustments in fabrication process. For this purpose the generation of behaviour maps of the grout mix has been useful, involving the materials that compose it and the rheological properties specified. For example, Figure 8 shows the behaviour map of two mixes, the shaded region indicates the dosages that meet the originally specified values (Marsh viscosity less than 32 s and free water under 4%); in this case the main variables were the cement fineness and the origin of the materials for cement fabrication, the results show that the cement with smaller particle size meets the specification in a greater range of dosages which could also represents lower sensitivity to external factors. These graphics must be constantly updated in order to optimize any necessary adjustment during the grouting process. 1.25

Type II Portland Cement (Plant A) Fineness 5000 cm2/g

Superfluidificant additive (% by weight of cement)

Superfluidificant additive (% by weight of cement)

1.25

1.00

0.75

0.50

0.25

0.00

Type II Portland Cement (Plant B) Fineness 4000 cm2/g

Marsh viscosity isocurve (s) Sedimentation isocurve

1.00

Specified properties

0.75

0.50

0.25

0.00 0.6

0.7

0.8

W/C

0.9

1.0

0.6

0.7

0.8

0.9

1.0

W/C

Figure 8 – Example of behaviour maps for two different mixes The properties of the grout mix also must be monitored through time to identify in advance any necessary adjustment, to this end the continuous evaluation of the properties with more variation or those that determine to a greater extent the effectiveness of the treatment has been appropriate. For example in Figure 9 the evaluation of viscosity and stability of a mix for a given observation time is presented, Figure 9a shows the results concentration for a period where the mix properties fluctuated significantly, Figure 9b shows the behaviour after adjusting the proportion of additives and the fabrication process. Such controls


have operated continuously during the execution of the work in order to identify deviation in properties and implement prevention or correction measures.

Sedimentation (% free water)

Accepted

b)

Optimal

Accepted

Optimal

Marsh viscosity (s)

a)

Sedimentation (% free water)

Figure 9 – Results concentration by percentage (a) after and (b) before adjust dosages and fabrication process. Pre-specified limits for each property are shown. CONCLUSIONS The design methodology for rock grouting in dam projects presented is founded on updating the information generated at each implementation stage of the treatments, mainly that related to hydrogeological zoning which derives from the engineer’s experience and is assisted by soft computing and artificial intelligence to facilitate decision making. By means of simple application criteria based on the structural geology and the identification of potential water flow paths, vulnerable areas to seepage can be identified that allow comprehensive waterproofing designs. The results obtained at each treatment stage should be reviewed for several scrutiny scales in order to perform opportune adjustments to the grouting parameters; these assessments must cover the consumption evolution, residual permeability and the grout mixtures properties. REFERENCES Bonin G.R., Rombough V.T., Carter T.G., Jefferies M.G. (2012). Towards better injection control and verification of rock grouting, Proc. 4th Int. Conf. Grouting and Deep Mixing, New Orleans, 14601471 . Bremen, R. (1997). The use of additives in cement grouts. Int J. Hydropower Dams, 4(1), 71-76. Bruce, D. A. (2005). Glossary of grouting terminology. Journal of geotechnical and geoenvironmental engineering, 131(12), 1534-1542. Bruce, D. A. (2011). Rock grouting for dams and the need to fight regressive thinking. Geotech. News, 29(2), 23-30.


Carter T.G., Dershowitz W., Shuttle D.A., Jefferies M.G. (2012). Improved methods of design for grouting fractured rock, Proc. 4th Int. Conf. Grouting and Deep Mixing, New Orleans, 1472-1483. De Paoli, B., Bosco, B., Granata, R., & Bruce, D. A. (1992). Fundamental observations on cement based grouts (1): traditional materials. Grouting, Soil Improvement and Geosynthetics, ASCE, New Orleans, 474-485. Deere, D. U. (1982). Cement-bentonite grouting for dams. In Grouting in Geotechnical Engineering (pp. 279-300). ASCE. El Tani, M. (2012). Grouting rock fractures with cement grout. Rock mechanics and rock engineering, 45(4), 547-561. Ewert, F. K. (1985). Rock grouting with emphasis on dam sites. Springer-Verlag Berlin. Ewert, F. K. (1997). Permeability, Groutability and Grouting of Rocks Related to Dam Sites. Dam Engineering, 8, 31-75. García, S., López-Molina, J. A., Castellanos-Pedroza, V. (2015a). Artificial Hydrogeological zonation. 13th International ISRM Congress, Montreal-Quebec, Canada. García, S., López-Molina, J. A., Castellanos-Pedroza, V. (2015b). Intelligence for grouting balance. 13th International ISRM Congress, Montreal-Quebec, Canada. Høien, A. H., & Nilsen, B. (2014). Rock Mass Grouting in the Løren Tunnel: Case Study with the Main Focus on the Groutability and Feasibility of Drill Parameter Interpretation. Rock Mechanics and Rock Engineering, 47(3), 967-983. Houlsby, A.C. (1990). Construction and Design of Cement Grouting - A Guide to Grouting in Rock Foundations. John Wileys & Sons, Inc., New York. Lombardi, G., & Deere, D. (1993). Grouting design and control using the GIN principle. International water power & dam construction, 45(6), 15-22. López-Molina, J. A., & Espinosa-Guillén, J. A. (2013). Rock mass enhancement by grouting: tools for design optimization and decision making. In 3rd Sinorock symposium, Rock characterisation, modelling and engineering design methods. Naudts, A. (1995). Grouting to Improve Foundation Soil. Practical Foundation Engineering Handbook, 5277. Stille, H., Gustafson, G., & Hassler, L. (2012). Application of new theories and technology for grouting of dams and foundations on rock. Geotechnical and Geological Engineering, 30(3), 603-624. Weaver, K. D., & Bruce, D. A. (2007). Dam foundation grouting. ASCE Publications. Wilson, D. B., & Dreese, T. L. (2003). Quantitatively engineered grout curtains. Grouting and Ground Treatment (pp. 881-892). ASCE.