Evaluation of Apparent Co-efficient of Friction between ... - CiteSeerX

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The 12 International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India

Evaluation of Apparent Co-efficient of Friction between Soil and Nails Meenal Gosavi Deptt. Of civil Engineering, College of Technology, G. B. P U. Agri and Tech., Pantnagar, Udam Singh Nagar, India. 263 145.

Swami Saran Emeritus Fellow, Deptt. of Earthquake Engg., IIT, Roorkee, India. 247 667.

Satyendra Mittal Associate Professor, Deptt. of Civil Engg., IIT, Roorkee , India. 247 667,

Keywords : Pull-out tests, Nails, coefficient of friction, Normal stress

ABSTRACT: Soil nailing is a method of reinforcing the soil with steel bars or other materials. The purpose is to suppress the tensile and shear stresses in soil and restrain its lateral displacements. Apparent coefficient of friction between nail and soil (f*) plays an important role in determining the stability of nailed cuts. To obtain the value of f* laboratory pull-out tests were performed. The purpose of pull out tests was to study the effect of nail diameter, nail length, surcharge intensity and method of placing of nails on the coefficient of friction (f*) between soil and nail interface. Two types of tests were performed namely i) on driven nails and ii) on pre buried nails. One of the main findings of these tests was that if surcharge intensity was more than 15 kPa, the effect of nail diameter and length became marginal. For this case, the value of f* ranged between 0.5 to 0.6. However for very low surcharge intensities, the values of f* were found even more than unity. Further it was found that in pre buried nails the value of f* is about 10 to 15 % higher than for driven nails.

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Introduction

Soil nailing is a method of reinforcing the soil with steel bars or other materials. The purpose is to suppress the tensile and shear stresses in soil and restrain its lateral displacements. The nails are either placed in drilled bore holes and grouted along their total length to form ‘grouted nails’, or simply driven into the ground, called as ‘driven nails’. The technique helps stabilization of both natural slopes and vertical or inclined cuts. Many investigators (Gässler and Gudehus, 1981, 1983; Shen et al., 1981; Schlosser, 1982; Juran and Elias, 1992, Raju, 1996; Gupta, 2003) have proposed methods for investigating the stability of vertical/nearly vertical excavations. In each method, the assumed geometry of the slip surface is based on observations in either small scale model tests or full scale structures. The methods vary in the geometry of the assumed failure surface, the definition of the factor of safety and the forces assumed to act on the active zone. The soil-nail interface friction is a very important parameter in the design of soil nailing, as the force transfer from soil to nail is through the friction mobilised at the interface. The pull-out resistance is defined as the force mobilized along the length of nail which lies beyond the failure surface when an assumed failure surface cuts a layer of soil nails. For finding out the values of pull-out resistance, pull-out test on nail is conducted. Pull-out test on nail is done by two different methods e.g., 1) laboratory pull-out test, wherein interaction mechanism between the nail and soil and the influence of various parameters on apparent friction co-efficient ( f*) between nail and soil is investigated. Results obtained for pull out resistance form experimental pull-out tests are used for the design of nail wall and 2) In-situ pull-out tests which are carried out in the field as a routine matter on all soil nailing projects to verify the pull-out resistance assumed at the design stage.

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From the pull-out test the apparent friction co-efficient (f*) is calculated for cohesion less soil as

F = p Where,

p ⋅ L. σ ⋅ f ∗ v

Fp is pull-out force, p is perimeter of the nail, L is

(1)

length of the nail and

σv

is= normal stress = γ ⋅ z + q, q

is surcharge intensity, and z is depth at which nail is provided. Pull-out resistance of a nail or the value of the apparent co-efficient of friction f* is influenced by several factors, such as properties of soil, roughness and stiffness of the nail, boundary conditions of the tests, diameter of nail, length of nail and the normal stress acting on nail. Apparent coefficient of friction between nail and soil (f*) plays an important role in determining the stability of nailed cuts. In the present research work laboratory pull-out tests were performed to evaluate the value of f* for different diameter of nails. Effect of change in nail diameter, confining pressure and length of nail on the value of f* has been studied through these pull-out tests and presented herein (Gosavi, 2006).

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Soil Used

The soil used in the study was dry sand collected locally from river Solani bed. According to the Indian Standard (IS: 1498-1970) classification, the soil is classified as poorly graded sand (SP). The maximum and minimum void ratios were determined in accordance with the procedures laid down in Indian Standard IS: 2720 (Part XIV, 1968). Table 1 provides the relevant properties of the soil. Table 1 Properties Of Soil Used In Experimental Program S. No. 1 2 3 4 5 6 7 8

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Property Effective Size (D10) Uniformity Coefficient (Cu) Specific Gravity Minimum Void Ratio (emin) Maximum Void Ratio (emax) Unit Weight of sand (γ) Relative Density Angle of Internal Friction of Soil (φ)

Value 0.16 mm 5 2.54 0.45 0.79 3 16.5 kN/m 70% o 38

Nail Material and Dimensions

Usually in full scale nailed open cuts, tor steel bars of diameter varying from 16 mm to 36 mm are used as nails. Keeping this in view, it was decided to use 16 mm, 25 mm and 32 mm diameter tor steel bars as nails in pull-out tests. Lengths of nails were kept as 1.0 m, 1.5 m, 2.0 m, 2.5 m and 3.0 m. A MS hook (6 mm diameter) was welded on the front side of each nail to facilitate pulling of the nail. A small MS flat piece of 40 mm x 20 mm and 4 mm thickness was also welded on the nail, through which the readings for the displacement of nails were obtained during test. The yield strength of tor steel bars was 4.15x105 kPa.

4 4.1

Pull-out Tests Tank

Pull-out tests were performed in a wooden tank with dimensions 3.25 m x 0.4 m x 1.0 m (high). The length of the tank was reduced to 2.25 m and 1.25 m for the pull-out tests on nails of length 2 m, 1.5 m and 1 m respectively. Length of the tank was kept as 3.25 m for the pull-out test on nails of length 2.5 m and 3.0 m. The sides of the tank were made up of 25 mm thick wooden planks suitably stiffened by cast iron angles at corners, top and bottom of tank. The concrete floor below the tank, acts as a bottom of a tank. A circular hole of diameter 50 mm was cut on the front side of the tank with its centre at a height of 200 mm above the base of the tank. To reduce the effect of front wall friction on nail, a MS hollow pipe sleeve (external diameter De = 88 mm and internal dimeter Di = 80 mm) of length 145 mm was fixed in this hole. The sleeve was fixed in such a way that 40 mm of its length remained inside the tank and 80

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mm remained outside the tank to facilitate straight insertion of the nail from outside the tank. Through this sleeve the nail end to be pulled out, could be taken out for attachment to the pulling device. To keep the nail centrally in the sleeve and to avoid the friction between sleeve and nail, the space around the nail was filled with 25 mm thick foam piece having length same as that of sleeve length. Schematic diagram of pull-out tank and photograph are shown vide Figures. 1 (a) and (b).

(a)

(b) Figure 1 Experimental Pull-Out Test Set Up

4.2

Pull-out Device

Pull-out device consists of two frictionless pulleys. Lower pulley was fixed in the base frame of steel angles. Base frame was fixed in front of the sleeved opening. The other frictionless pulley was fixed to a channel section, which was fixed to the loading frame at 3.0 m height. MS tendon of 4 mm diameter was allowed to pass over these two pullies. One end of the tendon was fixed to the hook welded in front side of the nail. Rear end of the tendon was fixed to loading hook, on which dead loads were applied in uniform increments. The base frame was fixed in position with the help of four bolts grouted in the concrete floor, which provide reaction to the pull. The base frame with lower frictionless pulley is shown in Figure 2.

Figure 2 Details of Base Frame with Lower Pulley

4.3

Placing of Sand and Reinforcement

Sieved dry sand was used in the investigation was stored in a clean dry place. The desired density of sand was achieved by depositing the sand by rainfall technique. In rainfall technique, sand was allowed to fall freely through a specially fabricated sieve with holes of 3 mm diameter spaced at 25.4 mm centre to centre. The height of fall was decided by conducting a series of test in which known volume of sand was dropped from different heights and its density was determined. The sand was filled in tank up to the centre of the sleeved hole. The top surface of the sand was levelled properly and the nail was placed on the sand surface (as a pre-buried nail) and hooked end of it was

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taken out of the sleeve. To prevent leakage of sand through the sleeved hole, sleeve was packed with foam from outside after placing of nail. Nail was pulled out by placing dead loads in uniform known increments.

4.4

Surcharge Application

To study the effect of normal stress on the pull out force Fp and to find apparent coefficient of friction between nail 3 and soil (f*), uniform surcharge was applied on the sand. A wooden sleeper (density = 27.34 kN/m ) of 0.36 m width, 3.0 m length and 152.4 mm height was placed centrally on the levelled sand. Length of the wooden sleeper was curtailed as 2 m and 1 m for respective reduction in tank length as 2.25 m and 1.25 m. On the sleeper three equally spaced channel sections (ISMC 150) were placed in transverse direction (one at centre and two on its either side) (Fig. 1a.). One square beam (fabricated by welding two channel sections ISMC 150 face to face) was placed longitudinally above these transverse channel sections. At the centre of the longitudinal beam, a proving ring of 50 kN capacity was placed to measure the surcharge applied by hydraulic jack of 100 kPa capacity as shown in Figure 1 (b). The loading frame was made with four ISMC 150 (150 mm x 75 mm x 160.9 N/m) columns, two nos. on either side of the tank and spaced at 3 m centre to centre. These columns were grouted in the floor by making a proper foundation. These columns supported the cross beams of ISMB 125 (125 mm x 75 mm x 127.5 N/m), duly fastened rigidly with nuts and bolts. Surcharge load was applied in the increment of 2.5 kN.

4.5

Tests Performed

Total 135 tests were performed on pre-buried nails of diameter 16 mm, 25 mm and 32 mm with their lengths as 1.0 m, 1.5 m, 2.0 m, 2.5 m and 3.0 m respectively. Details of the pull-out tests performed on pre-buried nails in the laboratory are given in Table 2. In the pull-out tests the effect of placing of nails on the value of f* was also studied. For this study, additional 27 tests were performed on driven nails of 16 mm, 25 mm and 32 mm diameter and 1.0 m length. In these pull-out tests, pull-out test tank was filled up fully by sand using rain fall technique to achieve unit weight of sand as 16.5 kN/m3 (relative density of sand as 70%). Nail was then driven in the pull-out tank from outside by hammering the nail. After placing the nail to its position, MS hook and MS flat were welded on its front end and pull out arrangements were made similar to the pull-out tests on pre buried nails. Details of the pull-out tests performed on the driven nails are given in Table 3. Table 2 Smmary of pull-out tests (pre-buried nails) Test Nos.

Height of Overburden (m)

Uniform Surcharge kN

Length of Nail (m)

1-20 21-45

Nail Dia. (mm) 16 16

0.2 ,0.4, 0.6, 0.8 0.8

Nil 5, 10, 15, 20, 25

1, 1.5, 2, 2.5, 3.0 1, 1.5, 2, 2.5,3.0

46-65

25

0.2 ,0.4, 0.6, 0.8

Nil

1, 1.5, 2, 2.5, 3.0

66-90

25

0.8

5, 10, 15, 20, 25

1, 1.5, 2, 2.5, 3.0

91-110

32

0.2 ,0.4, 0.6, 0.8

Nil

1, 1.5, 2, 2.5, 3.0

111-135

32

0.8

5, 10, 15, 20, 25

1, 1.5, 2, 2.5, 3.

(Surcharge intensity can be calculated by dividing uniform surcharge by the surface area above with surcharge is 2 2 2 applied i.e. 0.5 m for length of nail 1 m, 0.9 m for length of nail 1.5 m and 2.0 m and 1.3 m for length of nail as 2.5 m and 3.0 m.) Table 3 Smmary of pull-out tests (driven nails) Test Nos.

Nail Dia. (mm)

Height of Overburden (m)

1-4 5-9 10-13 14-18 19-22 23-27

16 16 25 25 32 32

0.2 ,0.4, 0.6, 0.8 0.8 0.2 ,0.4, 0.6, 0.8 0.8 0.2 ,0.4, 0.6, 0.8 0.8

Uniform Surcharge intensity (q) 2 kN / m Nil 10, 20, 30 Nil 10, 20, 30 Nil 10, 20, 30

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Length of Nail (m)

1 1 1 1 1 1

5

Results

For 135 pull-out tests on pre-buried nails and 27 pull-out tests on driven nails (listed in Table2 and 3) pull-out load Vs displacement of nail plots were drawn. Typical pull-out Vs displacement of nail plots for the nails of diameter 25mm are presented in Figures 3 to 7. In Figs. 3 to 7 pull-out load represents the load at which the nail showed excessive displacement . With the help of the pull-out loads given in Figs. 3 to 7 apparent coefficient of friction between soil and nail (f*) was calculated as, f*

=

Pull out load {π .d .L (γ .z + q )}

(2)

3 Where, d is diameter of nail in m, L is length of nail in m, γ is unit weight of soil (kN/m ), z is sand fill above the nail 2 in m and q is Surcharge intensity (kN/m ).

Pull-out Load Vs Displacement of Nail (d = 25 mm and L = 3.0 m)

5000

Pull-out Load (N)

4000

3000 q = 16.2 kPa 2000

q = 20.6 kPa q = 23.9 kPa

1000

q = 27.7 kPa q = 31.6 kPa

0 0

2 4 6 8 Displacement of Nail (mm) (b)

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Figure 3 Pull-out Load Vs Displacement of Pre-buried Nail of 25 mm diameter (L= 3.0 m) Pull-out Load Vs Displacement of Nail

4000

(d = 25 mm and L = 2.5 m)

3500

Pull-out Load (N)

3000 2500 2000 q = 16.2 kPa q = 20.6 kPa q = 23.9 kPa q = 27.7 kPa q = 31.6 kPa

1500 1000 500 0 0

2

4 6 8 10 Displacement of Nail (mm) (b)

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Figure 4 Pull-out Load Vs Displacement of Pre-buried Nail of 25 mm diameter (L=2.5m)

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Pull-out Load Vs Displacement of Nail (d = 25 mm and L = 2.0 m) 4000

Pull-out Load (N)

3000

q = 17.9 kPa

2000

q = 23.4 kPa q = 29.0 kPa 1000

q = 34.5 kPa q = 40.1 kPa

0 0

2

4 6 Displacement of Nail (mm) (b)

8

10

Figure 5 Pull-out Load Vs Displacement of Pre-buried Nail of 25 mm diameter (L=2.0m)

Pull-out Load Vs Displacement of Nail (d = 25 mm and L = 1.5 m) 3000

Pull-out Load (N)

2500 2000

q = 17.9 kPa q = 23.4 kPa

1500

q = 29.0 kPa 1000

q = 34.5 kPa q = 40.1 kPa

500 0 0

2 4 6 8 Displacement of Nail (mm) (b)

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Figure 6 Pull-out Load Vs Displacement of Pre-buried Nail of 25 mm diameter (L=1.5m)

Figure 7 Pull-out Load Vs Displacement of Pre-buried Nail of 25 mm diameter (L=1.0m)

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5.1

Effect of Overburden Pressure on f*

The values of apparent coefficient of friction (f*) for dry sand and pre buried steel nails of diameter 16 mm, 25 mm and 32 mm and length as 2.0 m and 3.0 m, is determined by Eq. 2 for different ranges of overburden pressures in the pull-out tests and presented Figs. 8 (a) and (b). From the Figs 8 (a) and (b) it was observed that as the overburden pressure increases the value of f* decrease but as the overburden pressure increased above 15 kPa the value of f* becomes somewhat constant irrespective of the length and diameter of the nail. It was also observed that for the lower overburden pressures i.e. less than 5 kPa the value of f* is greater that 1.0.

Overburden Pressure Vs f* (Length of nail - 3.0 m) Apparent coefficient of friction f*

D = 25 mm D = 32 mm

1.5 1 0.5 0 0

5

15

20

25

30

35

Overburden Pressure in kPa

3.5

D= 16 mm

3

D= 25 mm

2.5

D= 32 mm

2 1.5 1 0.5 0 10

0

20

30

40

50

Overburden Pressure in kPa

Figure 8

5.2

10

Overburden Pressure Vs f* (Length of nail - 2.0 m) Apparent coefficient of friction f*

D = 16 mm

2

(a) (b) Effect of Overburden Pressure on Apparent Coefficient of Friction, f* (L = 3.0 m and 2.0 m)

Effect of Diameter of Nail on f*

From the perusal of Figs. 8 (a) and (b) it was observed that the value of f* is greater with the greater diameter of nail. At higher overburden pressures the f* becomes somewhat constant and the increase in f* with increase in the diameter of nail is very marginal.

5.3

Effect of Length of Nail on f*

Effect of length of pre-buried nail on the values of apparent coefficient of friction (f*) with different overburden pressures are given in Figs. 9(a) and (b). From figs. 9 (a) and (b), it was observed that the value of f* increases with increase in the length of nail. At higher surcharge intensities (i.e. greater then 17 kPa) the increase in f* was observed to marginal with increase in the length of nail. Length of nail vs f*

0.7 0.6

0.8 16.22 kPa 20.06 kPa 23.91 kPa 27.75 kPa 31.60 kPa

0.5 0.4

f*

f*

Lenth of nail vs f*

0.9

16.22 kPa 20.06 kPa 23.91 kPa 27.76 kPa 31.60 kPa

0.7 0.6 0.5

0.3 1

1.5

2 2.5 Length of nail in m

3

1

3.5

1.5

2

2.5

3

length of nail in m

(a)

(b)

Figure 9 Effect of Length of Nail, L on Apparent Coefficient of Friction, f* (D=-25 mm and 32 mm)

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3.5

5.4

Effect of Placing of nails of the value of f*

0.80

Apparent Coefficient of friction (f *)

Apparent Coefficient of friction (f *)

Effect of method of placing of nail on the value of f* is shown in Fig. 10 a and b. From these Figures, it was observed that the pre-buried nails shows higher values of f* then the values of f* obtained for the driven nails. It was also observed that the values of f* becomes near about constant at higher overburden pressures apart from the method of placing the nails.

d=25 mm Pre-buried Nails d=25 mm Driven Nails

0.60 0.40 0.20 0.00 0

10

20

30

40

50

Overburden Pressure (kPa)

2.00

d=32 mm Pre-buried Nails

1.50

d=32 mm Driven Nails

1.00 0.50 0.00 0

(a)

20

40

Overburden Pressure (kPa)

60

(b)

Figure 10 Effect of Method of Placing of Nail on Apparent Coefficient of Friction, f * a) Dia. 25 mm, b) Dia. 32 mm (Length of Nail – 1.0 m)

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Summary of Results

The laboratory pull-out studies helped in understanding the bond developed between nail and the surrounding sand and evaluating the value of apparent coefficient of friction f* between nail and soil. The following points were observed from pull-out tests. i)

Values of f* calculated using the pull-out loads were found to depend on the value of overburden pressure, length of nail, diameter of nail and method of nail installation.

ii)

Values of f* were found to be greater than unity for lower overburden pressures (i.e. less than 5 kPa) irrespective of change in length and diameter of nails.

iii)

Values of f* were found to become constant at higher overburden pressures (i.e. greater than 15 kPa) irrespective of change in length and diameter of nails.

iv) Values of f* were observed to be higher for greater diameter of nails. v)

At higher overburden pressures (i.e. greater than 15 kPa) increase in the value of f* with increase in the diameter of nail was marginal.

vi) Values of f* were found to increase with increase in the length of nail. vii) Values of f* obtained for pre-buried nails were found to be greater than the value of f* obtained for driven nails.

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Acknowledgement

The financial assistance provided by CSIR, New Delhi. India for the research work is deeply acknowledged by the authors.

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References th

Gässler, G. and Gudehus, G. (1981), “Soil Nailing – Some Aspects of New Technique”, Proc. of X ICSMFE, 665-670. th

Stockholm, 3,

Gässler, G. and Gudehus, G. (1983), “Soil Nailing Statistical Design”, Proc. of VIII ECSMFE, Helsinki, 2, 491-494.

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Gosavi, Meenal, 2006, “Behaviour of Nailed Open Cuts”, Ph.D. Thesis, Indian Institute of Technology, Roorkee, India. Gupta, R.P. (2003), “A Study on Soil Nailing with Respect to Open Excavations and slopes”, M.Tech Thesis, Indian Institute of Technology, Roorkee. Juran, I. and Elias, V. (1992), “Ground Anchors and Soil Nails in Retaining Structures “, Foundation Engineering Handbook, Van Nostrand Company, Second Edition, 868-905.7. Raju, G. V. R. (1996), “Behaviour of Nailed Soil Retaining Structures”, Ph. D. Thesis, Nanyang Technological University, Singapore. Schlosser, F. (1982), “Behaviour and Design of Soil Nailing”, Symposium on Recent Developments in Ground Improvement techniques, Bangkok, 399-413. Shen, C.K., Bang, S., Romstad, K.M., Kulchin, L. and Denatale, J.S. (1981), “Field Measurements of an Earth Support System”, ASCE Journal of Getotechnical Engineering, 107(GT12), 1625-1642.

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