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Australian Geomechanics Vol 49 No 3 September 2014 79 ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL NAILS, DINMORE TO GOODNA, SE-QLD J. A. Williams 1 and L. Yang 2 1 Principal Engineer – Geotechnical, Thiess, Brisbane, Australia (formerly Parsons Brinckerhoff) 2 Principal Geotechnical Engineer, Parsons Brinckerhoff, Brisbane, Australia (formerly SMEC) ABSTRACT This paper presents results of pull out tests on soil nails to assess ultimate bond stresses of residual clay soil, alluvium, weathered rock and embankment fill on the Ipswich Motorway Upgrade – Dinmore to Goodna (IMU – D2G) project. Vertical sacrificial soil nails were ‘pulled out’ or loaded to failure in a range of materials to assess bond stresses for design and construction of soil nail walls up to 13 m high. Testing was undertaken in materials for which there are no known published records of bond stress, including alluvium of the Brisbane River bank, historic fill of parts of the former Ipswich Motorway embankment and soil and rock of the Ipswich Coal Measures. This paper aims to share some of the ultimate and mobilised bond stress results obtained on the IMU – D2G project. 1 INTRODUCTION The Ipswich Motorway Upgrade – Dinmore to Goodna (IMU – D2G) involved reconstructing 8 km of the Ipswich Motorway from Dinmore to Goodna in the outer suburbs of Ipswich City, west of Brisbane. The project was delivered by an Alliance (Origin Alliance) between Transport and Main Roads, AbiGroup, Fulton Hogan, Seymour Whyte, SMEC and Parsons Brinckerhoff. The new motorway incorporates six lanes with provision for eight lanes. Origin Alliance constructed numerous retaining walls along the new motorway including soil nail walls in cuts and fills. Pull out testing of soil nails was carried out as part of the soil nail design process and this paper presents results from that soil nail testing. Soil nail walls were designed based on limit state principles outlined in BS8006 considering ultimate and serviceability limit states for internal design (pull-out resistance) and external design (global slip). The highest soil nail wall in weathered rock at IMU – D2G is up to 13 m high and 255 m long and incorporates nails 7– 10 m long at 1.5 m vertical and 1.2 m horizontal spacing with 28 mm bar diameter and 150 mm grout diameter. The highest soil nail wall constructed in fill on the project is 7 m high and 153 m long and uses nails 8–12 m long at 1 m spacing vertical and horizontal, with 24 mm bar and 150 mm grout diameter. The aforementioned soil nail wall in fill (Figure 1) was built under live motorway traffic using a carefully staged construction approach. Figure 1: A soil nail wall in historic fill on the IMU – D2G project, up to 7 m high beneath live motorway traffic.

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Page 1: ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL · PDF fileFigure 2: Ultimate soil nail test set up apparatus, ... Results from the soil nail pull out testing are presented in

Australian Geomechanics Vol 49 No 3 September 2014 79

ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL NAILS, DINMORE TO GOODNA, SE-QLD

J. A. Williams1 and L. Yang2 1Principal Engineer – Geotechnical, Thiess, Brisbane, Australia (formerly Parsons Brinckerhoff)

2Principal Geotechnical Engineer, Parsons Brinckerhoff, Brisbane, Australia (formerly SMEC)

ABSTRACT This paper presents results of pull out tests on soil nails to assess ultimate bond stresses of residual clay soil, alluvium, weathered rock and embankment fill on the Ipswich Motorway Upgrade – Dinmore to Goodna (IMU – D2G) project. Vertical sacrificial soil nails were ‘pulled out’ or loaded to failure in a range of materials to assess bond stresses for design and construction of soil nail walls up to 13 m high. Testing was undertaken in materials for which there are no known published records of bond stress, including alluvium of the Brisbane River bank, historic fill of parts of the former Ipswich Motorway embankment and soil and rock of the Ipswich Coal Measures. This paper aims to share some of the ultimate and mobilised bond stress results obtained on the IMU – D2G project.

1 INTRODUCTION The Ipswich Motorway Upgrade – Dinmore to Goodna (IMU – D2G) involved reconstructing 8 km of the Ipswich Motorway from Dinmore to Goodna in the outer suburbs of Ipswich City, west of Brisbane. The project was delivered by an Alliance (Origin Alliance) between Transport and Main Roads, AbiGroup, Fulton Hogan, Seymour Whyte, SMEC and Parsons Brinckerhoff. The new motorway incorporates six lanes with provision for eight lanes.

Origin Alliance constructed numerous retaining walls along the new motorway including soil nail walls in cuts and fills. Pull out testing of soil nails was carried out as part of the soil nail design process and this paper presents results from that soil nail testing.

Soil nail walls were designed based on limit state principles outlined in BS8006 considering ultimate and serviceability limit states for internal design (pull-out resistance) and external design (global slip).

The highest soil nail wall in weathered rock at IMU – D2G is up to 13 m high and 255 m long and incorporates nails 7–10 m long at 1.5 m vertical and 1.2 m horizontal spacing with 28 mm bar diameter and 150 mm grout diameter. The highest soil nail wall constructed in fill on the project is 7 m high and 153 m long and uses nails 8–12 m long at 1 m spacing vertical and horizontal, with 24 mm bar and 150 mm grout diameter. The aforementioned soil nail wall in fill (Figure 1) was built under live motorway traffic using a carefully staged construction approach.

Figure 1: A soil nail wall in historic fill on the IMU – D2G project, up to 7 m high beneath live motorway traffic.

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ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL NAILS, DINMORE TO GOODNA, SE-QLD WILLIAMS AND YANG

Australian Geomechanics Vol 49 No 3 September 2014 80

2 GEOLOGY The regional geology of the site comprises alluvium overlying a Late Triassic sedimentary basin (Cranfield et al., 1989), including the Bundamba Group and the older Ipswich Coal Measures. Rock types at the site comprise sandstone and siltstone with some shale, coal, conglomerate, mudstone and claystone. Minor basaltic dykes or sills are present. The dip of the strata is gentle related to minor folding and faulting. Residual soil is typically about 1–2 m thick comprising high plasticity clay which is commonly stiff and fissured. Historic motorway embankment fill comprised variable quality remoulded weathered shale, siltstone and sandstone. Coal rejects occur in some of the historic fill comprising mixtures of coal, ash, clay and shale and potentially a range of other rock materials left over from past eras of nearby coal mining.

Geotechnical engineering classifications of materials on the project were undertaken using the Unified Soil Classification (USC) for soils and rock was classified according to the system shown in Table 1.

Table 1: Project rock mass classification system

IMU - D2G rock classa Rock strength (MPa) Strength Block size (mm)

R1 SAN-1/SIL-1 UCS > 200MPa, Is(50) > 13.3 Extremely high (EH) >300

R2 SAN-2/SIL-2 UCS > 60MPa, Is(50) > 4 or Very high (VH) <300

UCS = 20–60, Is(50) = 1.3–4 High (H) >300

R3 SAN-3/SIL-3 UCS = 20–60, Is(50) = 1.3–4 or High (H) <300

UCS = 6–20, Is(50) = 0.4–1.3 Medium (M) >300

R4 SAN-4/SIL-4 UCS = 6–20, Is(50) = 0.4–1.3 or Medium (M) <300

UCS = 2–6, Is(50) = 0.13–0.4 Low (L) >300

R5 SAN-5/SIL-5 UCS = 2–6, Is(50) = 0.13–0.4 or Low (L) <300

UCS < 2, Is(50) < 0.13 Very low (VL) >300

R6 SAN-6/SIL-6 UCS < 0.6, Is(50) < 0.04 Extremely low (EL) <300

Notes: a. SAN = Sandstone; SIL = Siltstone; UCS = Unconfined Compressive Strength; Is(50) = Point Load Strength Index

3 EMPIRICAL BOND STRESS ESTIMATES Some of the empirical methods available in literature for estimating ultimate bond stress (τBSult) in soil and in weathered rock are presented below.

3.1 SOIL For clay soil using undrained shear strength,

τBSult = α.Cu (1)

where Cu is undrained shear strength and α = 0.25 to 0.75 depending on undrained shear strength (Byrne et al. 1998). For soils using effective stress,

τBSult = c’ + σv’.tanǾ (2)

where c’ and Ǿ’ are effective cohesion and effective friction angle respectively; σv’ is the effective normal stress typically averaged over the length of the nail. At IMU – D2G the bond stresses were capped at a maximum (= Cu.α, generally using α = 0.5).

3.2 WEATHERED ROCK For rock up to a maximum of 4 MPa UCS (PTI 1996),

τBSult = 0.1 x UCS (3)

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ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL NAILS, DINMORE TO GOODNA, SE-QLD WILLIAMS AND YANG

Australian Geomechanics Vol 49 No 3 September 2014 81

Kulhawy et al. (2005) summarised various methods of assessing unit side resistance (τf) or ‘bond stress’ in weathered rock highlighting the common relationship in Equation (4),

τf / Pa = α.(UCS / Pa)β (4)

where α is a constant, β is an exponent and Pa is atmospheric pressure (1 atm = 0.1013 MPa). Refer also Pells et al. (1998), Pells (1999) and Seidel and Collingwood (2001) if wishing to incorporate roughness or other effects, although sidewall effects may be difficult to confirm over any reasonable length in soil nail holes.

Other useful references fitting the above general Equation (4) include Rosenberg and Journex (1976), Horvath (1978), Meigh and Wolski (1979), Williams et al. (1980), Rowe and Armitage (1984).

4 TEST METHOD Testing of soil nails commonly includes ultimate or pull-out testing, verification testing, acceptance or proof testing and creep testing (Lazarte et al., 2003). Each of the various soil nail test types is conducted for different purposes and involves different loading concepts, although typically loads do not exceed 80% of the tendon yield stress for safety reasons. For a more detailed overview on soil nail test types refer to Lazarte et al. (2003).

Testing undertaken at IMU – D2G included ultimate or ‘pull-out’ testing and acceptance testing. Ultimate testing provides the ultimate adhesion or ultimate bond stress of sacrificial nails if failure or pull-out is reached and verifies design adhesion factor of safety plus can give an indication of the soil nail load corresponding with excessive creep (Lazarte et al., 2003).

Testing was performed by Team Rock Anchors Pty Ltd and by Keller Ground Engineering Pty Ltd. Generally the testing was undertaken using a 110 tonne hydraulic jack with dial gauges for measuring settlement (to 0.01 mm) and load (to 2 kN) plus a 60 kN digital load cell accurate to 0.05 kN as shown in Figure 2.

Figure 2: Ultimate soil nail test set up apparatus, courtesy Team Rock Anchors.

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Nails were typically tested by increasing the load until a displacement of more than 40 mm was recorded or the maximum load reached at which the test can be terminated if pull-out does not occur. Loading was kept under 80% of the nail tendon minimum ultimate or ‘characteristic’ tensile strength.

The load and displacement were recorded at 5 kN load or 5 mm displacement increments, whichever occurred first and loading rates were within 5-10 kN/minute. Readings were taken only after load and displacement were stable but generally not less than 1 minute per 10 kN.

Ultimate soil nail tests were carried out in purpose drilled vertical boreholes which were each grouted to form a 1 m or 2 m bonded interval, including HDPE sheath. The grout strength minimum requirement was 40 MPa. Free lengths of the nail tendons ranged from 2.3 m to 14.4 m depending on the depths of soil or rock layer to be tested.

One soil or rock layer was targeted for testing in each drill hole and generally duplicates were drilled next to each other allowing for testing at 7 day and 28 day grout curing. The number of discrete ultimate soil nail tests at any one location was a maximum of 12 tests. In total, 37 ultimate soil nail tests were carried out testing the following materials:

• Existing embankment fill (old motorway) • Alluvium on a bank of the Brisbane River (typically sandy clay and silt) • Residual clay soil • Weathered siltstone and sandstone

5 RESULTS The ultimate bond stress of the test results was calculated by

τBSult = Tult/πdL (5)

where Tult is the test load, d is the drillhole diameter (=150 mm) and L is the bond length (1 to 2 m). It is assumed that the test load was distributed uniformly through the short bond length. As the tested bond lengths were short the load transfer is believed to be efficient. According to Barley (1995), an efficiency factor of 1 could be adopted if the bond length is less than 2.3 m.

Results from the soil nail pull out testing are presented in Table 2 and in Figures 3 to 7. Standard penetration test (SPT) ‘N60’ results from nearby geotechnical boreholes are included in Table 2 and in Figures 3 to 5.

Table 2: Results of pull out tests on soil nails on the IMU – D2G project.

Material Test ID

Curing (days) Location

Test depth

(m)

Maximum

load or failure load

(kN)

Ultimate bond stress (kPa)

Fail* SPT N60

SPT depth (m)

Fill

Sandy Gravelly CLAY

(CL-CI)

8D

8F

8B

8E

8A

8C

7

7

7

7

7

7

Church

St

2-3

2.1-3.1

3.5-4.5

3.7-4.7

3.7-4.7

5.5-6.5

70

160

75

150

97

30

149

339

159

318

306

64

Y

Y

Y

Y

Y

Y

7 2.5

8 4

14 5.5

Alluvial

‘Stiff’ Silty/ Sandy CLAY

(CH)

1 7

Lower James St – Brisbane

River

3-5 102 108 Y - -

2 7 11-13 110 116 Y 15 12

Alluvial ‘Stiff’ SILT (MH)

3 7 14.4-16.4 161 170 Y 13 15

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ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL NAILS, DINMORE TO GOODNA, SE-QLD WILLIAMS AND YANG

Australian Geomechanics Vol 49 No 3 September 2014 83

Residual ‘Stiff’ Sandy CLAY

(CH)

1B

1C

1D

1A

7

28

28

7

Church

St

6.5-7.5

6.5-7.5

6.5-7.5

6.5-7.5

50

65

55

35

106

137

117

74

Y

Y

Y

Y

15 7.5

SAN4 7A

7B

7C

7D

7

7

28

28

Law

St

7-8

7-8

7-8

7-8

165

264

290

353

350

562

615

>749

Y

Y

Y

N

- -

SAN5 4A

4B

4C

4D

7

7

28

28

Donald

St

2.4-3.4

2.4-3.4

2.4-3.4

2.4-3.4

225

345

327

220

477

732

693

467

Y

Y

Y

Y

- -

3A

3B

3C

3D

7

7

28

28

Church

St

12.5-13.5

12.5-13.5

12.5-13.5

12.5-13.5

353

353

353

353

749

>749

>749

>749

Y

N

N

N

- -

SIL5 5A

5B

5C

5D

7

7

28

28

Donald

St

5.7-6.7

5.7-6.7

5.7-6.7

5.7-6.7

353

353

353

229

>749

>749

>749

488

N

N

N

Y

- -

6A

6B

6C

6D

7

7

28

28

Donald

St

7.8-8.8

7.8-8.8

7.8-8.8

7.8-8.8

270

353

353

353

572

>749

>749

>749

Y

N

N

N

- -

* Although it would be ideal for the pull out tests to reach the point of pull out failure, this is not possible for safety reasons when the loads reach 80% of the tendon yield stress (denoted by Fail = N). In these cases, the bond stress reported is the bond stress mobilised and may not reflect the ultimate bond stress.

Figures 3 to 7 present bond stresses and movements along the bonded interval for the various materials tested, noting theoretical extension of the bar in the unbonded interval has been subtracted from total extension to arrive at movement along the bonded interval. Test locations and depths are included in the legends in Figures 3 to 7 and the stipulated or nominated failure points are circled on the graphs. The lowest test result in each material type is labelled, i.e. ‘8C – 30 kN @ Failure’ in Figure 3, indicating the lower bound for that material.

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ULTIMATE BOND STRESSES FROM PULL OUT TESTING OF SOIL NAILS, DINMORE TO GOODNA, SE-QLD WILLIAMS AND YANG

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Figure 3: Results of ultimate soil nail testing in embankment fill.

Figure 4: Results of ultimate soil nail testing in alluvium.

Figure 5: Results of ultimate soil nail testing in residual soil.

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Figure 6: Results of ultimate soil nail testing in sandstone (Note tests 3B,C & D and 7D did not fail).

Figure 7: Results of ultimate soil nail testing in siltstone (Note tests 5A, B & C and 6B, C & D did not fail.

6 DISCUSSION Bond stresses can vary substantially depending on various factors including physical properties of the ground, presence of groundwater and construction methods. Carrying out ultimate soil nail testing is generally a necessity however opportunities for such testing may not be possible in early design phases.

Estimating bond stresses from empirical relations can result in a significant variation between lower and upper bound values. Table 3 below shows the estimated ultimate bond stress values for soils on the IMU – D2G project using equations (1) and (2) along with ultimate bond stress values assessed from pull out testing and the ultimate values adopted for design.

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Table 3: Comparison of ultimate bond stresses for soils on the IMU – D2G project

Material Ultimate bond stress (kPa)

Empirical Estimate Test Result Adopted

Historic embankment fill ‘Sandy Gravelly CLAY’ (CL-

CI) 13 to 87

63 to 339 80

Alluvial ‘Stiff’ Silty/ Sandy CLAY and

SILT (CH/MH) 50 to 150 108 to 170 110

Residual ‘Stiff’ Sandy CLAY (CH) 50 to 150 74 to 137 110

Ultimate bond stress results from pull out testing in soils showed significant variation between lower and upper bound values. Generally our approach was to adopt ultimate bond stress values for design that were close to lower bound from testing. Notwithstanding this, the ultimate bond stress test results for soils supported adoption of ultimate design values toward the mid-range to upper bound of the empirical estimates. The nails were not exhumed in this assessment limiting our ability to investigate whether some of the variance in bond stress results might have been related to grout or annulus inconsistencies, which might have been possible particularly in fill.

Table 4 below shows the estimated ultimate bond stress values for rock on the IMU – D2G project along with ultimate bond stresses from pull out testing and the ultimate values adopted for design. Empirical estimates were made using equations (3) and (4) plus a range of other references (Figure 8).

Table 4: Comparison of ultimate bond stresses for weathered rock on the IMU – D2G project

Rock Ultimate Bond Stress (MPa)

Rock Ultimate Bond Stress (MPa)

Empirical Estimate

Test result Adopted Empirical

Estimate Test

result Adopted

SAN-6 0.1 to 0.45 - 0.4 SIL-6 0.1 to 0.45 - 0.16

SAN-5 0.2 to 0.71 0.467 to >0.749^ 0.5 SIL-5 0.2 to 0.71 0.488 to

>0.749^ 0.4

SAN-4 0.2 to 2.95 0.35 to >0.749^ 0.55 SIL-4 0.2 to 2.95 - 0.5

SAN-3 0.5 to 6 - 0.6 SIL-3 0.5 to 6 - 0.55 SAN-2 1.3 to >6 - 1.0 SIL-2 1.3 to >6 - 0.9 SAN-1 >9 - 1.6 SIL-1 >9 - 1.2

^ Maximum mobilised bond stress as pull out failure was not reached

Figure 8: Estimated ultimate bond stress using unconfined compressive strength and empirical relations.

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Ultimate bond stress estimates and test results for rock showed significant variability between lower and upper bound (refer Figure 8 and Table 4). Some pull out tests in rock failed within the range estimated empirically whilst other tests in the same materials reached the maximum safe pull out force without failing, at almost double the load of the tests that failed.

When carrying out ultimate soil nail testing it is important to ensure sufficient pull out force is available or can be safely implemented if wishing to test the ultimate bond stress. In some pull out tests in weathered rock, the bar diameter chosen for some of the testing limited the maximum pull out force that could be applied. In future the authors will use high tensile steel bar to enable higher testing forces even though in this case the objectives of the testing were successful, i.e. confirm design parameters.

The ultimate bond stress test results for rock generally supported adoption of ultimate design values toward the mid-range to upper bound of empirical estimates. This was particularly relevant for Class 4 and Class 5 rock.

Ultimate soil nail testing in rock in this case only occurred in Class 4 and in Class 5 siltstone (refer Table 1 for rock class definitions) as there were generally only sparse occurrences of better class rock in soil nail walls at the site.

The pull out tests were undertaken in vertical holes on this project. The impacts of soil or rock anisotropy were not specifically investigated in this paper. However it is observed bond stress test results vary significantly for some rock materials, i.e. class 4 and Class 5 rock and some of this variability potentially reflects macro scale anisotropy of the tested materials, e.g. interbedded sandstones and siltstones with weathering gradations. Plans to exhume nails to check the physical form of the grouted section were not able to be carried out due to construction program issues, which is also a possible explanation for some of the variance in bond stress results. Further study is recommended about possible differences in bond stress between vertical test nails and inclined production nails, plus potential bond stress variations depending on other features such as nail hole stability.

The pull out testing occurred on discretely bonded intervals (1 or 2 m) aimed at assessing ultimate bond stress of individual soil types or rock classes. We applied a bond stress efficiency factor of 1, however it is noted that our test nails were short and that alternative efficiency factors may apply for longer nails (Barley, 1995). There could also be a range of considerations for the design of long nails, such as effects of progressive debonding.

Bond stresses were attempted to be assessed for each of the materials anticipated overall at the soil nail walls, with the design process relying on detailed ground investigations at each wall to develop detailed ground models.

7 CONCLUSION Ultimate soil nail testing was carried out on the IMU – D2G project in materials including embankment fill of part of the former Ipswich Motorway, stiff sandy clay residual soil and alluvium plus weathered sandstone and siltstone of the Ipswich Coal Measures. The pull out testing program was successful enabling valuable comparison and refinement of empirically estimated ultimate bond stresses. The pull out testing generally confirmed the anticipated ultimate bond stresses or in some cases supported use of ultimate design values toward the mid-range to upper bound of empirical estimates resulting in more economic designs.

Figure 9: A soil nail wall in weathered rock on the IMU – D2G project.

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No known published bond stress information was known to exist for the Ipswich Coal Measures when this paper was prepared.

The approach used for soil nail design and pull out testing was integral to successful construction of soil nail walls in fill up to 7 m high beneath live motorway traffic and soil nail walls in weathered rock up to 13 m high (Refer Figure 9).

It is recommended ultimate soil nail testing is carried out at all sites where soil nail walls are proposed to help assess ultimate bond stress values for design.

8 ACKNOWLEDGEMENTS Thanks are extended to Origin Alliance and to Transport and Main Roads — Queensland Government for permissions to use project data from the IMU – D2G project.

9 REFERENCES Barley, A.D. (1995). Theory and Practice of the Single Bore Multiple Anchor System. Anchors in Theory and Practice,

International Symposium on Anchors in Theory and Practice, Saltzburg, October 9-10. Byrne, R.J., Cotton, D., Porterfield, J., Wolschlag, C., and Ueblacker, U. (1998). Manual for Design and Construction

Monitoringof Soil Nail Walls. FHWA-SA-96-069R. Federal Highway Administration, U.S. Department of Transportation.

BS8006 (1995). Code of Practice for Strengthened/Reinforced Soils and Other Fills. British Standard. Clayton, C.R.I. (1995). The Standard Penetration Test (SPT): Methods and Use. Report 143. Construction Industry

Research Association (CIRIA). Cranfield, L.C., Hutton, L.J., and Green, P.M. (1989). Ipswich Sheet 9442, 1:100 000 Geological Map Commentary,

Queensland Department of Mines. Horvath, R.G. (1978). Field load test data on concrete-to-rock bond strength for drilled pier foundations. Publication

78-07. Toronto: University of Toronto. Kulhawy, F.H., Prakoso, W.A., and Akbas, S.O. (2005). Evaluation of Capacity of Rock Foundation Sockets. American

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Walls. FHWA0-IF-03-017. Federal Highway Administration, U.S. Department of Transportation. Meigh, A.C., and Wolski, W. (1979). Design parameters for weak rock. Proceedings of 7th European Conference on

Soil Mechanics and Foundation Engineering, Brighton, Sep. 1979, 5: 59–79. London: British Geotechnical Society.

Pells, P.J.N. (1999). State of Practice for the Design of Socketed Piles in Rock. 8th ANZ Geomechanics Conference, Hobart.

Pells, P.J.N, Mostyn, G., and Walker, B.F. (1998). Foundations on Sandstone and Shale in the Sydney Region. Australian Geomechanics, December 1998: p. 17-29.

Porterfield, J.A, Cotton, D.M., Byrne, J. (1994). Soil Nailing Field Inspectors Manual – Soil Nail Walls. FHWA-SA-93-068. Federal Highways Administration, U.S. Department of Transportation.

PTI (1996). Recommendations for Prestressed Rock and Soil Anchors. 3rd Edition. Post-Tensioning Institute, Phoenix, Arizona.

Rosenberg, P., and Journeaux, N.L. (1976). Friction and end bearing tests on bedrock for high capacity socket design. Canadian Geotechnical Journal, 13(3): p. 324–333.

Rowe, R.K., and Armitage, H.H. (1984). Design of piles socketed into weak rock. Report GEOT-11-84. London: University of Western Ontario.

Seidel, J.P, and Collingwood, B. (2001). A new socket roughness factor for prediction of rock socket shaft resistance.Canadian Geotechnical Journal, 38(1): p. 138–153.

Stroud, M.A. (1989). The Standard Penetration Test – Its application and interpretation. Proceedings of ICE Conference on Penetration Testing in the UK, Birmingham. Thomas Telford, London.

Williams, A.F., Johnston, I.W., and Donald, I.B. (1980). Design of socketed piles in weak rock. In (Ed.) P.J.N. Pells, ”Structural Foundations on Rock”. Balkema, Rotterdam, p. 327–347.