Comparative study on pullout behaviour of pressure grouted soil nails ...

J. Cent. South Univ. (2013) 20: DOI:

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Comparative study on pullout behaviour of pressure grouted soil nails from field and laboratory tests 洪成雨，殷建华，裴华富

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HONG Cheng-yu , ZHANG Yi-fang , YIN Jian-hua , PEI Hua-fu3

补中文名

1. Department of Civil Engineering, Shantou University, Guangdong, China; 2. Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China; 3. Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2013

加“a” Abstract: Pullout resistance of soil nail is a critical parameter in design and analysis for geotechnical engineers. Due to the complexity of field conditions, the pullout behaviour of cement grouted soil nail in field is not well investigated. In this work, a number of field pullout tests of pressure grouted soil nails were conducted to estimate the pullout resistance of soil nails. The effective bond lengths of field soil nails were accurately controlled by a new grouting packer system. Typical field test results and the 加“a” related comparison with typical laboratory test results reveal that the apparent coefficient of friction (ACF) decreases with the increase of overburden soil pressure when grouting pressure is constant, but increases almost linearly with the increase of grouting pressure when overburden pressure (soil depth) is unchanged. Water contents of soil samples at soil nail surfaces show obvious reductions compared with the results of soil samples from drillholes. After soil nails were completely pulled out of the ground, surfaces of the soil nail and surrounding soil were examined. It is found that the water content of the soil at the soil-nail interfaces decreases substantially compared with that of soil samples extracted from drillholes. In addition, all soil nails expand significantly in the diametrical direction after being pulled out of ground, indicating that the pressurized cement grout compacts the soil and penetrated into soil voids, leading to a corresponding shift of failure surface into surrounding soil mass significantly. Key words: soil nail; apparent coefficient of friction; overburden soil pressure; grouting pressure

1 Introduction Grouted soil nail is a popular reinforcement for stabilizing slopes, excavations and retaining walls. Pullout shear resistance of the soil-nail interface is a critical parameter, which is dependent on a number of influencing factors, such as drilling method, ground conditions, overburden soil pressure, grout injection method (under gravity or pressure). Two typical measures have been used to evaluate the pullout behaviour of soil nails, i.e., the laboratory pullout test and field pullout test. The laboratory test is commonly used to estimate the pullout resistance of grouted soil nails. Testing condition can be strictly controlled so that reliable experimental environment can be ensured and testing uncertainties and variations arising from complicated field conditions are avoided. Interaction mechanism between soil and soil nail has been widely investigated using laboratory pullout tests and a number of critical issues have been fully addressed. These typical

laboratory tests involved different types of soil nails placing in various fill materials, such as compacted soil or loose soil in saturated or unsaturated conditions [1−8]. These extensive test results indicate certain correlations between pullout resistance and a number of influencing parameters such as overburden soil pressure, grouting pressure, constrained soil dilation, soil water content, etc. [6−7, 9−12]. In comparison with laboratory tests, full-scale field tests provide reliable evaluation of the field pullout behaviour of soil nails. Although scattered test results may be obtained due to the complicated ground conditions, field tests are still necessary for understanding the practical interaction mechanism between soil and soil nails. Typical test results suggest that the use of grouting pressure is a simple and effective measure to increase the interface shear resistance of soil nails [10, 13]. Pullout resistance of soil nail increases linearly proportional to the increase of grouting pressure [14] due to the highly infiltration effect of cement grout into surrounding soil, motivating a highly interlocking effect between soil nail

Foundation item: Project(NTF12015) supported by the STU Scientific Research Foundation for Talent, China; Project(polyU 5320107E ) supported by the Research Grants committee General Reseath Fund, China Received date: 2012−09−10; Accepted date: 2013−04−10 Corresponding author: HONG Cheng-Yu, PhD; Tel: +86−754−82902967; Fax: +86−754−82902005; E-mail: [email protected]

J. Cent. South Univ. (2013) 20:

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and surrounding soil. ELIAS and JURAN [14] also reported that there exists a limited grouting pressure, beyond which the grouting pressure may not increase the pullout resistance of soil nails significantly. But there is inadequate typical test results supporting this conclusion. YEUNG et al [15] found that the grouting pressure strengthens the bond strength of the soil-cement grout interface in field pullout test of a glass fiber reinforced polymer (GFRP) system. Due to the limited number of field tests, the effect of overburden soil pressure or grouting pressure on the pullout resistance of GFRP nails was not quantified. This work presents a systematic comparative study on the laboratory and field pullout behaviour of soil nails grouted with different pressures. Effect of grouting pressure on the shear resistance of the soil-nail interface is quantified and typical observations are analyzed. Variations of critical parameters such as soil water content and soil nail diameter before and after the pullout tests are discussed and interpreted. All these test findings and the related explanations are useful for better understanding of the practical pullout behaviour of grouted soil nails.

3 Pullout test setup Field pullout tests involved installation of packers, placement of steel bars in drillholes, sealing of grouted part, pressure grouting, removal of packer, pullout test, and water content and diameter measurements after pullout tests. Figure 1 shows a basic field test setup of the nail bar in drillhole. A loading device and load transfer frame are used for compressing the packer, which will expand and seal the drillhole in diametrical direction. Then the grout pipe starts to inject cement slurry into the drillhole under the effect of certain pressure. After a specified pressure is approached, both outlet pipe and grout pipe are sealed. During the whole process, two pressure gauges are used for monitoring the pressure change of cement slurry, as shown in Fig. 1. Detailed information about the packer the related operation work can be found in Ref. [16]. For simplification, two key test procedures are introduced as below.

2 Field conditions Experimental program was carried out in a uniform natural slope, namely feature No.11SW-C/F220 under the Landslip Preventive Measure (LPM) Program of Geotechnical Engineering Office (GEO) of Hong Kong Government at South of Pok Fu Lam Kennels, Pok Fu Lam, Hong Kong, China. The slope height was 12 m and the slope length was 26.5 m. Slope angle varied from 15° to 48° to the horizontal. Pullout tests were conducted on ten soil nails, which were placed at different soil depths underground. The diameter, grouted length and total length of the nail bar were 100 mm, 1.2 m and 2.5 m, respectively as presented in Table 1. Soil nails were completely pulled out of the ground to examine the soil-nail interface conditions, measure water content of soil at soil nail surface and the diameters of different soil nail locations. Table 1 Data of soil nails in field pullout tests Soil nail Steel bar Grouted Soil No. length/m length/m depth/m SN1 2.5 1.2 6

Grouting pressure/kPa 40

SN2

2.5

1.2

6

0

SN3

2.5

1.2

6

80

SN4

2.5

1.2

6

140

SN6

2.5

1.2

2

0

SN7

2.5

1.2

2

140

SN8

2.5

1.2

2

80

SN9

2.5

1.2

2

40

Fig. 1 A schematic view of grouting process on a steel bar in field

3.1 Installation of grouting packers on steel bars To control the adherence length of grouted soil nail, a special grouting packer system was adopted to maintain the grouting pressure inside the drillhole. Figure 2(a) shows the grouting packers installed on soil nail bars. The packer is a hollow cylinder with 480 mm in length, 90 mm in external diameter, and 50 mm in internal diameter. Packer material is rubber which could expand significantly under axial compression effect, sealing the grouted part of the soil nail bar (marked in Fig. 2(a)). Calibration test results indicate that the maximum grouting pressure maintained by the packer was 160 kPa. The packer can be easily removed after cement grout has been cured, so that the real friction between soil nail and soil can be measured. 3.2 Soil properties and field pullout test Laboratory tests were conducted on a number of


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Fig. 2 Field photos of full instrumentation of nail bars: (a) Nail bars installed with grouting packers; (b) Basic field setup of pullout test; (c) Grouted parts after pulled out of ground for visual examination

soil specimens extracted from different drillholes. Table 2 shows basic properties of soils in field. Measurement results show that the average values of specific gravity, unit weight, and water content are 2.6, 21.5 kN/m3, and 8.1%, respectively. The effective overburden soil pressures of soil nails placing at average soil depths of 2 m and 6 m are 43 kPa and 129 kPa calculated using the average unit weight. Table 2 Soil properties Soil property

Value

Average specific gravity (Gs)

2.6

Average total unit weight/(kN·m−3)

22

Average water content, w/%

8.1

Average plastic limit, wp/%

21

Average liquid limit, wl/%

29

Field pullout tests involved a number of testing equipments as presented in Fig. 2(b). A hydraulic jack, a load cell, and two LVDTs are used for applying pullout force, monitoring the change of pullout force, and measuring the displacement of soil nail heads and steel pad, respectively. Pullout test was carried out 7 d after grouting in the field and pullout force increment was 5 kN and maintained for 15 min for each loading step. After approaching the maximum pullout force, the soil nail was pulled out continuously at a speed of 1 mm per min to achieve a maximum pullout displacement of 50 mm. Then the whole grouted part was completely

pulled out for the measurement of diameter and soil water content at soil nail surface. Figure 2(c) shows a grouted part of a soil nail placing on the ground after pullout test. An independent grouted part is successfully obtained.

4 Comparisons between laboratory and field pullout test results 4.1 Effect of grouting pressure and overburden soil pressure on shear strength of soil nail interface Average shear stress τ is calculated by dividing the pullout force of soil nail head with the contact area between cement grout and soil:



P πDL

(1)

where P is the pullout force applied on the soil nail head; L and D are the grouted length and nail diameter, respectively. It is noted that the diameter D used in the above equation is the averaged soil nail diameter measured after being pulled out of ground. If a soil nail was not completely pulled been out, the diameter of the soil nail segment that partially pulled out was measured and used in Eq. (1). Figure 3 shows relationships between average shear resistance and pullout displacement at different depths in field. Four grouting pressure values 0, 40, 80, and 140 kPa were adopted in present tests. Most pullout resistance against displacement curves of soil nails are softening, that is, pullout resistance decreases after the peak shear


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resistance is approached. As grouting pressures vary at 0, 40, 80, and 140 kPa, the corresponding maximum average shear strength are 90, 110, 123, and 128 kPa for the soil depth of 2 m, and 82, 82, 93, and 121 kPa for the soil depth of 6 m, respectively. The maximum pullout shear stress at the soil/nail-soil interfaces mostly increases as the increases of grouting pressure, so the use of grouting pressure is an effective measure for enhancing the pullout resistance of soil nails in real field.

Average soil depth = 6 m

Fig. 3 Average shear stress of nail-soil interface against pullout displacement of soil nail head when average soil nail depth is (a) 2 m; (b) 6 m

Apparent coefficient of friction is a common parameter for reflecting the correlation between pullout resistance and overburden soil pressure or embedded depth of soil nail [2]. As proposed by SCHLOSSER [17], the soil-inclusion interface friction is evaluated by an apparent coefficient of friction (ACF), η*, defined by dividing the maximum shear stress of soil-nail interface (τmax) with the effective overburden soil pressure (  v ):

* 

 max  v

(2)

where effective overburden pressure  v is calculated by multiplying the effective unit weight of soil with the average depth of the soil nail. It is noted that for drill and grouted soil nails, the normal stress at the soil-nail interface is a complicated parameter. Due to the stress release caused by hole drilling and the consequent arching effect of the drillhole, the overburden soil pressure around the soil nail no longer exists. The pressure grouting process disturbs the normal stress on the soil nail surface before pullout test [10], resulting in stress re-distribution surrounding the soil nail. In addition, the actual normal stress surrounding the soil nail changes progressively due to possible volume change tendency (expansion or contraction) of the soil-nail interface during pullout test. Therefore, the normal stresses at the soil-nail interface are actually complex and difficult to determine. The use of Eq. (2) is simple and ease of understanding. Past typical test results also show that the ACF value decreases as the increase of overburden soil pressure [2], that is, the higher the overburden soil pressure, the lower the ACF value. However, for the case of pressure grouted soil nails, is it reasonable to use ACF value to indicate the effect of overburden soil pressure on the pullout resistance of soil nails? Figure 4 shows relationships of measured ACF values against overburden soil pressure from test results of Ref. [7, 10, 12] and present tests. It is noted that the total soil

Fig. 4 Relationships of apparent coefficient of friction against overburden soil pressure


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overburden pressure is used in Eq. (2) rather than the effective value. The calculated ACF values vary substantially at constant overburden pressures due to the existence of grouting pressure. That is, the ACF values increase as grouting pressure increases. For example, as marked by circles in Figure 4, test results of YIN and ZHOU [7] show that, at the overburden soil pressure of 120 kPa, the ACF value increases from about 0.5 to 1.0. Similarly, the corresponding increments measured by YIN et al [10] are 0.4 (from 0.4 to 0.8) and 1.1 (from 0.6 to 1.7) at the overburden soil pressures of 80 and 200 kPa, respectively. For the test results by ZHOU [12], the maximum increments of ACF values due to grouting pressure is 0.5 (from 0.2 to 0.7) at the overburden soil pressure of 350 kPa. The present test results also show a corresponding rise of ACF value from 2.0 to 3.1 at the overburden soil pressure of 40 kPa. These phenomenon indicates that the ACF value, which is a coefficient correlating the pullout resistance with overburden soil pressure, is not unique at constant values of overburden soil pressure. Relationships between ACF values and overburden soil pressures are scattered due to the adoption of grouting pressure, so that the ACF value is difficult to reveal the real variation of pullout resistance associated with overburden soil pressure. In case grouting pressure is used, the interaction interface between soil and soil nail is more dependent on grouting pressure than overburden soil pressure, as the grouting pressure directly acts on the soil nail surface, while the overburden soil pressure is effective at a certain distance away from soil nail surface. In present study, a new parameter which clarifies the correlation between the pullout resistance and grouting pressure is adopted, namely ACF value related to grouting pressure, denoted by uG. The common coefficient used in literatures relating to overburden soil pressure is renamed by “ACF value related to overburden soil pressure”, denoted by uo. Therefore, two new parameters are given as below: uO  uG 

 max Pos

 max PQ

(3) (4)

where Pos and PG are overburden soil pressure and grouting pressure, respectively. It is noted that when uo and uG are used, the corresponding grouting pressure and overburden soil pressure must be constant, so that their correlations with pullout resistance can be clearly identified. Figure 5 shows relationships between ACF value and overburden soil pressure at different grouting pressures. For the specified test under the same testing condition, the ACF value uo still decreases with the increase of overburden soil pressure when grouting

pressure is unchanged. For example, as overburden soil pressure increases from 80 to 200 kPa [10], the corresponding values of uo reduce from 1.7 to 1.0 when PG=0 kPa, from 1.25 to 0.55 when PG=80 kPa, and from 0.6 to 0.35 when PG=140 kPa. In present tests, ACF values uo also reduce from 2.9 to 1.0 when PG=140 kPa, from 2.2 to 0.8 when PG=80 kPa and from 1.9 to 0.6 when PG=0 kPa. Typical variation trends can also be found in test results of ZHOU [12]. This is consistent with observations in LUO et al [2] where no grouting pressure was involved.

Fig. 5 Relationships of apparent coefficient of friction against overburden soil pressure at different grouting pressures

Figure 6 shows the variations of ACF values with respect to grouting pressure at different overburden soil pressures. It is clear that the most values of the coefficient uG increases linearly with the increase of grouting pressure as found from test results by YIN et al [10] and present test results. From the test results of YIN et al [10], the increases of grouting pressure results in a related increase of ACF value uG about 0.9 (from 0.7 to 1.6) and 0.3 (from 0.3 to 0.8) for the grouting pressure of 80 and 200 kPa, respectively. For the present test results, the grouting pressure which increases from 0 to 130 kPa leads to a corresponding increments of ACF value about 1 (from 1.8 to 2.8 at OSP of 43 kPa) and 0.4 (from 0.6 to 1.0 at OSP of 128 kPa). Therefore, the higher the grouting pressure, the larger the ACF value uG at the same level of overburden soil pressure. The relationship between pullout resistance and grouting pressure can be approximated as pure linear. Typical increase of ACF values reveals the effect of grouting pressure on the maximum shear resistance of the soil-nail interface. The drilling process releases all initial overburden soil pressure, but as a result of pressure grouting, the cement grout pressure works actively on the internal hole surface, so that more normal stress on the soil nail surface is triggered. At the same time, the pressurized cement grout slurry infiltrates into soil voids, increasing both strength

加 in Ref.[21] J. Cent. South Univ. (2013) 20:

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and roughness of soil-nail interface. All these factors strengthen physical bonding effect between soil and soil nail and consequently lead to the substantial rise of ACF values. Pressure grouting is an active and effective measure to increase the resistance of soil-nail interface.

and 0.15 to 0.95. The consistent reductions of water content values may be due to the hardening effect of cement grout, which absorbs water from surrounding soil. On the other hand, the real field test results show that the reductions of water content are much more obvious and scattered than laboratory test results. This reflects that the field uncertainties may be difficult to avoid, while the laboratory testing condition is stable and can be strictly controlled.

Fig. 6 Relationships of apparent coefficient of friction against grouting pressure at different overburden soil pressures (OSP= Overburden soil pressure)

4.2 Comparison of water content and soil nail diameter change Water content at the soil nail interaction interface is also a critical parameter affecting the pullout resistance of soil nails substantially. The higher the water content (or the wetter the interface), the lower the pullout resistance. To better understand the pullout resistance associated with the change of water content, a new coefficient, namely water content ratio, is defined by dividing the original soil water content (in drillholes) with the soil water content measured at the soil nail surface after pullout test. Water content ratio is defined as: 应该是 Wsurface

w 

winterface worignal

(5)

应该是 Wsurface

where wwurface and worignal are water content values of soil samples at the soil nail surface and in drillholes, so that remain proportion of soil water content after pullout test can be identified using δw. Figure 7 shows values of water content ratio for different pullout tests in publications. Calculated δw values from laboratory tests by CHU [18] and ZHOU [12] are slightly smaller than 1, indicating that the water content values at the soil nail surface decrease slightly after pulled out ground (water content ratios are around 0.95) compared with the original soil in pullout box. While the obtained δw values in present field tests and field test results by CHAI and HAYASHI (2005) all or partially decrease significantly. The measured water content ratios vary from 0.3 to 0.5 in present field tests 引用文献？

Fig. 7 Water content ratios of soil specimens in laboratory and field pullout tests

4.3 Measurement of soil nail diameters before and after pullout tests In this paper Soil nail diameter change is highly related to the occurred shear strength of soil nail interface. In the work, the soil nail surface conditions were evaluated by comparing original soil nail (drillhole) with soil nail diameters after completely (or partially) pulled out of ground. Figure 8 shows comparisons between soil nail and Ref. drillhole diameters from laboratory and field tests. It is noted that laboratory pullout tests of grouted soil nails from ZHOU [12] were also conducted under different grouting pressures, similar to the present testing conditions. Diameters of original drillholes are stable (around 100 mm) but approximately 8 mm larger in average than drillhole diameters in Ref. [10]. The soil nail diameters in present field tests after pullout test (lie between 115 and 130 mm) are also substantially larger than the original drillhole diameter (generally less than 110 mm), implying that soil nails expand substantially in diametrical direction. This is also a result of the pressure grouting effect, which increases the bond strength of the soil in the vicinity of soil nail. Consequently, the shear failure surface surrounding the soil nail which generally occurs at a location with weakest bond strength shifts outwards away from the original drillhole surface, leading to comparatively larger soil nail diameters after pulled out tests.


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Fig. 8 Comparisons between drillhole diameters and average soil nail diameters in laboratory and field pulled out tests

5 Summary and conclusions 1) Most curves of field pullout resistance against soil nail head displacement are softening. Field test results also indicate the maximum shear stress of soil-nail interface increases with the increase of grouting pressure. 2) All laboratory and field pullout test results show that apparent coefficient of friction (ACF) related to overburden pressure uo decreases with the increase of overburden soil pressure at the same magnitude of grouting pressure. 3) Apparent coefficient of friction (ACF) uG increases linearly with the increase of grouting pressure, as the pressurized cement grout infiltrates into soil voids, increasing both strength and roughness of soil-nail interface, and further leads to high bond strength of the interface between soil nail and soil. 4) Values of soil water content after pullout tests are smaller than the original soil water content in dirllholes in both laboratory and field tests. But the field test results have more uncertainties than laboratory tests, which only show slightly reductions of all water content values after pullout test. The water content reduction is possibly attributed to the water absorption process in hardening effect of cement grout in drillholes. 5) Pressurized cement grout enhance the soil strength in the vicinity of soil nail by significant infiltration and compaction effect. Therefore, the actual failure surface will shift deeper into shrouding soil, resulting in larger soil nail diameters after pullout tests in both laboratory and field tests. 6) The present investigation involves a limited number of field pullout tests. More field pullout tests

considering various grouting pressures are recommended in comparative study.

Acknowledgements The authors wish to thank Mr. Albert Ir N L HO, Mr. RAYMOND W T CHEUNG and Mr. C K TSE for their kind support in carrying out the field pullout tests. Financial supports of a grant from STU Scientific Research Foundation for Talents (SRFT) (Project No: NTF12015) is acknowledged. Financial support from Research Grants Committee (RGC) General Research Fund (GRF) (Grant No: PolyU 5320/07E) of the Hong Kong Special Administrative Region Government of China, The Hong Kong Polytechnic University and the support-in-kind from Ove Arup & Partners Hong Kong Limited are gratefully acknowledged. Notation τ shear stress of soil-nail interface P pullout force of soil nail head D soil nail diameter L soil nail length τmax maximum shear stress of soil-nail interface  v effective overburden pressure uo apparent coefficient of friction (ACF) related to overburden soil pressure uG apparent coefficient of friction (ACF) related to grouting pressure δw water content ratio Pos overburden soil pressure PG grouting pressure worignal original water content value of soil sample in drillhole wsurface water content value of soil sample at soil-nail surface after pullout test


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References [1]

[2]

[3] [4]

[5]

[6]

[7]

[8] [9]

[10]

LEE C F, LAW K T, THAM L G, YUE Z Q, JUNAIDEEN S M. Design of a large soil box for studying soil-nail interaction in loose fill [J]. Soft Soil Engineering, 2001: 413−418. 卷期号 LUO S Q, TAN S A, YONG K Y. Pull-out resistance mechanism of a soil nail reinforcement in dilative soils [J]. Soils and Foundations, 2000, 40(1): 47−56. MILLIGAN G W E, TEI K. The pull-out resistance of model soil nails [J]. Journal of Soils and Foundations, 1998, 38(2): 179−190. PRADHAN B. Study of pullout behaviour of soil nails in completely decomposed granite fill [D]. Hong Kong, China: The University of Hong Kong, 2003. PRADHAN B, THAM L, YUE Z, JUNAIDEEN S, LEE C. Soil-nail pullout interaction in loose fill materials [J]. International Journal of Geomechanics, 2006, 6: 238. 页码范围？ SU L J, CHAN T C F, YIN J H, SHIU Y K, CHIU S L. Influence of overburden pressure on soil-nail pullout resistance in a compacted fill [J]. Journal of Geotechnical and Geoenvironmental Engineering, 2008, 134(9): 1339−1347. YIN J H, ZHOU W H. Influence of grouting pressure and overburden soili pressure on the interface resistance of a soil nail [J]. Journal of Geotechnical and Geoenvironmental Engineering, 2009, 135(9): 1198−1208. WEATHERBY D E. Tiebacks [R]. Washington D.C. Report No. FHWA-RD-82-047, Federal Highway Administration, 1982. CHU L M, YIN J H. Comparison of interface shear strength of soil nails measured by both direct shear box tests and pullout tests [J]. Journal of Geotechnical and Geoenvironmental Engineering, 2005, 131(9): 1097−1107. YIN J H, SU L J, CHEUNG R W M, SHIU Y K, TANG C. The influence of grouting pressure on the pullout resistance of soil nails in compacted completely decomposed granite fill [J]. Geotechnique, 2009, 59(2): 103−113.

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

ZHANG L L, ZHANG L M, TANG W H. Uncertainties of field pullout resistance of soil nails [J]. Journal of Geotechnical and Geoenvironmental Engineering, 2009, 135(7): 966−972. ZHOU W H. Experimental and theoretical study on pullout resistance of grouted soil nails [D]. Hong Kong, China: Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, 2008. SHIELDS D R, SCHNABEL H, WEATHERBY D E. Load-Transfer in pressure injected anchors [J]. Journal of the Geotechnical Engineering Division, ASCE, 1978, 104(9): 1183−1196. ELIAS V, JURAN I. Soil nailing for stabilization of highway slopes and excavations. Publication FHWA-RD-89-198 [R]. Washington D.C. Federal Highway Administration, 1991. YEUNG A T, CHENG Y M, THAM L G, AU A S K, SO S T C, CHOI Y K. Field evaluation of a glass-fiber soil reinforcement system [J]. Journal of Performance of Constructed Facilities, 2007, 21(1): 26−34. HONG C Y. Study on pullout resistance of cement grouted soil nails [D]. Hong Kong, China: The Hong Kong Polytechnic University, 2011. SCHLOSSER F. Behaviour and design of soil nailing. International Symposium on Recent Development in Ground Improvement Techniques, Bangkok, 1982: 399−413. CHU L M. Study on the interface shear strength of soil nailing in completely decomposed granite (CDG) soil [D]. Hong Kong, China: The Hong Kong Polytechnic University, 2003. SCHLOSSER F, ELIAS V. Friction in reinforced earth [C]// Proceedings of the ASCE Symposium on Earth Reinforcement, ASCE, Pittsburgh, Pennsylvania, USA, 1978: 735−763. SU L J, CHAN T C F, SHIU Y K, CHEUNG T, YIN J H. Influence of degree of saturation on soil nail pull-out resistance in compacted completely decomposed granite fill [J]. Canadian Geotechnical Journal, 2007, 44(11): 1314−1328. (Edited by HE Yun-bin)

加参考文献：[21]. CHAI X J. HAYASHI S. Effect of constrained dilatancy on pull-out resistance of nails in sandy clay. Ground Improvement, 2005, 9(3): 127-135.