Lateral Load Test on Composite Piles

Jianwei Yue*, Zhenfeng Wang, Xiaotong Liu，Xu Chen Professor, Dept. of Civil. Engineering Henan University, Kaifeng, 475001, China *Corresponding Author, e-mail: [email protected]

Guangrong Ling Professor, Design ＆Research Institute of Tianjin University Tianjin 300072，China

ABSTRACT The purpose of this investigation is to perform full-scale pile load tests on new type composite piles formed by inserting a precast concrete pile into a cement soil mixed pile to evaluate the lateral bearing capacity and failure mode of the composite pile in the soft soil and the effect of pile cap of the composite pile. The goals of this investigation are accomplished by performing field tests on full-size composite piles with or without pile cap under cyclic load. The H-TYO curves, H-ΔY/ΔH curves and failure mode of piles are obtained. Critical and limit loads, critical load method and turning point method for lateral bearing capacity of a composite pile are analyzed according to test results. Test results show the lateral bearing capacity of a composite pile with pile cap is higher than that of a composite pile without pile cap and that the precast concrete pile plays an important role in resisting the lateral load.

KEYWORDS:

Lateral load; composite pile; recast concrete pile; pile cap; soft soil.

INTRODUCTION The characteristics of soft clay are low shear strength, high compressibility and low permeability. As construction over such soft clay is known to cause large settlement (Mesri and Choi, 1985), structural failure may be caused, if suitable measures are not taken to control the settlement and improve the bearing capacity of the weak soil foundation. In fact, many methods have been adopted to treat soft clay ground, such as partial or whole ground replacement, preloading, injection grout, deep-cement-mixing, sand column, timber or concrete piles, cast-insitu piles, etc. These improvement methods all have their advantages and disadvantages (Lin and Wrong, 1999). Deep-cement-mixing, sand column and other ordinary earth reinforcement methods, which cannot provide big subgrade bearing capacity, are not suitable for medium or high constructions. However, concrete or cast-in-situ pile is uneconomical as a friction pile, for the strength of pile materials isn’t utilized when the low capacity ground fails (Ping, D. RAN, Q. and Zheng, Z.C, 2004).

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In fact, many methods for reinforced subgrade, such as pin pile (Xanthakos etc, 1994) and SMW (Soil Mixing Wall) (Wang etc, 1998) are used in practice. Pin pile is a 100-200 mm castin-situ pile reinforced by inserting a steel pipe or steel bar, which is used for the restoration and strengthening of historical buildings on rock, sand or gravel grounds, SMW method is inserting H-shape steel into the center of cement-mixing wall usually used as a retaining wall in deep pit excavation. This method is originally developed in Japan and broadly used in pit excavation support system in China to control lateral deformations of pits. Tests and practice have proved its feasibility for excavation support in soft subgrade. However, pin piles are only suitable for sand grounds, and SMW method is only used as excavation supports, which are unsuitable to bear vertical loads in soft clay ground. Therefore, based on the pin pile and SMW, a new type composite pile has been invented to support high designed loads in soft ground and reduce construction cost. The new type composite pile consists of an external cement-mixing pile socket and an inner precast reinforced concrete pile (Fig.1) that is inserted right after the completion of the cement-mixing pile. The new type composite pile transfers load down and outward to the surrounding soil by inner concrete pile and big surface of the external cement-mixing pile, and bears higher working load than other methods of reinforced subgrade (Ling, G.R, 2001). However, because the composite pile have not yet gained wide acceptance in practice, no information regarding its lateral-load resistance behavior is reported. This paper presents the details of lateral load tests on the piles and gives an elementary analysis of lateral bearing capacity of the new type composite pile.

PILE CHARACTERISTICS AND CONSTRUCTION A simple illustration of the composite pile is shown in Fig.1. Precast reinforced concrete pile called inner pile is designed to resist pile’s compression and transfer the vertical load, because of its high strength and elastic modulus of concrete. According to test results (Ling, G.R, 2001) and FEM analysis, the length of inner pile is almost 80% that of external cement mixing pile which is also called external cement soil socket. The design can obtain better load-transfer capacity and reduce material. External socket is designed to transfer the vertical load to surrounding soil through skin frictions. Actually, cement-mixing pile is very effective to improve ground bearing capacity in silt, clay and peat with medium water content, and also cheaper than other type piles in China. However, the main deficiency of the cement-mixing piles is the low cement-soil strength and low bearing capacity, because their side friction can’t fully operate and the top cement-soil is crushed. Now, this deficiency can be avoided by inserting pracast reinforced concrete pile into the mixing pile. The cement-soil strength of external socket not only has important influence on load distribution ratio of inner pile to external socket, but also controls the skin friction strength and distribution of skin friction. Typically, cement content is 15-20%. The laboratory tests have found that unconfined compression strength of soil-cement increases with curing time and cement content, so the concrete pile is inserted into the socket as quickly as possible. To compare the side friction of a cement-mixing pile with that of a concrete pile, the distribution of the side friction is shown in Fig.2 (Ling,G.R,2001). Fig.2 shows that the side friction of a cement-mixing pile is very big compared with that of a concrete pile and that the inner pile of a composite pile changes the side friction of a cement-mixing pile and makes full use of the friction. Therefore, the characteristic of vertical bearing capacity of a composite pile is super.

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Precast concrete

Cement-mixing

Soil ground (a) Actual illustration of a composite pile

Precast concrete

Cement-mixing il

Soil ground

(b) Test illustration of a composite pile Figure 1: Illustration of a composite pile To master the lateral bearing capacity of the composite pile, in-situ tests of three full-scale piles are carried out. The detailed parameters of the test piles are shown in Tab. 1 and Fig.3. No.1 and No.2 piles have pile cap and No.3 pile has no pile cap to study pile-cap influence on lateral bearing capacity. The cement soil is mixed three times up and down. When the mixing pile is completed, precast concrete pile is inserted at once. If the inner pile isn’t inserted into the cementmixing pile within half an hour, the trouble will occur when the inner pile is inserted. The reason is that unconfined compression strength of soil-cement increases and that the side friction of the inner pile is enhanced with the rebound of soil around the mixing pile. A vibratory hammer assists the insertion of an inner pile. The installation equipment is shown in Fig.4. Fig. 5 and Fig.

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6 show only a little soil is extruded in the process of inserting of the inner pile. Thus, the new type composite pile has little environment pollution in actual application.

qs Cement-mixing pile Composite pile

Concrete pile

Z Figure 2: The side friction of three kinds of piles Table 1: Parameters of tested piles External Pile No . Diameter Length /m /m 0.6 0.6 0.6

14.0 14.0 14.0

pile

Design Water Mixing cement cement number content/% ratio 17.1 17.1 17.6

0.75 0.85 0.85

6 6 6

Inner concrete pile Diameter Concrete of Pile Square Steel Length compression cap section Strength /m strength /m /m /MPa /MPa 0.27 13.5 40.2 475 0.6 0.27 13.5 40.2 475 0.6 0.27 13.5 40.2 475 -

270 mm

1 2 3

mixing

270 mm Figure 3: Transverse section of an inner pile

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Inner concrete pile

Figure 4: Pile-driving machine

Figure 5: Site before inserting inner pile

Figure 6: Site after inserting inner pile

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SUBSURFACE CONDITIONS The test piles are carried out in the site of Tianjin University. The subsurface conditions in the vicinity of the test piles are based on the results of nearby borings. A summary of the geotechnical parameters of the subsurface materials is provided in Table 2. Table 2: Geotechnical Properties of Subsurface Materials Layer Fill Clay Power clay Power clay Power clay Power soil

Thickness (m) 0.90 2.20 1.40 4.00 5.90 1.1

Moisture content(%)

Void ratio

31.8-33.7 30.3 30.6 23.3-28.7 26.6

0.91-0.94 0.84 0.86 0.65-0.83 0.73

Compression Liquidity module (MPa) index 4.48 4.29 4.28 6.54 4.27

Plasticity index

0.52-0.66 0.97 0.81 1.00-1.35 0.77

19.0 11.5 14.0 10.4 11.0

LOAD TEST SETUP AND PROCEDURES The lateral load test adopted load-unload method of many circulations. Each grade load was 1/10—1/15 limit lateral bearing capacity. Each load sustained four minute and then horizontal deflection was read from dial indicator. Load was then unloaded and residual deflection was read in two minute. Five loops were carried out within a load grade. To assure the direction of acting force, hydraulic jack with ball socket applied lateral load applied at the ±0.000 position. The lateral load was measured by a load cell. Dial indicator measured the lateral deflection of piles. Maximum range of dial indicators is 50 mm. Four dial indicators were respectively located on pile’s two sides (left and right) in level of force-action line and level 50 mm above force-action line.

RESULTS AND ANALYSIS Load test results The lateral load and lateral deflection (H-T-YO curve) at the top of piles are shown in Fig.7. The failure mode at the top of piles is shown in Fig.8-11. Tests of three piles ended when lateral deflections at the top of piles exceed 40 mm. The test results showed that the No.1 and No.2 piles with pile cap have the same failure mode that soil around the piles is crushed and pile’s body isn’t broken, that their inner piles don’t separate from external cement-soil socket, and that external cement-soil socket also isn’t crushed. However, the inner pile in No.3 pile without pile cap separate from external cement-soil socket. According to the observation from failure mode of piles and the results from deflection, the pile cap, which make the inner pile and external cement-soil socket cooperate with each other, should be designed in the actual application to get higher lateral and vertical bearing capacity, especially in seismic zone.

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(a) The H-T T-YO curve off No.1 pile

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(b) Thee H-T-YO cuurve of No.2 ppile

(b) The H-T-YO O curve of Noo.3 pile Figurre 7: The H-T T-YO curves of piles

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Composite pile

Figure 8: Failure mode of No.1 pile

Composite pile

Figure 9: Unloading failure shape of No.2 il

Figure 10: Failure shape of No.3 pile

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Precast concrete pile Cement mixed pile Soil

Figure 11: Unloading failure shape of No.3 pile

Pile’s analysis under lateral load The analysis under lateral load was performed prior to the load test for the purpose of determining the maximum allowable lateral load and estimating the lateral deflection of the piles under lateral load. The analysis is also performed after the test to evaluate differences between the experimental results and the results of FEM analysis with ANSYS7.0. The FEM analysis can help us to detect weakness of test design and to master deflection and stress characteristic of piles (Mazen E. A.2001 ). The pre-load test analyses were performed by varying the lateral loads at the point of force application until the maximum deflection was estimated. The inner pile and external soil-cement socket were considered as an integral pile in resisting the lateral force. The composite pile parameters used in the analyses were as follows: Concrete Young’s modulus (Ec) is 3.0x104 MPa; soil Young’s modulus (Es) is 10 MPa; soil Poisson’s ratio is 0.35; soil cohesive strength is 15kPa; The internal friction angle of soil is 22 degrees; Cement soil Young’s modulus (Ecs) is 80 MPa and cohesive strength is 150 kPa, The internal friction angle of cement soil is 33 degrees. The pile’s calculations also indicated that the piles should be fixed at a depth of approximately 12.0m in the soil. During the lateral load test, the pile deflections were observed to be greater than the pile deflections estimated from the pre-load test analyses. As shown in Table 3, the analysis results indicate much lower deflections than the measured values. Table 3: Calculated and measured results Lateral load /kN 60 80

Calculated deflection /mm #1,#2,#3 2.42 3.58

Measured deflection /mm #1 #2 #3 7.69 5.29 8.79 12.88 9.82 19.84

Note: all pile deflections are from the point of loading at el. ±0.00.

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Analysis after test is performed to evaluate the probable reasons for the higher measured deflections as compared to the calculated deflections. The elastic modulus of the composite pile has an important effect on the lateral deflection of piles. Adopting small elastic modulus of cement soil in FEM analysis, calculated deflection is similar to the measured. Considering this, it is believed that the uncertainty on the elastic modulus of a composite pile accounts for the difference between the measured and estimated pile deflections. In addition, analysis is also performed by reducing the soil strength to simulate the effects of the soil’s disturbance caused by the mixing soil. The results from the analyses using small soil strengths indicate that soil strength also influences the deflection of the composite pile. Therefore, the elastic modulus of the composite pile and soil strength are important parameters to control and calculate the deflection of the composite pile.

SIMPLE SOLUTION FOR LATERAL BEARING CAPACITY OF A COMPOSITE PILE Decision rule for critical and limit loads According to Technical Code for Building Pile Foundations (JGJ 94-94 China ),one rule for critical value is that the H-t-Y0 curve appear obvious displacement increase, the former load of which is critical load. The other rule is that the load of end-point of the first line in

H 0 − Δy 0 / ΔH 0 curve is critical load. Similarly, one rule for limit value is that the H-t-Y0 curve appears steep increase of displacement, the former load of which is limit load. The other rule is that the load of end-point of the second line in

H 0 − Δy 0 / ΔH 0 curve is limit load. Based on the

H − Δy / ΔH

0 0 curves are drawn and shown in Figure 12. According to the critical and test, 0 limit rules, the critical and limit loads of piles are given in Table 4.

Table 4: Critical and limit loads of piles Pile No.

H cr /k N

y cr /mm

H u /k N

yu /mm

1 2 3

80.0 69.0 62.5

12.88 6.13 9.89

110.0 118.0 87.0

23.44 27.26 21.97

H /H

H u / H cr 1.375 1.710 1.392

y u / y cr 1.82 4.45 2.22

cr are discrete. The main reason is that the strength of Table 4 shows that the values of u cement soil is also discrete and the position of an inner pile in a composite pile is different, all which affect the bearing capacity of a pile.

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(a) H-ΔY/Δ ΔH curve of No.1 N pile

(b) H H-ΔY/ΔH currve of No.2 pile

(c) ( H-ΔY/ΔH curve of N o.3 pile

Figure F 12: The H-ΔY//ΔH curve oof piles

Design D va alue for lateral be earing ca apacity Based d on the forrmula of the cast-in-placee pile, consiidering the ddamage characteristic of a composite pile, thee design valuee for lateral beearing capaciity of a compoosite pile can be written ass

Rh =

α=5

mb0 EI

α 3 EI y0a νx

(1) (2)

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( m=

5 H cr νx) 3 ycr

b0 ( EI )

2

(3)

3

(4)

b0 = 0.9(1.5 D + 0.5)

where Rh is the design value for lateral bearing capacity of a composite pile . α is the lateral deformation coefficient of a pile. EI is the flexural rigidity of a pile consisting of external socket and inner pile. y0 a is the permissible lateral displacement at the top of a pile . ν x is the deformation coefficient at the top of a pile. m is the proportional factor of lateral resistance on foundation soil. b0 is the computational width of a composite pile. H cr is the critical load of a pile. ycr is the corresponding displacement of lateral critical load of a pile. D is the diameter of a pile. Based on the formulas, the design values for lateral bearing capacity are given in Tab.5. Here, y0 a is 10.0 mm. According to the test results, we can draw the m − Δy / ΔH curves (Figure 13) existing an obvious turning point. The m value is an asymptote of exponential series above the turning point and is a relatively stable value under the point. Therefore, the m value is more reasonable and more convenient in engineering application than critical load method. The design values for lateral bearing capacity based on turning point is listed in Tab.5.

Δy / ΔH / mm /k N

0.0 0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

20

m /M N/m 4

40 60 80

1# 2# 3#

100 120 140 Figure 13: The m- Δy / ΔH curve

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Table 5: The results from the two methods Critical load method Pile No. 1 2 3

m / MN / m 11.87 31.98 12.21

4

Rh /kN

Turning point method Rh /kN m / MN / m4

62.3 112.9 63.4

14.0 17.0 13.0

69.8 77.3 65.8

CONCLUSIONS Here we introduced a new type composite pile, its lateral bearing capacity and failure characteristics under lateral load. Test and primary analysis proved its effectiveness on soft ground. The following conclusions are obtained: As a kind of new type composite pile, the behavior of the pile has many difference compared with a pile of single material, especially lateral bearing capacity. The cement-soil strength of external socket and the position of inner pile influence on the lateral bearing capacity. The external socket of cement soil and inner pile resist the lateral force. The failure mode of composite piles with pile cap is different with that of a composite pile without pile cap. Composite piles with pile cap didn’t occur the damage between the external socket and inner pile compared to the composite pile without pile cap. Therefore, the pile cap should be designed in actual application. The inner pile didn’t take place failure. Post-load analysis show that the deflection of a composite pile is affected by many factors, such as strength of external cement-soil, the position of inner pile, soil strength around the pile, etc. The formulas of design value for bearing capacity of a composite pile are given by two methods: critical load method and turning point method.

ACKNOWLEDGEMENTS This paper is supported by scientific Committee of Henaan Province, China (496260015). Help received from Dr. Li Jinjun and Dr. Liu Bopeng during testing phase is also acknowledged.

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