Numerical Modeling of Pile Load Test

4th China International Piling and Deep Foundations Summit (26-28March 2014), Shanghai, China

Numerical Modeling of Pile Load Test Naveen, B.P Research Scholar, Indian Institute of Science, Bangalore 560012, India. Email: bpnaveen@ civil.iisc.ernet.in

Parthasarathy, C.R Director, Sarathy Geotech & Engineering Services Pvt Ltd, Bangalore. Email: [email protected]

Sitharam, T.G Professor in Civil Engineering and Chairman, Centre for infrastructure and Sustainable transportation & urban planning, Indian Institute of Science, Bangalore 560012. Email: [email protected]

Keywords: Pile foundations, High Strain Dynamic load testing, Static pile load test ABSTRACT: Pile foundations are intended to transmit heavy loads into the ground and are gaining popularity among the preferred foundation system. Variety of construction methods is available for the installation of piles. Pile load tests are generally performed to estimate the load carrying capacity of piles and also to verify the values determined by theoretical methods in designs. Static pile load tests being most commonly used and routinely performed test to measure the load carrying capacity of piles. The static load test involves the direct measurement of pile head displacement and the test provides a direct measurement of pile capacity. High Strain Dynamic load testing using PDA (Pile Driving Analyzer) is becoming popular for the assessment of pile bearing capacity. It is applicable to drilled shafts, cast-in-situ and driven piles. Two sets of strain gauges and accelerometers are attached to pile shaft. The pile is impacted with a driving hammer or with a drop weight. Gauge signals are sent to PDA where they are conditioned, digitized, analyzed and displayed. In the field test, obtained force and velocity signals are post-processed using CAPWAP software. The output of the analyses results is presented as an equivalent static load-settlement curve. In this paper, by comparison of numerical model result with the equivalent static load settlement curve, it has been observed that the result of the dynamic pile load test compares reasonably well with the static load test results. Dynamic testing allows contractor and other agencies to reliably test quickly and cost effectively all the types of piles. Dynamic load testing using PDA is an efficient and reliable alternative to static load test. This paper presents the numerical simulation of load carrying capacity of piles in residual soils. PLAXIS 2D software is used in this study. The simulation is carried out for single pile with vertical load at pile top, so as to evaluate the settlement of the pile in residual soils. The numerical results have been presented with equivalent static load-settlement curve generated by CAPWAP analysis. 1 INTRODUCTION A pile is intended to transmit a structural load into the ground without risk of shear failure or excessive settlement. Generally there are two basic types of piles viz., end bearing piles and friction piles. Almost all piles receive support from both end bearing and shaft resistance. Static load test on a pile is one of the methods for determining the load-carrying capacity of a pile. It can be conducted on a driven pile or cast-in-situ pile on a working pile or a test pile, and on a single pile or a group of piles (Rausche, 1991). Dynamic load testing using PDA is a quick method to analyze the bearing capacity of pile. It can be used

for prefabricated piles, cast-in-place concrete piles, steel piles and wooden piles. Dynamic pile load test takes less time to carry out than the static pile load tests and also costs a fraction of the cost of conducting the static pile load cost. This paper presents the numerical simulation of load carrying capacity of piles in residual soils. The numerical results are compared with the equivalent static load-settlement curve generated by CAPWAP analysis. For the numerical modeling PLAXIS 2D software is used. The modeling is carried out for single pile with vertical load at the pile top so as to evaluate the settlement of pile in residual soils.

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4th China International Piling and Deep Foundations Summit (26-28March 2014), Shanghai, China 2 DYNAMIC PILE LOAD TEST Dynamic load testing is a quick method to analyze the bearing capacity of pile for vertical loads. It can be used for cast-in-situ place concrete piles also. In Dynamic pile load test, many sensors (two strain gauges and accelerometers) are connected to the pile near the pile head. These sensors can be able to measure strain and acceleration on concrete pile. These sensors are connected to the concrete pile with anchor bolts. These sensors are generally recovered after the testing. Once, the sensors have been connected to the pile dynamic analyzer (PDA) monitoring system, this system becomes automated and it can be used to direct the test controls. A drop-weight weighing 5.0 ton (approximately) was used to give impacts on the pile top with a height of fall of about 0.5 to 2.0m as shown in Fig 1. The generated compression wave travels down the pile and reflects from the pile toe upward. The reflected wave mainly consists of information about the shaft, toe resistance and pile defects. The measured signals are processed automatically and stored by the PDA monitoring system (Rausche et.al 1991).

Fig. 1. Dynamic pile load testing

This data can be retrieved easily for further simulation by using Case Pile Wave Analysis Program (CAPWAP) software. CAPWAP is a signal matching software, it can separate static and damping soil characteristics and also allows for an estimation of the side shear distribution and the piles end bearing. CAPWAP is based on the wave equation model, which analyses the pile as a series of elastic segments and the soil as a series of elasto-plastic elements with damping characteristics, where the stiffness represents the static soil resistance and the damping represents the dynamic soil resistance. Typically the pile top force and velocity measurements acquired under high strain hammer impacts, which can be analyzed utilizing the signal matching procedure yielding forces and velocities over time and along the pile length (Linkins and Rausche, 2004). Fig. 2. The Pile Dynamic Analyzer (PDA) monitor

This CAPWAP analysis generates the equivalent static load-settlement curve. CAPWAP results are used to compare with the static pile load test results estimated from the numerical model. Fig 1 shows the arrangement for dynamic pile load testing. Fig 2 shows the pile dynamic analyzer monitor used in our field test. The cast-in situ concrete pile considered for dynamic test using PDA in this study is of diameter (D) 0.5m, length 11m and made of M30 grade concrete. The site soil is composed of top clay and residual weathered rock up to 15m. Table.1 lists the parameters of the soil stratums at the test site. The same parameters have been used for the numerical simulations.

Dynamic pile load test has been conducted as explained above. The sensors results have been fed to CAPWAP software along with soil and pile properties. Utilizing the signal matching procedure the forces and velocities over time and along the pile length has been estimated. Fig 3 shows the load-settlement curve obtained from CAPWAP software as the equivalent static load-settlement curve on 0.5m diameter pile. The dynamic load applied on the pile top is about 50 kN, however the estimated pile loads have been up to about 1200 kN.

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4th China International Piling and Deep Foundations Summit (26-28March 2014), Shanghai, China Load (kN) 0

200

400

600

800

1000

1200

0

2

Settlement (mm)

CAPWAP 4

6

8

10

12

Fig. 3. Equivalent static Load –Settlement curve from dynamic pile load test

3 NUMERICAL MODELING OF THE PILE An attempt to study the load-settlement behaviour of a pile under vertical load applied at the pile head is modeled in this paper. The finite element package PLAXIS 2D, Version 9 is used for this purpose. The pile is 0.5m in diameter and 11m in length. The incremental loads have been applied in the model and the settlements are evaluated for the simulated pile. Under static loading conditions, the complete load-settlement curve has been generated. The PLAXIS 2D results are compared with the equivalent static load –settlement curve obtained using dynamic pile testing using PDA. The PLAXIS 2D model consists of 2 layers of soil: clay (upto 6m), and soft-weathered rock(6m to 15m). Table 1. shows the physical and mechanical properties of the soil layers and also the pile and the same has been adopted in the PLAXIS 2D model. Table 1: Soil Parameters Linear Elastic

Pile concrete

Soil

Type

-

Drained

Soft weathered rock Drained

γunsa

[kN/m³]

16.00

18.00

24.00

γsat

[kN/m³]

16.00

18.00

24.00

Eref

[kN/m²]

30000000.0

ν

[-]

C

[kN/m²]

Ǿ

3000.0

50000.0

0.200

0.300

0.300

-

21.00

42.00

-

30.00

40.00

Model parameters for PLAXIS Model Vertical pile in soil has been modelled as an axi-symmetric problem. In PLAXIS 2D 15 noded

triangular element has been chosen to model the soil, which results in a two-dimensional finite element model with two translational degrees of freedom per node. The 15-noded triangle provides a fourth order interpolation for displacements and the numerical integration involves twelve Gauss points. Fig 4 shows geometry of the problem along with pile in an axi-symmetric problem. The pile is made up of reinforced cement concrete and it is also modelled using triangular elements. The behaviour is assumed to be linear-elastic. The soil behaviour as described by Mohr-Coulomb for top clay soil and hardening soil models for the bottom weathered rock are selected for preliminary analyses. After conducting multiple trials and sensitivity analyses, it is concluded that the Mohr-Coulomb model is suitable model for both the layers of soil. Kanimozhi and Ilamparuthi (2004) have also indicated that Mohr-Coulomb is an ideal model for both soils and rock. The Mohr-Coulomb model can be considered as a first order approximation of real soil behaviour. The elasto-plastic model adopted requires 5 basic input parameters, namely Young’s Modulus, E, Poisson’s ratio, ν, cohesion, c, friction angle, Ǿ and dilatancy angle, Ψ. Boundary conditions: The bottom boundary is rigid, i.e., both horizontal (u) and vertical displacement (v) are zero. Standard fixities are used at the left and right boundaries of the model. These side boundaries act like rollers such that u=0 but v≠0. Interface element: The soil-structure interaction is modelled using an elastic-plastic model to describe the behaviour of interfaces. For the interface to remain elastic, the shear stress, |τ| < σn tanΦi + ci, where Φi and ci are the friction angle and cohesion (adhesion) of the interface respectively and σn, the effective normal stress. The interface element properties are linked to the strength properties of the soil layers. The main interface parameter is the strength reduction factor Rinter. A strength reduction factor of 1 is used in this analysis. Fig 5 shows the generated mesh along with interface element between pile and the soil. A global coarseness parameter as well as a local parameter is used while generating the mesh for the problem considered. The average element size and the number of generated triangular elements depend on the global coarseness setting. The global coarseness setting of medium with 320 triangular elements was found to be most suitable. 158

4th China International Piling and Deep Foundations Summit (26-28March 2014), Shanghai, China Situations where some areas have large stress concentrations or large deformation gradients require the use of the local coarseness parameter. This gives the element size relative to the average element size as determined by the global coarseness setting. Local coarseness parameters are generated automatically by PLAXIS 2D. A geometry dimension of the model with 5m x 15m was found to be optimum.

soil weight so that the state of stress is slightly altered. This approach is generally used to model the installation process of bored cast in-situ pile (Yang and Jeremi, 2000; Wehnert and Vermeer,2004). In phase 3, the loading were set to zero and applied in increments. The observed settlement for the corresponding loads was noted. The stage wise loading is applied by load increments of 200,400,600,800,1000,1200kN at the pile head. Figure 6 shows the estimated load settlement curve from PLAXIS 2D model.

Load (kN) 0

200

400

600

800

1000

1200

0

2

Settlement (mm)

PLAXIS 2D 4

6

8

10

Fig. 6. Load –Settlement curve from PLAXIS 2D model. Fig. 4. Model with applied Vertical Load Fig 7 shows the deformed mesh of the pile model at the end of the numerical simulation.

Fig 5. Shows the generated mesh

Each calculation is divided into 9 phases, in the first phase the initial soil stress is generated. Then the pile takes its position in the identified mesh. However, the concrete properties are adopted by Changing the parameter sets in the elements that represents the pile. The pile weight is larger than the

Fig.7. Deformed mesh

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4th China International Piling and Deep Foundations Summit (26-28March 2014), Shanghai, China COMPARISON OF EQUIVALENT STATIC LOAD –SETTLEMENT CURVE BY CAPWAP AND STATIC LOAD –SETTLEMENT CURVE OBTAINED BY PLAXIS2D MODEL

The results of the PLAXIS 2D model of the pile are now compared with the results of the dynamic pile load test Fig 7 shows the result of dynamic pile load test(equivalent load settlement curve estimated by CAPWAP) along with static load settlement curve estimated from the PLAXIS 2D model. The results compare reasonably well. It is also noticed that the field curve has shown a different shape starting from 5.5mm, which may be attributed to some problems in the field test. Table 2. show the comparison of vertical settlement data corresponding to the applied ultimate load of 1200 kN.

distribution with a higher level of confidence and estimate the equivalent load –settlement data for evaluating static capacity of the pile. After high strain dynamic data was collected in the field, it was processed utilizing the signal matching software CAPWAP (Case Pile Wave Analysis Program). The dynamic pile load test was performed in a site where in bored cast insitu pile has been installed in residual soil. Static load and displacement characteristics at the pile top predicted by CAPWAP are compared to numerically simulated static pile load test results with two layered soil media consisting of top clay and residual weathered rock up to about 15m. The numerical model of pile –soil system was analyzed using PLAXIS 2D model. The numerically simulated vertical load tests were analyzed using an axi-symmetric finite element model. Following are the some of the conclusions from the paper:

Load (kN) 0

200

400

600

800

1000



1200

0

Settlement (mm)

2

PLAXIS 2D CAPWAP

4



6

8

10

12

Fig. 7. Load –Settlement curve from dynamic pile test(CAPWAP) and numerical model (PLAXIS 2D)

 

Table 2. Comparison of vertical settlement data corresponding to applied ultimate load of 1200 kN Methods CAPWAP

Vertical Settlement (mm) 10.00

PLAXIS 2D analysis

9.41



 CONCLUSIONS



A new system of dynamic strain sensors and accelerometers embedded on the test pile provides direct synchronous measurement of force and acceleration at various locations within a pile during high strain dynamic pile testing. This instrumentation system creates an opportunity to explore side and end bearing pile resistance 160

A dynamic testing procedure utilizing standard PDA top transducers and accelerometer plus the Case Pile Wave Analysis Program (CAPWAP) was devised to calculate with an increased degree of confidence the equivalent load-displacement displacement plot for a single pile embedded in residual soil. CAPWAP allows for a computation of a simulated load set curve utilizing the static components of the calculated total resistance. Thus, after signal and dynamic sensor matching was achieved a simulated load-set curve was generated and compared to the results of a static load test simulated using PLAXIS 2D model. Dynamic test is an efficient and reliable alternative to field static pile load test. It can be concluded that there is good comparison of load–displacement relation obtained through CAPWAP and that obtained by numerical model using PLAXIS 2D. From above study it can also be concluded that the Mohr-Coulomb (MC) model with a medium mesh is optimum to simulate the settlement of a vertical loaded pile. PLAXIS 2D simulation results are close to dynamic pile load test results. Additional comparison with mode field dynamic tests and modeling attempts are needed to further characterize the potential benefit of using dynamic tests to evaluate the static capacity of the pile.

4th China International Piling and Deep Foundations Summit (26-28March 2014), Shanghai, China REFERENCES Kanimozhi, T. & Ilamparuthi, K. (2004). Performance of Pile under Lateral Load. Indian Geotechnical Conference, Dec, Bangalore, pp 102-105. Likins, G. E., & Rausche, F., August, (2004). Correlation of CAPWAP with Static Load Tests. Proceedings of the Seventh International Conference on the Application of Stress wave Theory to Piles 2004: Petaling Jaya, Selangor, Malaysia; 153-165. Likins, G. E., Rausche, F.,& Thendean, G., Svinkin, M., September, (1996). CAPWAP Correlation Studies. Proceedings of the Fifth International Conference on the Application of Stress-wave Theory to Piles 1996: Orlando, FL; 447-464. Rausche, F., Likins, G. E., Liang, L., Hussein, M.H., January, (2010). Static and Dynamic Models for CAPWAP Signal Matching. The Art of Foundation Engineering Practice, Geotechnical Special Publication No. 198, Hussein, M. H., J. B. Anderson, W. M. Camp Eds, American Society of Civil Engineers: Reston, VA; 534-553. Rausche, F., August, (1991). The 1991 CAPWAP Program, Recommended CAPWAP Matching Procedure, Remedies Against Fault Capacity Predictions. PDA Users Day: Cleveland, OH; 1-15. Wehnert, M. & Vermeer, P.A. (2004). Numerical Analyses of Load Tests on Bored Piles. 9th Int. Symp. on Numerical Models in Geomechanics, August, Ottawa, Canada. Yang, Zhaohui & Jeremi, Boris (2000). Numerical Analysis of Pile Behavior under Lateral Loads in Layered Elastic-Plastic Soils. International Journal for Numerical and Analytical Methods in Geomechanics, June, University of California, USA.

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