Numerical Analysis of Single Pile in Soft Clay

Numerical Analysis of Single Pile in Soft Clay

*Neha George

Student Dept. of Civil Engineering,

Mar Athanasius College of Engineering, Kothamanaglam Ernakulam, India

Berlin Sabu

Asst. Professor Dept. of Civil engineering

Mar Athanasius College of Engineering, Kothamangalam Ernakulam , India

Abstract:- This paper aims at analyzing the load deformation characteristics of a single pile of varying length(5,10,15m) and

1. MODEL VALIDATION

   varying diameter (0.4,0.5,0.6,0.8,1m) in cohesive soil with A. Validation of soil model

   varying cohesive values (10,20,30,40 kN/m²) using PLAXIS 3D

   software. The results of the analysis has been compared based on the safe bearing capacity of the pile under different conditions. A 3D numerical model was developed to simulate the single pile in clay soil. Variation in safe bearing capacity of pile with cohesion for piles of different diameter and different length were studied. The extensive parametric study concluded that there is a linear variation between SBC and diameter of pile, SBC and cohesion of clay soil and SBC and length of pile

   KeywordsSingle pile, safe bearing capacity , cohesion, PLAXIS 3D

   1. INTRODUCTION

      Piles are deep foundation used when shallow foundations do not satisfy the settlement and bearing capacity criteria. Piles are classifies based on the material of the pile, the displacement properties and the nature of load support. The pile load displacement characteristics of single pile are affected by pile properties, soil properties, installation methods and loading conditions. Pile foundations transfer the superstructure loads to deeper part of the soil strata .Based on the nature of load support (end bearing and friction pile), the piles transfer loads either to an end bearing stratum or via the skin friction developed along the shaft of the pile. In soft soil the pile makes use of the strength of the soil by skin resistance along the shaft of the pile. Here the load transfer of piles is the sum of the toe resistance and skin resistance in soft soils [1]. Methods adopted to find the bearing capacity of piles are static formula, dynamic formula and field load tests. The static formula method relates soil shear strength to the skin friction along the pile shaft and to end-bearing below the pile point. Both the skin friction and the end bearing resistance are used to calculate the bearing capacity of piles of any dimension in cohesive and cohesion less soils [2].

      Many studies have been performed to determine the behavior of axially loaded pile in cohesive soils. In this study a comparison of the Safe bearing capacity of single pile of different length and diameter are tested in cohesion soil of different cohesion values.

      The Alzey Bridge pile load test is an axially loaded validation case, which is widely used by researchers for validating new soil models. The Alzey Bridge pile load test was carried out near Frankfurt. During the test, load cells were installed at the foot of the pile to measure the loads that are carried directly by the pile base. The test results and other parameters are presented by Engin et al.[13]. The Alzey Bridge pile load test considers an axially loaded pile with a diameter of 1.3 m and a lengthof 9.5 m. The ground water table is approximately 3.5 m below the ground surface. The pile was modeled in the Mohr -Coulomb soil model using PLAXIS 3D software. Embedded pile was used to model the pile. Point load was assigned to the pile top and the soil was meshed. Medium mesh was use for meshing. Figures 1 and 2shows the modeling of soil and pile using PLAXIS 3D.

      Fig. 1 Fig. 2

      Fig.1and 2: Model of Embedded Pile and Point Load in soil in PLAXIS 3D

      Table 1 and 2 presents the data available for modeling the pile and soil mass, taken from Engin et al.[3].

      Table 1 Soil Properties for Validation [3]

      Property	Value	Unit
      Saturated unit weight	20	/3
      Dry unit weight	20	/3
      Poissons ratio	0.2
      Cohesion	20	/2
      Internal friction angle	20	degree
      Dialatancy angle	0	degree
      Interface ratio	1

      Table 2 Pile Properties for Validation [3]

      Property	Value	Unit
      Pile diameter	1.3	m
      Pile length	9.5	m
      Modulus of elasticity	1×10	kN/m²
      Poissons ratio	0.20
      Base resistance	1320	kN
      Skin friction	201.37	kN/m

      The pile was modeled in the Mohr -Coulomb soil model using PLAXIS 3D software. Embedded pile was used to model the pile. Point load was assigned to the pile top and the soil was meshed. Medium mesh was use for meshing. Figure 3 compares the experimental load displacement behavior of piles with that obtained using PLAXIS 3D software.

      From Fig. 3, it may be noted that, except at the initial linear elastic stage, the experimental and theoretical load displacement behavior of pile are comparable. Hence, the modeling of soil and pile using PLAXIS 3D has been validated and the same approach can be followed for further study.

      Fig.3 Comparison of Validation of the Soil Mode

2. PARAMETRIC STUDY SAFE BEARING CAPACITY OF PILE

   The load deformation analysis on a single pile of varying length(5,10,15m) and varying diameter (0.4,0.5,0.6,0.8,1m)

   in cohesive soil with varying cohesive values (10,20,30,40 kN/m²) using PLAXIS 3D software was studied. The results of the analysis has been compared based on the safe bearing capacity of the pile under different conditions. A 3D numerical model was developed to simulate the single pile in clay soil.

   As the first step of the modeling process, the geometry of the single pile and the soil block were defined. The soil volume was modeled as a 10-noded tetrahedral element and has a depth twice the length of the pile in Mohr- Colomb soil model. The soft clay was modeled in the undrained condition.

   The piles are modeled as embedded beam element with special interface element. The pile-soil interaction for an embedded pile involves a skin resistance and a tip resistance.

   The input parameters that are required for modeling the soil are modulus of elasticity, poisons ratio, angle of friction, dialatancy angle and unit weight of soil[4]. The modulus of elasticity of cohesive soil has been determined as per the following relation [5]

   Modulus of elasticity =500C .. (1) Where, C is the cohesion of the soil.

   With respect to the pile, the parameters like Modulus of elasticity, poissons ratio, skin friction per unit length and end resistance are required for the analysis. The skin friction per unit length and end resistance of pile were determined as per the guidelines specified in BIS for the calculation of pile capacity in cohesive soils[6] and accordingly, following equations were used.

   Skin friction per unit length of pile =As××C . (2) End resistance of pile = Ap×Nc×Cp ..(3) Where,

   <>Ap- cross-sectional area of pile tip, in m² Nc- bearing capacity factor (taken as 9) Cp- average cohesion at pile tip, in kN/m² As- surface area of pile shaft, in m²

   – adhesion factor of soil, depending on the consistency of soil (taken as 1 up to C = 40kN/m2)

   1. average cohesion of soil, in kN/m²

      Sl. No	Pile Diamet er (m)	Cohesio n ( kN/m²)	Modulus of elasticity of soil[15] (kN/m²)	End resista nce of pile (kN)	Skin friction in pile (kN/m)
      	d	C	500×C	Ap×Nc ×C	As××C
      1	0.4	10	5000	11.3	12.56
      		20	10000	22.61	25.12
      		30	15000	33.91	37.68
      		40	20000	45.22	50.24
      2	0.5	10	5000	17.66	15.7
      		20	10000	35.33	31.4
      		30	15000	52.99	47.1
      		40	20000	70.65	62.8
      3	0.6	10	5000	25.43	18.84
      		20	10000	50.87	37.68
      		30	15000	76.3	56.52
      		40	20000	101.74	75.36

      Sl. No	Pile Diamet er (m)	Cohesio n ( kN/m²)	Modulus of elasticity of soil[15] (kN/m²)	End resista nce of pile (kN)	Skin friction in pile (kN/m)
      	d	C	500×C	Ap×Nc ×C	As××C
      1	0.4	10	5000	11.3	12.56
      		20	10000	22.61	25.12
      		30	15000	33.91	37.68
      		40	20000	45.22	50.24
      2	0.5	10	5000	17.66	15.7
      		20	10000	35.33	31.4
      		30	15000	52.99	47.1
      		40	20000	70.65	62.8
      3	0.6	10	5000	25.43	18.84
      		20	10000	50.87	37.68
      		30	15000	76.3	56.52
      		40	20000	101.74	75.36

      Table 3 gives the input parameters for pile in soft clay soil. Table 3 Input Data for the Analysis of Piles in Clayey Soil

      Figure 4 is an example of load displacement graph for pile of length 10m and diameter 0.8m

      Fig.4 Load vs. Displacement Graph for Pile of Length 10m and Diameter 0.8m

      The load capacity of pile for a specified displacement is higher for soil with higher value of cohesion. Similarly, as the diameter of pile is more, the load capacity of pile for a specified displacement is higher. These behaviors are expected, as the end bearing capacity as well as frictional resistance of pile shaft are proportional to the diameter of pile and the cohesion. Similar behavior has been observed for piles of different length.

3. SAFE BEARING CAPACITY OF PILE

   The Safe Bearing Capacity (SBC) of piles could be determined from the load- displacement graphs of piles and as per the guidelines specified in BIS [7]. According to this code of practice, the safe vertical load on single pile for initial test shall be least of the following :

   1. For piles up to and including 600 mm diameter

      1. Two thirds of final load at which the total displacement attains a value of 12mm

      2. Fifty percent of the final load at which the total displacement equal to 10 percent of the pile diameter in case of uniform diameter piles

   2. For piles of diameter more than 600 mm

   1. Two thirds of final load at which the total displacement attains a value of 18mm

   2. Fifty percent of the final load at which the total displacement equal to 10 percent of the pile diameter in case of uniform diameter piles

      Figure 5 explains the method of determining the safe bearing capacity of 10 m long, 0.50 m diameter pile in soil having different cohesion from the load-displacement curve, as a

      4	0.8	10	5000	45.22	25.12	typical example, and is self explanatory. Out of the two values, the least one is taken as the SBC.
      		20	10000	90.43	50.24
      		30	15000	135.65	75.36
      		40	20000	180.86	100.48
      5	1	10	5000	70.65	31.4
      		20	10000	141.3	62.8
      		30	15000	211.95	94.2
      		40	20000	282.6	125.6

      4	0.8	10	5000	45.22	25.12	typical example, and is self explanatory. Out of the two values, the least one is taken as the SBC.
      		20	10000	90.43	50.24
      		30	15000	135.65	75.36
      		40	20000	180.86	100.48
      5	1	10	5000	70.65	31.4
      		20	10000	141.3	62.8
      		30	15000	211.95	94.2
      		40	20000	282.6	125.6

      Fig. 5 Typical example showing the method of determination of safe bearing capacity of pile from load-displacement curve.

      In a similar way, the SBC of all other piles were determined, and the details are presented in tables 4,5 and 6 for piles of length 5m, 10m and 15m respectively. It may be observed from tables 4. 2 to 4.4 that, in all the cases considered, the criteria of (2/3)× load corresponding to 12mm is critical when compared with the criteria of 50% of load corresponding to deformation equivalent to 10% of pile diamter.

4. RESULTS AND DISCUSSION

   Table 4 Safe Bearing Capacity (SBC) of 5m Long Pile having Different Diameter in Soil with Different Cohesion

   Diameter(m)	Cohesi on (kN/m² )	(2/3)× load correspond ing to 12mm (kN)	50% of load corresponding to deformation equivalent to 10%of pile diamter (kN)	Safe Bearing Capacity (kN)
   0.4	10	62.74	78.10	62.74
   	20	146.02	170.16	146.02
   	30	225.25	261.34	225.25
   	40	249.09	289.25	249.09
   0.5	10	65.79	91.60	65.79
   	20	159.23	201.46	159.23
   	30	249.42	311.63	249.42
   	40	337.52	418.85	337.52
   0.6	10	68.96	103.34	68.96
   	20	171.64	230.01	171.64
   	30	323.62	393.12	323.62
   	40	370.05	482.07	370.05
   0.8	10	89.87	146.50	89.87
   	20	229.84	329.75	229.84
   	30	368.22	515.93	368.22
   	40	503.28	699.80	503.28
   1	10	102.21	215.67	102.21
   	20	378.02	877.75	328.54
   	30	447.83	537.71	447.83
   	40	583.00	1015.85	583.00

   Table 5 Safe Bearing Capacity (SBC) of 10m Long Pile having Different Diameter in Soil with Different Cohesion

   Diameter (m)	Cohe sion (kN/ m²)	(2/3)× load correspondi ng to 12mm (kN)	50% of load corresponding to deformation equivalent to 10%of pile diamter (kN)	Safe Bearing Capacity (kN)
   0.4	10	112.38	139.60	112.38
   	20	244.01	294.55	244.01
   	30	426.72	445.79	426.72
   	40	537.41	627.67	537.41
   0.5	10	150.48	182.53	150.48
   	20	271.50	342.37	271.50
   	30	438.12	535.39	438.12
   	40	608.57	735.29	608.57
   0.6	10	127.28	185.21	127.28
   	20	297.14	405.41	297.14
   	30	486.35	636.02	486.35
   	40	674.46	873.66	674.46
   0.8	10	154.34	219.50	154.34
   	20	386.08	503.44	386.08
   	30	639.08	796.44	639.08
   	40	896.18	1096.19	896.18
   1	10	152.67	262.41	152.67
   	20	525.41	620.95	712.65
   	30	847.55	989.22	847.55
   	40	1015.32	1365.94	1015.32

   Table 6 Safe Bearing Capacity (SBC) of 15m Long Pile having Different Diameter in Soil with Different Cohesion

   Diamet er (m)	Cohesio n (kN/m²)	(2/3)× load correspondi ng to 12mm (kN)	50% of load corresponding to deformation equivalent to 10%of pile diamter (kN)	Safe Bearing Capacit y (kN)
   0.4	10	159.39	193.71	159.39
   	20	336.73	403.28	336.73
   	30	592.96	649.55	592.96
   	40	439.72	626.74	439.72
   0.5	10	173.57	221.77	173.57
   	20	380.47	474.98	380.47
   	30	585.32	727.00	585.32
   	40	804.23	988.04	804.23
   0.6	10	181.91	245.37	181.91
   	20	424.16	538.00	424.16
   	30	664.04	832.13	664.04
   	40	915.98	1135.52	915.98
   0.8	10	217.78	297.26	217.78
   	20	545.01	683.86	545.01
   	30	875.76	1073.92	875.76
   	40	1224.49	1476.31	1224.49
   1	10	212.68	351.59	212.68
   	20	603.42	842.18	603.42
   	30	987.31	1335.39	987.31
   	40	1407.75	1846.06	1407.75

   1. Variation of SBC with cohesion for different diameter of piles

      Figures 5,6 and 7 show the variation of SBC for different diameter of piles of length 5m, 10m and 15m.

      Fig.5 Variation SBC with Cohesion for different diameter of piles of Length 5m

      Fig.6 Variation SBC with Cohesion for Different Diameter of Pile of Length 10m

      Fig.7 Variation SBC with Cohesion for Different Diameter of Pile of Length 15m

   2. The variation of SBC with cohesion for different

   length of piles

   Figures 8,9,10,11 and 12 variation of SBC with cohesion for different length of piles having diameter of 0.40m, 0.50m, 0.60m, 0.80m and 1.0m respectively

   Fig.8 Variation of SBC with Cohesion for different length of piles having the diameter of 0.4m

   Fig.9 Variationof SBC with Cohesion for different length of piles having the diameter of 0.5m

   Fig.10 Variation of SBC with Cohesion for different length of piles having the diameter of 0.6m

   Fig.11 Variation of SBC with Cohesion for different length of piles having the diameter of 0.8m

   Fig.12 Variation of SBC with Cohesion for different length of piles having the diameter of 1.0m

5. CONCLUSIONS

   Based on present study the following conclusions could be made:

   1. There is a linear variation between SBC and diameter of pile, SBC and cohesion of clay soil and SBC and length of pile.

   2. For low value of cohesion (10 kN/m2), difference in the SBC of pile with different length and diameter is not significant. However, as the value of cohesion increases (up to 40 kN/m2), the difference in the SBC of pile with different length and diameter becomes significant.

6. ACKNOWLEDGEMENT

I would like to express my sincere gratitude to my guide Prof. Berlin Sabu for her valuable guidance, care and timely support throughout the thesis work. I am extremely grateful to the principal Prof. Mathew K., Mar Athanasius College of Engineering, for providing me with best facilities and atmosphere. I take this opportunity to thank Prof. Leni Stephen, Head of the Department, Department of Civil Engineering. Above all, I owe my gratitude to the Almighty for showering abundant blessings upon me; acknowledgement would not be complete without acknowledging my gratitude to my beloved parents who have been pillars of support and constant encouragement throughout the course of my thesis work.

REFERENCES

1. Krzysztof arkiewicz1, Pile bearing analysis based upon ultimate values of toe and skin resistance as well as their mobilization with settlement, MATEC Web of Conferences 284, 2019

2. Harry M. Coyle and Ibrahim H. Sulaiman, Bearing Capacity of Foundation Piles: State of the Art, Foundations of Bridges and Other Structures and presented at the 49th Annual Meeting.

3. H.K. Engin, E.G. Septanika & R.B.J. Brinkgreve, Improved embedded beam elements for the modelling of piles, Taylor and Francis Group,

   Lonon, UK, 2007

4. PLAXIS Connect V-20- PLAXIS 3D Reference Manual

5. Joseph E. Bowles, Foundation Analysis and Design, The McGraw Hill Companies,Inc., International Edition, Fifth Edition

6. BIS, IS:2911(part1-4) 2010, Indian Standard Design and Construction of Pile Foundations Code of Practice Part 1 Concrete Piles

7. BIS, IS:2911(part4) 1985, Code Of Practice For Design And Construction of Pile Foundations Part 4 Load Test On Piles