NUMERICAL EXPERIMENTS INTO PILES IN IMPROVED GROUND ON THE RESPONSE TO LATERAL LOADING RICARDO BERGAN BORN – Ph.D. Candidate Federal University of Rio Grande do Sul – UFRGS –
[email protected] VITOR PEREIRA FARO – DSc Federal University of Rio Grande do Sul – UFRGS –
[email protected] NILO CESAR CONSOLI – Ph.D Federal University of Rio Grande do Sul – UFRGS –
[email protected] ABSTRACT The behavior of laterally loaded piles is well known to be straightly related to characteristics of the upper part of the soil. Recommendations of over 30 years in past already dealt with this behavior {i.e. Simons and Menzies [1]; Broms [2]}, and treated with solutions that improved the lateral resistance, by substituting the upper part of the soil with a more rigid material. Besides those solutions improved the lateral resistance, the technique reflects a practice of material replacement. Here, a ground improvement technique, dealing with cemented-soil is presented, studying numerically the behavior of piles subjected to lateral forces. The proposed model analyses a large diameter bored pile which the results of loading test are documented in the literature. With the load tests results in natural soil, the model was calibrated, and then applied a layer of cemented-sand varying its geometry in length and width. The results shows first a good correlation between the numerical model and the load tests. When applied the layer of cemented sand, the lateral resistance of piles reached a improvement of over 100% compared to when in natural state. RESUMO O comportamento de estacas carregadas lateralmente é conhecido por ser diretamente relacionado com as características da parte superior do solo. Recomendações de mais de 30 anos já citavam este comportamento {i.e. Simons and Menzies [1]; Broms [2]}, e tratavam com soluções que melhoravam a capacidade lateral, através de substituição da parte superior do solo por um material mais rígido. Apesar que estas soluções melhoravam a resistência, esta técnica reflete uma prática de substituição de material. Neste trabalho, uma técnica de melhoramento de solo, trabalhando com solo-cimento, é apresentada, estudando numericamente o comportamento de estacas submetidas a esforços laterais. O modelo proposto analisa uma estaca escavada de grande diâmetro, a qual tem seus resultados de ensaios de carregamento publicados na literatura. Com os resultados dos ensaios de carregamento da estaca em solo natural, o modelo foi calibrado, e então uma camada de areia cimentada com variações geométricas de comprimento e largura foi aplicada. Os resultados mostraram uma boa correlação entre o modelo numérico e os ensaios de carregamento. Quando aplicada a camada de solo-cimento, a resistência lateral das estacas apresentou melhorias acima de 100%, ante comparada com o solo em estado natural.
1.
INTRODUCTION
The behavior of laterally loaded piles is well known to be straightly related to characteristics of the upper part of the soil. Recommendations of over 30 years in past already dealt with this behavior (i.e. Simons and Menzies [1]; Broms [2]), and treated with solutions that improved the lateral resistance, by substituting the upper part of the soil with a more rigid material. Besides those solutions improved the lateral resistance, the technique reflects a practice of material replacement. Here, a ground improvement technique, dealing with cemented sand is presented, studying numerically the behavior of piles subjected to lateral forces. Due to the planning of a high-speed rail system in Taiwan, full scale load tests were performed in a well characterized soil strata. This data was published by Huang et al. [3], and here used to provide a database from lateral load tests, enabling the calibration of a numerical model. 2.
GEOTECHNICAL SITE CHARACTERIZATION
The test site located in Taiwan was within a sugarcane field. Previous boreholes showed a depth of soil higher than 100m. Standard Penetration Tests, followed by Flat Dilatometer Tests and Seismic Cone Penetration Tests (which include the measurement of the seismic shear wave velocity) were carried out, as can be seen in Huang et al. [3]. The results of the soil investigation considered for the pile B7 are shown in figure 1. SPT
SCPT
N (blows / 30cm) 0
0
20
40
DMT
qc (MPa) 60
0
10
20
G (kPa) 30
0
50 100 150 200 250 300
P0 / P1 (kPa) 0
1000 2000 3000 4000
5
Depth Below Surface (m)
10 15 20 25 30 35 40 45
P0 P1
50
Fig. 1 – Results of soil investigation, standard penetration test, cone penetration test and flat dilatometer from Huang et al. [3]. The soil profile was divided in 4 layers, as can be seen in figure 2, based on the change in the soil classification and resistance. The relative density (Dr) based on CPT was computed using the equation 𝑞𝑞𝑐𝑐 𝑝𝑝𝑎𝑎
0.5
𝐷𝐷𝑟𝑟 = � 305 �
(1)
developed by Kulhawy and Mayne [4], where pa = atmospheric pressure. The angle of internal friction was estimated using three correlations, the De Mello [5] method, the Bolton [6] Method, and the API [7] method, all based on relative density. For the De Mello method, the angle φ’ is given by the equation: (1.49 − 𝐷𝐷𝑟𝑟 ) × tan 𝜑𝜑′ = 0.712 (2)
By the Bolton method, the angle (φ’) is given by the equation 𝜑𝜑′ = 33 + {3[𝐷𝐷𝑟𝑟 (10 − 𝑙𝑙𝑙𝑙 𝑝𝑝′) − 1]}
(3)
Finally, for the API method, the angle (φ’) is given by the equation 𝜑𝜑′ = 16𝐷𝐷𝑟𝑟 2 + 0.17𝐷𝐷𝑟𝑟 + 28.4
(4)
For the estimations of the Young’s modulus, three non-direct methods were used. The proposed correlation by Baldi et al. [8] which use the data from DMT, the correlation proposed by Schnaid et al. [9] which use data from CPT, and the correlation with the seismic shear wave velocity from the SDMT. The correlation proposed by Baldi et al. [8] uses the Dilatometer Modulus ED which can be obtained as (5) 𝐸𝐸𝐷𝐷 = 34.7(𝑃𝑃1 − 𝑃𝑃0 ) in which P0 is the “lift-off” pressure in which the membrane starts to expand, and P1 is the required pressure to move the centre of the membrane by 1.1 mm against the soil. To obtain the E25 the dilatometer modulus must be multiplied by a conversion factor as: 𝐸𝐸25 = 𝐹𝐹 × 𝐸𝐸𝐷𝐷 (6)
In Baldi et al [8] proposure, the F value is 3.5 for a OC soil. In the study of Schnaid et al. [9], besides complaining about cemented and uncemented sands, and making a straight relation to Shear Modulus G0, has been used in terms of comparison with the others method as follows 3 𝐺𝐺0 = 110�𝑞𝑞𝑐𝑐 𝜎𝜎′𝑣𝑣0 𝑝𝑝𝑎𝑎 (7)
this expression refers to the lower bound of uncemented sands. With the value of G0, provenient from Schnaid et al. [9] study and from the result of the SDMT, making use of elasticity theory, and fixing Poisson’s ratio ν as 0.3, the E modulus is given by: 𝐸𝐸 = 2𝐺𝐺(1 + 𝜈𝜈) (8) Results of the correlations of resistance and stiffness parameters can be seen and compared in figure 2. Interpreted Soil Profile 0 5
Depth Below Surface (m)
10
0
20
40
φ’ (°)
Relative Density (%) 60
0%
25% 50% 75% 100%
25
30
35
E (kPa) 40
45
0
100 200 300 400 500
Silt (layer 1) Silty Sand (layer 2) Sandy Silt (layer 3)
15 20 25 30
Silty Sand (layer 4)
35 40 45 50
De Mello (1971)
Baldi et al. (1986)
Bolton (1986)
SCPT
API
Schnaid (2004)
(1987)
Fig. 2 – Soil profile based on standard penetration test, cone penetration test and flat dilatometer test, along with interpreted relative density, friction angle and elastic modulus.
3.
PILE LATERAL LOAD TEST RESULTS AND NUMERICAL CALIBRATION
Looking for a optimization in the design of pile foundations for the construction of a high-speed rail system in Taiwan, full scale load tests were performed in a well characterized soil strata. The results of lateral load tests carried out on the pile denominated B7, a bored reinforced concrete pile, with 1.5m of diameter and 34.9m of length, prevenient from the work of Huang et al. [3], were considered in this study. The disposure of the pile into the soil, as the properties considered to each layer of soil, can be seen in figure 3. Pile Disposure Soil Profile 0 5
Depth Below Surface (m)
10
0
20
40
Load
60
Silt (layer 1) Silty Sand (layer 2) Sandy Silt (layer 3)
15 20 25 30
Material Properties
Silty Sand (layer 4)
35
Layer c’ φ’ E (kPa) ψ' (°) ν No (°) (kPa) 1 28 0 0 0.3 71.000 2 29 0 0 0.3 123.000 3 29 0 0 0.3 166.000 4 31 0 0 0.3 200.000 Soil 34.9 300 8.725 0.3 3.600.000 Cement Pile 0.15 28.000.000
40 45 50
Fig. 3 – Pile disposure within the soil profile, and material properties. To predict the response of this bored reinforced concrete pile to horizontal loading, the finite element code Abaqus© has been used. The constitutive model used for soil was the Mohr-Coulomb, which the input parameters are relatively easy to obtain, and the results showed a good agreement. While the r.c. pile was considered to be in a purely elastic regime, due to its stiffness. No plans of symmetry were used, the model utilized was full 3D. The same mesh element types were used to the soil layers and for the pile. Hexaedric elements, as a 8-node brick, with trilinear displacement, trilinear pore pressure, reduced integration, and hourglass control were used, as can be seen in figure 4.
Fig. 4 – Numerical model 3D mesh.
In the attempt to simulate the horizontal loading, the model was conceived to apply a displacement by the top of the pile, while making the readout of the reaction force by the top of the pile too. Due to convergence easiness, this sequence has been utilized. The exact disposure and material properties shown in figure 3 has been applied to the numerical model. The direct comparison between the measured by the full scale load test, and by the numerical model is presented in figure 5. 3500
lateral load (kN)
3000 2500 2000 1500 1000 Measured Numerical Model
500 0
0
10
20
30
40
50
60
70
80
90
100
horizontal displacement (mm) Fig. 5 – Results from the numerical model, simulating the soil in natural state.
110
120
130
140
The results of the numerical model showed a good agreement with the measured values, underpredicting the real values by the beginning, while overpredicting a bit by the end. The deviation from the measured values, as in figure 6, by the most of the points showed values under 5%, and few points slightly close to 10%, hence, the model has been considered satisfactory.
3500
3000
predicted lateral load (kN)
2500
2000
1500
1000
Numerical Model
500
5% Deviance 10% Deviance
0
0
500
1000
1500
2000
2500
measured lateral load (kN)
Fig. 6 – Measured vs. predicted load.
3000
3500
4.
IMPROVED GROUND MODELLING
Once the numerical model was considered calibrated, as showing a good agreement with the load test results, a layer of improved ground in the top and around the pile has been proposed, with the geometrics variations showed in figure 7. Simulation No.
W (m)
L (m)
t (m)
1
3
3
3
2
4,5
3
3
3
6
3
3
4
3
4,5
3
5
3
6
3
6
4,5
4,5
3
7
6
4,5
3
8
4,5
6
3
9
6
6
3
L
W
t
Fig. 7 – Sequence of numerical simulations. The proposed improved ground used, had its properties based in some previous studies, as the method proposed by Consoli [10], in which artificially cemented soils have a fixed internal friction angle, with a value of 34.9°. The dilation angle was simply defined as 25% of the internal friction angle, while cohesion intercept, and Young’s modulus were determined by the methodology proposed by Cruz [11], in this case considering Osorio sand with about 7 percent of volumetric cement content. Evaluating the overall improvement of the pile load capacity, comparing the pile in natural state soil, values over 100% of improvement appeared. For values of 3%, 6%, and 9% in relation to the pile diameter (45mm, 90mm, and 135mm), of horizontal displacement, the improvement prior to the pile in natural state soil is shown in figure 8. 3%
6%
9%
200% 180%
Improvement
160% 140% 120% 100% 80% 60% 40% 20% 0%
1
2
3
4
5
6
7
8
9
Simulation No.
Fig. 8 – General improvement of the pile lateral resistance. Due to the proposed variation in the geometrics of the improved ground layer, it was able to evaluate the relative efficiency between the enlargement of length and/or width. Fixing one dimension, and then enlarging the other {i.e. fixing the L dimension in 3m, and varying the W dimension from 3m, to 4.5m and 6m}, allowed to compare graphically, through figure 9, the advance in pile lateral resistance. When fixing the L dimension, and then enlarging the soil cement layer in the W dimension, the results from figure 9a, 9c and 9e shows a small improvement if compared to itself. While when fixing the W dimension, and enlarging in the L dimension, the results from figure 9b, 9d and 9f shows a more pronounced improvement, again compared to itself. Showing that in the direction of the load application, as due to the stress distribution, the layer of improved ground shows a more pronounced effect.
Lateral load (kN)
7000
7000
6000
6000
5000
5000
4000
4000
3000
3000 Measured Numerical Model SIM_001 SIM_002 SIM_003
2000 1000 0
0
25
50
75
100
Lateral load (kN)
150
1000 0
7000
7000
6000
6000
5000
5000
4000
4000
3000
3000
2000
Measured Numerical Model SIM_004 SIM_006 SIM_007
1000 0
0
25
50
75
100
125
150
0
6000
6000
5000
5000
4000
4000
3000
3000
2000
Measured Numerical Model SIM_005 SIM_008 SIM_009
1000
50
75
(e)
50
100
125
150
75
125
150
Measured Numerical Model SIM_002 SIM_006 SIM_008 0
25
50
75
100
125
150
(d)
2000
Measured Numerical Model SIM_003 SIM_007 SIM_009
1000 0
100
(b)
1000
7000
25
25
2000
7000
0
0
horizontal displacement (mm)
(c)
0
Measured Numerical Model SIM_001 SIM_004 SIM_005
horizontal displacement (mm)
(a)
Lateral load (kN)
125
2000
0
horizontal displacement (mm)
25
50
75
(f)
Fig. 9 – Results from numerical simulations, horizontal displacement vs. lateral load.
100
125
150
5.
CONCLUSIONS
Due to an appropriated soil investigation campaign, followed by a proper parametrization of soil layers, the numerical model presented here is considered to be a Class A prediction. The results from the numerical model have shown a good agreement with the lateral load test, and the authors conclude that there was no big effort to calibrate it. Since the results have shown a good agreement, the numerical model could be used to expand to analysis such group effect, and even in the application of improved ground layers of different resistances, and geometrics. The improvement reached by applying the soil cement layer is very perceivable, even from the smallest geometrics tested, already shown a improvement of over 40%. In which, the major effect is reducing the initial displacement, making the pile-soil set more rigid. 6.
REFERENCES
[1]
SIMONS, N. E.; MENZIES, B. K. A short course in foundation engineering. Guildford: IPC Science and Technology Press, 1975. 4 , iv, 159 p. ISBN 0902852426.
[2]
BROMS, B. B. Stability of Flexible Structures (Piles and Pile Groups). Fifth European Conference on Soil Mechanics and Foundation Engineering. Madrid, 1972. 239-269 p.
[3]
HUANG, A. B. et al. Effects of construction on laterally loaded pile groups. Journal of Geotechnical and Geoenvironmental Engineering, v. 127, n. 5, p. 385-397, 2001.
[4]
KULHAWY, F. H.; MAYNE, P. W. Manual on Estimating Soil Properties for Foundation Design. 1990
[5]
DE MELLO, V. F. B. The Standard Penetration Test State-of-the-Art Report. 4th Panamerican Conference on Soil Mechanics and Foundation Engineering San Juan (Puerto Rico). I 1971.
[6]
BOLTON, M. D. The strength and dilatancy of sands. Géotechnique. 36: 65-78 p. 1986.
[7]
API. Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms. American Petroleum Institute, 1987.
[8]
BALDI, G. et al. Flat dilatometer tests in calibration chambers. Use of In Situ Tests in Geotechnical Engineering, Geotechnical Special Publication (GSP) No. 6. Blacksburg, Virginia: ASCE, 1986. p. 431-446 p.
[9]
SCHNAID, F.; LEHANE, B. B.; FAHEY, M. In situ test characterisation of unusual geomaterials International Conference on Site Characterization. Porto: Milpress, 2004. 49-74 p.
[10]
CONSOLI, N. C. A method proposed for the assessment of failure envelopes of cemented sandy soils. Engineering Geology, v. 169, p. 61-68, 2014.
[11]
CRUZ, R. C. Influência de parâmetros fundamentais na rigidez, resistência e dilatância de uma areia artificialmente cimentada. 2008. (Doctor). Programa de pós graduação em Engenharia Civil, Universidade Federal do Rio Grande do Sul, Porto Alegre.
