Design chart of piled raft foundations on soft clay

					Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference Honolulu, Hawaii, USA, May 25 –30, 2003 Copyright © 2003 by The International Society of Offshore and Polar Engineers ISBN 1 –880653 -60 –5 (Set); ISSN 1098 –6189 (Set)

Design Charts of Piled Raft Foundations on Soft Clay
Young-Kyo Seo and Kyung-sik Choi
Div. of Ocean Development Engineering, Korea Maritime University Busan, Korea

Sung-Gyo Jeong
Department of Civil Engineering, Dong-A University Busan, Korea

For the structure foundation under the soft clay layer conditions, the design charts are first presented for the evaluation of both bearing capacity and total settlement in the raft alone foundation system. Loadsettlement relationship curves are used to evaluate the ultimate soilbearing capacity. The total settlement is evaluated by applying various traditional safety factors of the uniformly distributed loads. Then, the parametric studies are carried out for the piled raft foundation system. In the numerical analyses, elasto-plastic finite elements models are used to present the foundation response and the design charts, which enable the determination of the raft size and pile length and spacing if piles are available.

Based on their research work, an attempt is made to give insights of using such numerical results in the solution of certain problems connected with the piled raft foundation on layered soils in particular regions in Busan.

KEY WORDS: Piles; rafts; foundations; design; numerical modeling. INTRODUCTION
As Korean government has proposed a plan of new land development in the Nakdong river deltaic plane near the Busan, second largest city in Korea, infrastructures such as industrial and commercial facilities have being constructed since the early 1990’s. In this area, the soft clay layer about 20 to 30m thickness have been deposited over the sandy soil or directly on the undulated bedrock surface. On the top of the soft clay layer, typically about 5 to 10m’s of sand layer is placed. Figure 1 shows the typical results of laboratory tests on samples of Busan clay. The insitu groundwater table is around 0.5 to 0.8m below the ground surface. For the structure foundation under the soft clay condition in this area, piles are costumery used to transmit the structural loads to the bedrock. So far, the designer of the raft or the piled raft foundations in this area has very little information available for the evaluation of both bearing capacity and settlement in the preliminary design stage. The early work for the piled raft foundation design was carried out by Prakoso and Kulhawy (2001). They presented an approximate design procedure based on the results of numerical analyses and the focus was on the “floating” piled raft system in undrained conditions. For the design, various parametric studies of the piled rafts had been presented in homogenous soils. Fig. 1 Ground conditions and soil properties

Considering the behavior of piled raft foundation and soil system, it was decided to use the computer program, called PLASIS (Verneer and Brinkgreve, 1995) as a computer tool to develop the design charts. This geotechnical finite element program can analyze the behavior of a raft and piles on a layered soil profile. As for the element type, six node triangular elements were used. The side resistance was modeled by using interface elements between the piles and the soil layers if piles are available. The contact between the raft and the soil is assumed to be frictionless. In the numerical model as shown in Figure 2, the ground was divided into three layers and the ground water was assumed to be on the top of the ground. And also, the considered basic design parameters for the foundation were selected to incorporate relevant variables as follows [Pile group to raft width ratio ( B g / B r ) , Pile length (L) , Pile spacing (S ) , and Raft size (B) ]. The soil material properties required for the computer analysis were determined from laboratory testing. With the given data as shown in Table 1, the foundation behavior with regard to the settlement and bearing capacity was acquired from the results of computer analysis.


Total settlements in raft foundation The total settlement at the center of the raft is evaluated by applying different safety factors of the uniformly distributed loads. So, it can be chosen the limit allowable settlements to meet the local codes’ requirements. Figure 4 shows the immediate settlement after the loading and the consolidated settlement in different sizes of raft widths. The consolidated settlement was obtained when the excess pore pressure reached the nearly zero value.

Fig. 2 Typical finite element mesh. Table 1. Material properties in numerical analysis
Ε kN/m2 Upper Sand Clay Lower Sand Raft Pile 1.2e4 5.0e3 1.7e4 2.5e7 2.0e8 ν 0.3 0.2 0.3 0.2 0.25 γd kN/m2 17.0 11.11 17.0 23.0 γsat kN/m2 20.68 16.92 20.68 23.0 κx m/day 8.64e-3 8.64e-5 8.64e-3 κy m/day 8.64e-4 8.64e-6 8.64e-4 Cu kN/m2 2.0 30.0 2.0 φu 34.7 0 34.7

The foundations that can be considered first for minor structures on this area are reinforced concrete rafts. Rafts spread the loads over the widest possible area to reduce the settlement. There is, therefore a need for more information on the overall behavior of raft foundation on deep clay soil layer. This paper fist examines the bearing capacity and the total settlement of the raft foundation. Since the sand layer is typically placed on the soft clay layer in this area, it next examines the effects of the total settlement by improving the soil stiffness in the first sandy layer stiffness. The numerical analysis results in the paper are presented in the form of non-dimensional units. Bearing capacity in raft foundation The load-settlement relationships are used to evaluate the ultimate soilbearing capacity with different sizes of the rafts. Figure 3 shows the bearing capacity versus the raft width. It tells when the maximum raft width can be the depth of the fist sandy layer ( D f ) to get the maximum ultimate bearing capacity.

Fig. 4 Total settlements with respect to the raft widths Soil improvement in raft foundation The behavior of foundation load transfer into the supporting soil can be evaluated from the distribution of vertical stress with depth. When the upper layer is significantly stiffer than the lower layer, the stress in the lower layer is greatly reduced. Consequently, the consolidation settlement of the clay layer can be reduced by improving the first layer stiffness. Figure 5(a) shows the vertical stress distribution under the centerline of the loaded area. It is shown as the depth for ratios of Young’s moduli E c / E1 = 1,10,100 and 1000. Figure 5(b) shows the distribution of vertical stress along the horizontal beneath the raft according to the ratios of moduli. It shows the normalized values with the applied load (q o ) . From the numerical results, when the soil stiffness is increased to 100 times of original stiffness, the vertical stresses looks to be reduced effectively.

Fig. 3 Bearing capacity with respect to raft width

Fig. 5 (a) vertical stress under the center (b) horizontal stress beneath the raft Figure 6 shows the total settlement reduced with different sizes of raft


width when E c / E1 = 100 following the previous soil improvement results.

Effects of pile length in piled-raft foundation The pile length and the settlements are plotted in Figure 8. As expected, the total settlements are decreased as pile length increased. In the differential settlements, it sometimes becomes negative (upward dish shape) in particular B g / Br ratio. When the B g / Br is closed to the fully piled raft, it approaches to the zero values

Fig. 8 Effects of pile length Effects of pile diameter in piled-raft foundation To examine the pile diameter effects on the settlements, the results are plotted in Figure 9. It indicates that the pile diameter effect is minimal, regardless of B g / Br ratio in both total and differential settlements.

Fig. 6 Total settlements with the improvement of soil stiffness in different sizes of the raft widths

The use of piled raft foundations is considered to the situation that the raft alone can not satisfied the design requirements, and the piles are needed to reduce the overall and differential settlements of the structures. In the previous section, the results of the raft foundation showing that the total settlements will be greatly increased in the design using traditional values for the factor of safety. Settlements can be reduced by increasing the stiffness of the first sandy layer. When piles are used in conjunction with a raft, the applied loads are transferred to the supporting soil through the pile. In this section, the effects of pile spacing, numbers and length with the various pile group to raft width ratio ( B g / B r ) are studied for the total and differential settlements. The differential settlement for the foundation is defined as the difference between the center and the edge of the raft. In the results, the nondimensional ratios R and ∆R which are normalized by dividing the raft foundation settlements represent the total and differential settlements respectively. Effects of pile spacing in piled-raft foundation Figure 7 shows the effects of the pile spacing in terms of L / S . As the pile spacing decreased, the total settlement ratio R decreases until particular pile spacing, then it becomes independent of B g / Br ratio. The differential settlements ratio ∆R for the given case can be minimum when L / S is 1 to 1.5.

Fig. 9 Effects of pile diameter

In this study, the foundation for a particular area that has thick clay layer under the relatively less thickness sandy layer has been considered. A simple design charts for calculating the ultimate soil bearing capacity and the total settlement of the raft foundation first has been developed. Then, the effect of the soil stiffness has been studied to reduce the total settlement. Finally, various parametric studies were carried out for the pile raft foundation.

Prakoso, W. A. and Kulhawy, F. H. (2001). "Contribution to piled raft foundation design," J Geotechnical and Geoenvironmental Eng., ASCE, Vol 127, No 1, pp 17-24. Desai, C. S., Johnson, L. D., and Hargett, C. M. (1974). “Analysis of pile-supported gravity lock” J Getechnical Eng., ASCE, Vol 100, No 1, pp 1009-1029 Vermeer, P. A. and Brinkgreve, R. B. J. (1995). PLAXIS use’s manual version 6.1, Balkema, Rotterdam, The Netherlands. Clancy, P. and Randolph, M. F. (1993). “An approximate analysis procedure for piled raft foundations” Int. J. Numer. and Analytical Methods in Geomech. 17(12), pp 849-869 Hemsley, J. A. (2000) Design applications of raft foundations, Thomas Telford, London, England.

Fig. 7 Effects of pile spacing


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Description: Design Chart of Piled Raft Foundations on Soft Clay