ABSTRACT Foundation system improved with stone columns is many difficulties in quantitative analysis of soil-column interaction due that bearing capacity and consolidation behavior of stone column is affected by various parameters. Because, behavior of composite ground develops in the soil-foundation system interface, behavior of stone column is best investigated in term of various parameters which affect the horizontal resistance in the interface. In the present study, failure mechanism and various parameters for the behavior of end-bearing stone column groups are investigated by loading tests and unit cell consolidation tests. Results of model tests are verified through the failure behavior and bearing capacity by FEM analysis. Finally, the improving characteristics of soft ground and the influence of design parameters are investigated in this study using FEM results and PR value. From the PR value calculation and test results, bearing capacity of stone column is affected by undrained strength of surrounding ground, area replacement ratio of composite ground and installation of mat. And, behaviors of foundation are affected by diameter and spacing of pile rather than embedment ratio and mat. Also, the behavior of stone column is mainly affected by initial undrained shear strength of surrounding ground and column spacing. And, analyses by FEM using Mohr-Coulomb model and PR values are applicable to the prediction of stone column behavior. KEY WORDS: Stone columns; Composite ground; PR values; INTRODUCTION Stone column is one of the soft ground improvement methods, applicable to a wide range of soil strata and a economical method of support in compressible and cohesive soils for low-rise buildings, lightly loaded foundations, earth structures and storage tanks that can be tolerate appreciable movement. However, there are many difficulties in quantitative analysis of soil-column interaction due that bearing capacity and consolidation behavior of stone column-mat foundation system is affected by various

parameters. Therefore, it needs to evaluate the influence parameters to failure and horizontal resistance in the interface to estimate the foundation behavior using stone column. In the present study, failure mechanism and various parameters of behavior of end-bearing stone column-mat system are investigated by laboratory model tests. Also, the propriety of model tests is verified through the evaluation of the failure behavior by FEM analysis. MODEL PREPARATION The soil used for the model tests is silty marine clay in the western seashore of Korea. Table 1 shows the results of physical tests of marine clay.

Table 1 Physical properties of clay Physical properties Quantity

USCS CL�ML Percent passing through the No.200 sieve (%) 91 Liquid limit LL�(%) 30.6�33.8Plastic limit PL (%) 22.5�27.3Plasticity index (%) 7.1�6.5 Natural water contents (%) 35��� Specific gravity 2.65 Total unit weight (kN/m3) 16.87

The grain size of the crushed stone was chosen so that the ratio of column diameter to typical particle size was similar to that found in prototype stone columns and used well-graded crushed stone in this model tests. Clay beds were consolidated as slurry added water with 1.5 times of LL. And it was removed the organic matters and impurities by passing through the 0.074mm sieve, finally, pre-loaded with designated undrained strength. Loading test equipment used on the model test is shown in Fig. 1.

Behaviors of Foundation System Improved with Stone Columns Woo-Seok Bae, Bang-Woong Shin, Byung-Chul An

Department of Civil Engineering, Chungbuk National University Cheongju, Chungbuk, South Korea

Joo-Sub Kim Ki-Hwa Construction Co.

Cheongju, Chungbuk, South Korea

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Proceedings of The Twelfth (2002) International Offshore and Polar Engineering ConferenceKitakyushu, Japan, May 26–31, 2002Copyright © 2002 by The International Society of Offshore and Polar EngineersISBN 1-880653-58-3 (Set); ISSN 1098-6189 (Set)