Experimental study on the stress-strain-strength behaviour of a Completely Decomposed Granite soil and a geofoam

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Experimental study on the stress-strain-strength behaviour of a Completely Decomposed Granite soil and a geofoam


Author: Kumruzzaman, Md.
Title: Experimental study on the stress-strain-strength behaviour of a Completely Decomposed Granite soil and a geofoam
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Soil mechanics.
Strains and stresses.
Department: Dept. of Civil and Structural Engineering
Pages: xxxiv, 298 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2239712
URI: http://theses.lib.polyu.edu.hk/handle/200/548
Abstract: The weathering of the granite in situ has produced many Completely Decomposed Granite (CDG) soils in most soil slopes in Hong Kong. The CDG soils may be cut and used as fill materials with compaction for land formation works, marine reclamations, as back fills of retaining walls, etc. Therefore, re-compacted CDG soils are often encountered in most of the infrastructural projects, building projects, and slope stabilization projects. The measurement and study of the stress-strain-strength behaviour of CDG soils in general stress states are of both practical applications and research significance. No previous study has been conducted on the three-dimensional behaviour of the CDG soils in Hong Kong before. Results from the three-dimensional experiments are helpful for understanding the stress-strain-strength behaviour of the CDG soils and for the development of constitutive models in a general stress state. Geotechnical applications of expanded polystyrene (EPS) geofoam are abound even though EPS is not a material widely known within the geotechnical profession. This material has been extensively used for building road embankments, in flexible pavements and in fill constructed on top of very soft compressible soils. The lightweight attribute of EPS has been further utilized for optimal repair of failed slopes. Experimental results through more comprehensive triaxial tests under various stress/strain loading paths can best decide which yield criterion is the more appropriate for this material. It has been found that three-dimensional behaviour of EPS is unknown to the researchers and the observed behaviour of EPS under the true triaxial loading conditions will add the new findings to the research on EPS. To investigate the stress-strain and strength behaviour of a CDG soil in Hong Kong, a large number of tests on compacted CDG specimens were conducted in the laboratory. These tests include (a) conventional triaxial tests under drained and undrained condition, (b) both drained and undrained plane strain tests, (c) true triaxial tests, and (d) tests on hollow specimens. A new loading device developed in the laboratory is used to perform the plane strain and true triaxial testing. Drained and undrained triaxial and plane strain tests have revealed that a critical state could be reached for the CDG with a higher percentage of fine content under dense condition. Stress ratio at the critical state and the critical state line in q-p' plane under plane strain loading are approximately the same as those under triaxial loading. On the other hand, however, the critical state line in e-lnp' plane under a plane strain condition is not the same as that under a triaxial condition for the same CDG soil. True triaxial tests have been carried on CDG specimens under isotropically consolidated and drained compression conditions by maintaining the constant mean effective stress p'-value and b-value. Experimental results at failure stress states are compared with those calculated using the Mohr-Coulomb and the Lade-Duncan failure criteria. The Lade-Duncan failure criterion provides better agreement with the experimental failure points in lower and higher range of b-values. No failure criterion is matched in the mid range of b (0.2<b<1.0). Strong influence of principal stress direction as well as the value of b on the strength parameters in hollow cylinder specimens has been obtained. Friction angle is the highest (43o) for b=0.5 and a =1.3o whereas the lowest friction angle (29.1o) is under the condition of b=0.5 and a =90o The difference between the highest and the lowest friction angles is about 14o Comparison of strength parameters under principal stress rotation in extension condition with that of true triaxial testing shows pronounced effects of cross-anisotropy. The critical state has been achieved for all simple shear tests and the friction angle at the critical state (37.3o) under simple shear is very near to the condition of b=0.25 and a =45o A program of triaxial and true triaxial tests has been undertaken to investigate the mechanical behaviour of EPS geofoam. The results confirm that EPS is an elastoplastic hardening material which also softens strength-wise and stiffness-wise under increasing confining pressure and the stress loading has significant effects on the mechanical behaviour of geofoam. The study has also found that a Drucker-Prager yield locus can fit the experimental results on the deviatoric (pi) plane reasonably well. The originality of the study and main contributions may be summarized as follows: (a) An existing true triaxial loading system was modified, setup and calibrated for performing large strain true triaxial tests as well as plane strain tests. This modified true triaxial loading system has a number of new features such as using four sliding plates for large compression. (b) The existing hollow cylinder apparatus was modified by doubling the specimen height in order to reduce the end effects. This apparatus was calibrated using a specially calibration device designed by the author. This hollow cylinder apparatus was then setup and calibrated for performing tests to get the reliable experimental results. (c) The author designed and carried out a comprehensive experimental investigation program which included most tests on a typical CDG soil in general loading conditions. The results are valuable for the understanding of the stress-strain and strength behaviour and helpful for the development of constitutive models in a general stress state. (d) It was first time in the community that true triaxial tests were carried out on ESP geofoam. Results have revealed for the first time ever about the influence of Lode's angle on the stress-strain behaviour of the EPS geofoam.

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