The time-dependent stress-strain behaviour of natural Hong Kong marine deposits in different stress states (Vol. 2)

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The time-dependent stress-strain behaviour of natural Hong Kong marine deposits in different stress states (Vol. 2)

 

Author: Cheng, Chun-man
Title: The time-dependent stress-strain behaviour of natural Hong Kong marine deposits in different stress states (Vol. 2)
Degree: M.Phil.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Marine sediments -- China -- Hong Kong
Strains and stresses
Department: Dept. of Civil and Structural Engineering
Pages: 2 v. : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1733061
URI: http://theses.lib.polyu.edu.hk/handle/200/2050
Abstract: The study in this thesis focuses on six related aspects of the time-dependent stress-strain behaviour of Hong Kong marine deposits (HKMD): (1) strain-rate effects, (2) non-linear creep behaviour, (3) stress relaxation effects, (4) shear banding effects, (5) behaviour under pure shearing conditions, and (6) the dependence of the undrained failure behaviour on the initial stress states and the orientation of principal stresses. The prime objective of this thesis is to examine the time-dependent stress-stain behaviour of the HKMD under K0-consolidation conditions and different stress states by carrying out various types of laboratory tests and to gain a better understanding of the behaviour of the natural undisturbed HKMD. Testing programs consisted of a series of experimental tests on the natural undisturbed HKMD as well as on remoulded HKMD. These tests include: (a) K0-consolidated undrained triaxial tests for both compression and extension states shearing at step-changed strain rate ranging from +-0.2%/hr, to +-2%/hr, and to +-20%/hr (called CK0UC and CK0UE tests), (b) one-dimensional consolidation and creep tests (I -D CREEP tests), (c) K0-consolidated undrained triaxial stress relaxation tests during a CK0UC test or a CK0UE test and at a certain strain level (called CK0UCR tests and CK0UER tests here), (d) one-dimensional compression tests with constant rates of strain (CRS tests), (e) undrained simple shearing tests performed in a new hollow cylinder testing apparatus (SS tests), and (f) the rotation of principal stress tests carried out in the hollow cylinder testing apparatus (called CT and ET tests). It is found that the stress-strain behaviour of the natural undisturbed HKMD exhibits viscous characteristics, similar to other natural clay around the world. The present study shows that the strain effects are large for specimens in extension than those in compression and pure shearing. From the CK0UC and CK0UE tests, the undrained shear strength for the specimen of the natural undisturbed HKMD tested increases by 8.5% and 12.1% for one order increment of axial strain rate (i.e. p0.2) for the compression and extension modes respectively. The relationship of deviator stress and axial strain shows clearly the characteristics of strain softening for the compression tests and strain hardening for the extension tests on triaxial specimens. A clear shear band was observed for every specimen in compression and no clear shear band was seen for the specimens in extension. The formation of shear bands is related to the strain softening stress-strain behaviour of the soil. A hypothesis for the formation of banding softening is suggested, based on the results of the CK0UC, CK0UE, CK0UCR and CK0UER tests. In the 1 -D CREEP tests, the non-linear creep behaviour as well as the tertiary creep behaviour has been examined. Distinct non-linear creep behaviour has been seen for the natural undisturbed HKMD during the 1-D oedometer tests. The secondary compression index, Ca, is found to vary with vertical effective stresses and time. The non-linear creep function proposed by Yin (1999) for the remoulded HKMD has been adopted and modified to simulate the non-linear creep mechanism for the natural HKMD. The modelling results are in excellent agreement with data of tests on the natural undisturbed HKMD. Tertiary creep behaviour is observed for the natural HKMD under high vertical stresses. A hyperbolic equation has been thus proposed to simulate the tertiary creep behaviour. A new time-line theory on the basis of the modified creep function and a new empirical equation for the tertiary creep has been introduced. In CK0UCR and CK0UER tests, each initially K0-consolidated specimen was sheared with step-changed constant axial strain rates accompanied with multistage relaxation under a compression state or an extension state. The test results are used to examine the effects of the stress relaxation and strain rate on the stress-strain behaviour of the natural undisturbed HKMD. It is discovered that the undrained relaxation behaviour depends on the stress levels (SL), the strain rates prior to the relaxation, the permeability and creep potential of a soil. A power law equation is presented to compute the decay of the deviator stress during the relaxation. The decay of the normalized deviator stress (q/q0) is negatively correlated with the SL for the undrained compression tests, whereas q/q0 and SL are directly correlated to each other for the undrained extension tests. The strain-rate parameter, P0.2, does reveal that the strain-rate effects on the undrained shear strength in the extension tests are stronger than those in the compression tests. An average value 7.5% of P0.2 is obtained for the compression tests, while 14.8% for the extension tests. The stress-strain curves of the compression states also exhibit a strain-softening phenomenon and a clear shear band can be observed in every compressed specimen. The dependence of the undrained behaviour of the remoulded and natural undisturbed HKMD on different initial stress states (i.e. isotropic or K0 conditions), the behaviour during the pure shearing and the orientation of principal stresses is investigated using hollow cylinder torsional shear tests under direct loading paths. These tests simulated the elementary behaviour of the HKMD in-situ. The specimens were loaded along stress paths with a pure shearing condition (for example the slip failure at the base of an embankment where the soils are strained horizontally) and a rotation of the principal stress direction (simulating the stress condition during construction of a seawall on seabed for example). Strain-rate dependency on the undrained stress and porewater pressure is assessed in pure shearing condition. It is revealed that the undrained behaviour of the HKMD is dependent on the direction of the principal stress. The peak octahedral shear stress decreases when a varies from 0 to 45, and then increases again when a changes from 45 to 90, irregardless of the initial consolidation stress history. It is also found that failure states at large strain are not influenced by the stress system during consolidation, but the mode of shearing. The experimental results show that the strength of the HKMD is basically well described by the Lade's failure criterion. The significances and applications of the present study are presented in the thesis, with a special reference to soil conditions and geotechnical projects in Hong Kong. This thesis consists of two volumes:Volume I including text of all chapters, and Volume 2 including tables and figures of all chapters

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