Dilation and Stability of Sand in Triaxial Tests
A. Sawicki
ABSTRACT: Possible links between certain aspects of pre-failure instabilities of sand (instability line) and plastic dilation are studied. The starting point is experimental results obtained from triaxial investigations, which are approximated separately by analytical formulae for initially contractive and dilative sands. The irreversible strains are deduced from the condition that plastic work (dissipation) should be positive. Then, analytical formulae for plastic dilation are derived and presented in graphical form. In the case of initially contractive sand, a possible link between the instability line and maximum of the plastic dilatancy function is demonstrated. This condition is equivalent to the minimum dilation parameter or maximum of Rowe’s function. In the case of initially dilative sand, it is shown that the negative work done by a mean stress on expanding (dilating) soil should not be treated as dissipative because of thermodynamical requirements. Consequently, the plastic dilation function is zero for shearing stress exceeding the instability line.
Strength Anisotropy of a Residual Soil in Singapore
G. Meng and J. Chu
ABSTRACT: The undrained shear strength of soil is one of the most important parameters required for geotechnical design. Depending on the design situations, the undrained shear strength of soil may have to be determined by different tests. In this paper, some testing data on the determination of the undrained shear strength of a residual soil in Singapore are presented. Large blocks of undisturbed residual soil samples were taken from a construction site of the Deep Tunnel Sewerage System Project in Singapore. To study the inherent strength anisotropy, specimens cut in both vertical and horizontal directions were tests. The anisotropy behaviour of intact residual soil was also investigated under different major principal stress directions. The tests conducted included K0 consolidated undrained triaxial compression (CK0UC) and extension (CK0UE) tests, and K0 consolidated undrained direct simple shear (CK0UDSS) tests. Based on the experimental results, the cu/s10¢ versus OCR relationships of each type of tests are established for practical applications. The failure envelopes and friction angles determined from different types of tests are also compared.
Effect of Boundary Conditions on Shear Banding in True Triaxial Tests on Sand
P.V. Lade and Q. Wang
ABSTRACT: Shear banding in true triaxial tests on sand have been studied to determine its effects on failure under three-dimensional conditions. Experiments have been performed on rectangular prismatic specimens with height-to-diameter ratio H/D = 2.47 and on cubical specimens with H/D = 1.0 to determine the influence of slenderness ratio and stiff versus soft boundaries on the results of such tests. The experiments show clear differences between the two types of tests: Shear banding in the tall specimens occurs with a sharp peak, while the short specimens show smoother stress-strain behavior near the peak, and all friction angles from the short specimens are 1-2 degrees higher than those from the tall specimens. The analysis of shear banding indicates that the critical hardening moduli are closer to zero for the short specimens than for the tall specimens. Indications are that experiments should be performed on tall specimens in which the shear banding occurs freely, while the short specimens impede and delay the development of shear bands.
Behavioural Patterns of Fine Sands
V.N. Georgiannou
ABSTRACT: The behavioural patterns of five fine sands are investigated in the Hollow Cylinder and the triaxial apparatus under undrained loading conditions. The paper focuses on distinctive patterns of undrained response of sands, namely an unstable or brittle response associated with strength reduction after a transient peak and a stable response when a continuous increase in strength with loading is observed. The influence of various parameters such as particle shape, grading, addition of fines, consolidation history, stress level and loading conditions on sand behaviour is examined. Particle shape and angularity has much more significant influence on sand’s response pattern than small variations in the grading curves of uniform sands. However, larger variations in the grading curves or the addition of even small contents of fines (<5%) can also alter the behaviour of a sand from stable to brittle. The response of a sand to cyclic loading is related to its response to monotonic loading. Anisotropic consolidation does not appear to alter the mobilized angle at failure and phase transformation during torsional loading.
Simulating Shear Rate-Dependent Undrained Stress-Strain Behaviour of Natural Sedimentary Clay at Kobe Airport
M.-S. Jung and S. Shibuya
ABSTRACT: Effects of strain rate on undrained shear behaviour of seabed Holocene clay at Kobe airport site were examined in a series of triaxial compression and extension tests performed using different rate of axial straining. A comparative compression test in which the strain rate was changed in steps was also carried out. Similar tests were performed in constant-volume direct shear box (DSB) test. It was found that the undrained strength increased with increasing shear rate and increasing consolidation period. Isotach properties seemed a key to govern the undrained shear behaviour. The behaviour can be described such that the stress-strain response as well as the development of excess pore pressures was uniquely related to the axial strain rate, for which softer and more contracting response was examined when the rate of axial strain decreased, and vice versa as the axial strain rate increased. In this paper, a type of Isotach stress-strain response of the clay, together with the development of drained creep strain with time was successfully simulated by newly developed models.
Experimental Investigation on Settling Behavior of Hong Kong Marine Deposits in Settling Column Condition
F. Tong J.H.Yin and G.F. Zhun
ABSTRACT: Due to the high water content and viscous property, the Hong Kong marine deposit (HKMD) frequently poses challenge to geotechnical practice, particularly to the reclamation in Hong Kong. Formation of HKMD typically goes through sedimentation and selfweight consolidation. These behaviours are time-dependent and hard to describe. A series of one-dimensional settling column consolidation tests were conducted on the HKMD. Results reveal that the settling curve and settling rate are significantly affected by the sediment concentration in self-weight consolidation. Soils even exhibit viscosity in self-weight consolidation. After the primary self-weight consolidation, the settling rate is reduced in the “secondary consolidation”. Related parameters from sketches are evaluated for a better interpretation of the consolidation behaviour.
Development of a Hollow Cylinder Torsional Apparatus for Pre-failure Deformation and Large Strains Behaviour of Sand
E. Ibraim, P. Christiaens and M. Pope
ABSTRACT: A hollow cylindrical torsional apparatus (HCTA) recently developed at University of Bristol, UK is presented. The HCTA apparatus is testing granular soils in drained and undrained, in monotonic but also dynamic loading conditions, and it is equipped with a complex strain measurement system based on high resolution non-contact transducers. The experimental developments are designed to allow the study of the pre-failure deformation characteristics and the large strains behaviour, via a continuous test on a single specimen and, thus, analyze the soil stiffness with the evolution of the strain and stress levels. While the experimental developments and apparatus capabilities in large strains have been explored and validated in an earlier study, this paper describes mainly the precision measurement system, including the assessment of its performance with reference to the soil stiffness.
Effect of High Confining Pressure on the Behaviour of Fibre Reinforced Sand
S. Ud-din, A. Marri and D. Wanatowski
ABSTRACT: Several techniques of soil stabilisation and soil reinforcement are available for improving properties of geotechnical materials. However, the addition of fibre into soils has its unique potential. This is because friction between fibres and soil particles increases bonding between the particles. As a result, the stress-strain behaviour and failure characteristics of both cemented and uncemented soils reinforced with fibres can be improved. In this paper, the influence of fibre and cement on the behaviour of sand in a wide range of confining pressures is studied. Drained triaxial compression tests carried out on uncemented and artificially cemented Portaway sand with 0.5% randomly distributed discrete polypropylene fibres are presented. The experimental results show that stress-strain behaviour and strength characteristics of Portaway sand improve with addition of fibres. However, the effect of fibre reinforcement is more significant at lower confining pressures.
At high confining pressures, the contribution of fibres is suppressed and the effect of high confinement becomes dominant.
A Comment on the Ratio of the Maximum and Minimum Dry Density for Sand
E. Imre, S. Fityus, E. Keszeyne and T. Schanz
ABSTRACT: This work reveals an interesting feature of sand behaviour: the ratio of the minimum and the maximum dry density, D, is about constant. This statement is discussed here on the basis of the analysis of two databases: the database of the “Calibration Chamber” sands and the database of the Danube sands. Results show that although the D values are significantly and systematically different for the Calibration Chamber sands and the Danube sands, the coefficients of variation of the D values for each sample population is very small in each case. The difference in the characteristic D values in each case is explained in terms of differences in the experimental methods used and differences in the geological origin of the soils.
