Mladen Vucetic

A direct simple shear device for measuring small-strain behavior

A simple shear device named the double specimen direct simple shear (DSDSS) device was designed to investigate static and cyclic properties of soils at small strains, such as cyclic stress-strain loops, maximum shear modulus, Gmax, secant shear modulus, Gs, and the equivalent viscous damping ratio, λ. Cyclic shear strains between 5 × 10−4 and 2 × 10−2% were successfully applied and measured. Such measurements of very small strains were achieved by completely eliminating the friction of the load-transfer mechanism and by reducing the effects of the mechanical compliance of the device to practically zero. This was facilitated by using a special configuration of two parallel specimens. The performance of the DSDSS device was examined first by testing rubber dummy specimens of different stiffnesses and then by conducting a series of tests on kaolinite clay consolidated under different vertical consolidation stresses and over-consolidation ratios. The results produced by the new device show very consistent trends and are in good agreement with equivalent data obtained by others using other types of tests and equipment.

Cyclic Triaxial Strain-Controlled Testing of Liquefiable Sands

Two series of undrained cyclic triaxial strain-controlled tests were performed on two different Imperial Valley, California, silty sands which liquefied during an earthquake in 1981. Both intact and reconstituted specimens were tested, and the testing procedures are described. The experimental data confirm that cyclic shear strain is the fundamental parameter governing pore pressure buildup, because strain-controlled tests essentially eliminate the influence of specimen fabric and sample disturbance. Also, the results indicate that the cyclic triaxial test can be used to model cyclic simple shear (similar to seismic field conditions), if the cyclic simple shear strain, γ c y , is related to the cyclic triaxial axial strain, ∈ c y , by either of two similar analytical expressions: γ c y = 1.5 ∈ c y or γ c y = 3 ∈ c y . Consequently, a unique pore pressure model is developed and recommended to simulate the seismic pore pressure buildup at the site. This model is applicable to reconstituted and intact specimens of the two sands, despite their different void ratios and nonplastic silt contents, and is valid for both cyclic triaxial and cyclic simple shear strain-controlled conditions.

Effect of Frequency and Vertical Stress on Cyclic Degradation and Pore Water Pressure in Clay in the NGI Simple Shear Device

AbstractThe effects of the frequency of cyclic shearing and vertical consolidation stress on the cyclic degradation and cyclic pore water pressure in normally-consolidated kaolinite clay having PI=28 were investigated with the help of the cyclic strain-controlled Norwegian Geotechnical Institute (NGI) simple shear test. The testing program encompassed three cyclic shear strain amplitudes, γc=0.1,0.25,and 0.5%, two vertical effective consolidation stresses, σvc′=220 and 680 kPa, and three frequencies, f=0.001,0.01,and 0.1 Hz. The results reveal that the cyclic degradation parameter, t, which measures the rate of cyclic degradation with the number of cycles, increases substantially with f and decreases with σvc′. If f is increased 10 times, t may increase by 20–50%. If σvc′ is increased from 220 to 680 kPa, the t parameter may decrease by 20–38%. The cyclic pore water pressure normalized by σvc′ decreases with f and σvc′.

Dynamic centrifuge testing of soil-nailed excavations

A series of dynamic centrifuge tests was conducted on models of soil-nailed excavations. The models were subjected to various levels of horizontal shaking to investigate the seismic stability of their prototypes. The scaling factor was 50 in all tests, the depth of the prototype excavation was H = 7.6 m, and the length of nails in each test varied between 0.33 H and 1 H. The tests revealed the most probable failure mechanism under strong shaking and at the same time showed that soil nailing is an excellent method for earth support in seismic regions. The paper describes details of the testing procedure. The procedure includes selection and preparation of the soil, selection and design of the nails and facing, step-by-step filling of the model box, rough simulation of possible geologic history of the soil, excavation and installation of the nails and facing, cyclic shaking, measurements of accelerations and displacements, excavation and incisions of the nailed soil after the tests, and data processing. The investigation showed that centrifuge testing can successfully simulate dynamic soil-structure interaction for complex geotechnical systems such as soil nailing. However, to incorporate pertinent details of the prototype into the model and to provide the proper boundary conditions, a carefully planned step-by-step testing procedure must be implemented. Limitations of the testing and problems encountered are also discussed.

Small-strain Cyclic Testing With Standard NGI Simple Shear Device

In the standard Norwegian Geotechnical Institute type of direct simple shear (NGI-DSS) test, the horizontal load cell record includes several false loads which do not represent the soil shear resistance. In the case of cyclic loading, the relative magnitude of these false loads increases as the cycle shear strain amplitude γc decreases. Consequently, in order to obtain accurate cyclic soil behavior at small γc, the NGI-DSS test records must be corrected to address the errors due to these false loads. This paper describes how, with the help of modern computer technologies, the false loads can be identified, quantified, and then subtracted from the load cell record. In an example, the proposed correction procedure is successfully applied to values of γc as small as 0.026 %. With this procedure, the accurate equivalent viscous damping ratio, the variation of the secant shear modulus with the number of cycles, and other cyclic properties can be obtained at γc as small as 0.01 %.