Numerical Modelling of Shear Wave Propagation in Centrifuge Models

Shear modulus of a soil layer increases with the effective confining stress. This results in a reduction in the propagation velocity of shear waves as they travel from the bed rock towards the soil surface. In a centrifuge model prototype stresses and strains are recreated at homologous points. Thus the effective confining stress and hence the shear modulus will vary with depth in a centrifuge model. This results in a change in the propagation velocity of the shear waves as they travel from the base of the container towards the soil surface. This change in the propagation velocity was investigated by performing non-linear finite element analyses using simple single pulse and sinusoidal ground motion as well as more realistic bed rock accelerations. Based on the results from these analyses it was concluded that the variation of shear modulus with effective confining stress results in a reduction in the propagation velocity as the shear waves travel to oil surface. Also the frequency of the input bed rock motion suffers some dispersion

A Comparison of Wavelet Analysis of Strong Motion Data of Kocaeli, Duzce and Pulumur Earthquakes, Turkey

In recent years there were several earthquakes in Turkey with magnitudes 7.4, 7.2 and 6 in Kocaeli, Duzce and Pulumur respectively. The first two occurred in 1999 causing the most damage both structurally and economically. The last happened in January 2003. These were all on the North Anatolian Fault and the earthquake occurrence was propagating towards east on the fault. Damage due to surface faulting, slope stability, liquefaction occurred during these earthquakes. In this paper the strong motion data acquired from these earthquakes at various locations are inspected using both the commonly used Fast Fourier transform and more recent harmonic wavelet analysis developed at Cambridge. Harmonic wavelet analysis is applied to some of the strong motion data obtained to observe the variation of frequency contents with time as the earthquake proceeds. With this analysis a comparison of the accelerations on the same fault was obtained. All the earthquakes had their strongest component in east-west direction and this could be seen with peak accelerations occurring in that direction. The contribution from different frequencies at different time instants to the energy of the signal was observed with the wavelet analysis and energy changes in each of the earthquakes could be seen

CPT Assessment of Boundary Effects in Dynamic Centrifuge Modelling

Dynamic centrifuge modeling requires special boundary conditions in order to minimize the effects of model containers on the performance of the soil within them, especially due to reflection of stress waves. This concern has lead to the development of the Equivalent Shear Beam (ESB) model container, which matches container stiffness to that of the soil column, the performance of which is evaluated in this paper. A series of centrifuge test involving loose and dense, dry and saturated models of homogeneous horizontal sand layers have been carried out, and measurements taken to quantify the effects of the boundaries on soil behaviour. Miniature Cone Penetration Tests (CPT) were conducted in fIight, before and after earthquake loading to investigate boundary effects on the densification of sand near the end walls during dynamic loading, and arching of soil and shear transfer to the end-walls. The influence of boundary effects is shown based on centrifuge test data by comparing CPT profiles adjacent to the end walls with those taken near the center of the model container. The results verily the uniformity of the soil model prior to earthquake loading. Also they show that the penetration resistance changes after the earthquake loading. In case of loose dry sands, there is densification at the boundary relative to the center of model. In case of loose saturated models, the densification occurred at the middle of the model relative to the boundary region

Floatation of Tunnel in Liquefiable Soil

Underground structures such as tunnels have a lower unit weight than the surrounding soil and are commonly deemed to be susceptible to floatation in liquefiable soil. In the process of floatation, the tunnel has to possess ample buoyancy force to shear and carry the overlying soil upwards. This is aided by soil liquefaction resulting from the increase in water pressure with number of earthquake loading cycles. With onset of liquefaction, effective stress decreases which lead to a reduction in the shear strength of soil, hence assisting the floatation of tunnel. Conversely, the total stress exerted by the overburden soil suppresses the process. A series of centrifuge tests were conducted to investigate the floatation of tunnels in liquefiable sand deposits. This paper discusses the initiation and cessation of the floatation as well as the floatation susceptibility of varying depths of tunnels

Liquefaction Induced Settlement of Structures

Soil liquefaction following earthquakes leads to excessive damage to a wide variety of structures. Settlement and rotation of structures following liquefaction have been witnessed in many of the recent earthquakes. Investigation of the mechanisms of failure of structure when the foundation soil suffers either partial or full liquefaction is therefore very important. Dynamic centrifuge tests were conducted at Cambridge and elsewhere on different boundary value problems in which liquefaction of soil models was investigated. Excess pore pressure data and the settlement data for the particular structure that is being investigated are recorded during the centrifuge tests. In this paper the centrifuge test results from a range of structures will be considered. The co-seismic and post seismic settlement of structures will be considered separately along with the excess pore pressure recorded generated during the cyclic loading. It will be argued that the co-seismic component of the settlement is much larger than the post-seismic settlement in many of the structures considered. Accordingly a hypothesis that the hydraulic conductivity k of the liquefied soil during the earthquake shaking is much higher than the normal hydraulic conductivity is proposed. A discussion on the micro-mechanical reasons for this increased hydraulic conductivity is presented

Effect of Liquefaction on Pile Shaft Friction Capacity

Piled foundations are commonly used worldwide, and observed failures of these foundations during earthquakes has led to active research in this area. However, the way in which piles support axial loads during earthquakes is still not fully understood. In this paper, the results from centrifuge tests are presented which consider how axial loads are carried by piles during earthquake loading. It will be shown that the piles in dry soils mobilise additional shaft friction to carry the seismically induced axial loading. However, in the case of a pile group passing through a liquefiable soil layer and founded in a dense sand layer, the pile group suffered large settlements as it loses the shaft friction in the liquefied layer and attempted to mobilise additional end bearing capacity. Further, with the post-seismic dissipation of pore pressures and the consequent settlement of the soil, the piles register significant down drag forces. This resulted in a reduction of the loads being supported as shaft friction and required further end bearing capacity to be mobilised

Application of Wavelet Theory in the Analysis of Earthquake Motions Recorded During the Kocaeli Earthquake, Turkey 1999

The Marmara region of Turkey was shaken by an earthquake with a magnitude of 7.4 and epicentre in Golcuk on August 17, 1999. Structural damage of various degrees occurred in the region. In this paper the strong motion data acquired from this earthquake at various locations are closely inspected using Fourier transform and a time-frequency technique using harmonic wavelets developed at Cambridge, Newland (1993). The advantage of harmonic wavelet analysis when dealing with non-stationary signals like earthquakes is that one can plot the signal in a time-frequency space enabling the energy distribution in the signa4 to be observed. An introduction to wavelet theory will be presented along with various methods for applying this theory to earthquake acceleration signals for analysis. Conclusions are drawn based on the application of wavelet method to the Kocaeli Earthquake strong motion data. These data is analyzed for four locations with increasing distance from the epicentre. The energy of a signal can be broken into its constituents at different frequency bands and time locations via wavelet analysis, giving insight into the localised portions of the signal. The magnitude of accelerations decreases as one moves away from the epicentre. Wavelet transform allows us to see the discontinuities within the signal and zoom in for closer inspection. Using the wavelets, it was observed that in the Kocaeli earthquake ground motions, acceleration with same frequency occurred at different time instants. This could not have been observed by traditional DFFT methods

Successful transition from fed-batch to continuous manufacturing within a mAb process development cycle

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