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

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

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

Seismic Behaviour of Water Front Structures with Tyre Chip Backfill

Water front structures have suffered significant damage in many of the recent earthquakes. One of the primary causes for the poor performance of these classes of structures is the liquefaction of the foundation soil and in some instances liquefaction of the backfill soil. The liquefaction of the soil in-front of the quay wall tends to cause large lateral displacements and rotation of the wall. Full or partial liquefaction of the backfill can result in the increase of lateral earth pressure exerted on the wall that can cause additional lateral displacement of the wall. In this paper numerical analyses of a gravity wall type water front structure will be considered. Often such gravity walls are placed on rubble mound that is deposited onto the sea bed. The problem will be based on a generic model although the simplifications in the generic model were derived based on observed failures of quay walls following the 1995 Kobe earthquake. The paper presents finite element analyses of such a problem in which strength degradation of the foundation soil and the backfill material will be modelled using PZ mark III constitutive relationship. At the Port and Airport Research Institute (PARI) in Japan the possibility of using tyre chips from used car tyres as the backfill material is being researched using 1G underwater shaking table and dynamic centrifuge modelling. The finite element analyses will be repeated by including a zone of backfill consisting of the tyre chips. The properties of this material will be derived from the element tests carried out at PARI. Finally the results from the analyses of the gravity wall founded rubble mound with liquefiable foundation soil and backfill will be compared to those with tyre chip backfill. The improvement in the performance of the wall in terms of decreased lateral displacements and/or reduction in the rotation suffered by the wall will be compared

Role of Pile Spacing on Dynamic Behavior of Pile Groups in Layered Soils

This research investigates the influence of pile spacing on the dynamic behavior of pile groups by performing a series of specifically designed dynamic centrifuge experiments on pile foundations embedded in a two-layered soil profile. A single pile and two 3×1 row pile groups with different pile spacing were used as model pile foundations, and the soil models consisted of a soft clay underlain by dense sand. The influence of earthquake frequency on the dynamic behavior of two-layered soils is discussed using the centrifuge data and 1D site response analysis from DEEPSOIL. Further, the results of these centrifuge tests agreed with the conviction that the group effects will be diminished with the increase in pile-to-pile spacing in a pile group due to reduced pile-soil-pile interaction. However, these reduced pile group effects can lead to larger kinematic pile bending moments in the widely spaced pile group compared with a closely spaced pile group. Moreover, the single pile always has larger bending moments than both the tested pile groups—an exception to this is when there is a significant phase difference between the kinematic and inertial loads for a single pile but not for the widely spaced pile group. The influence of pile spacing on the shadowing effects and location of peak bending moments in the piles of a group are also discussed in this paper. Lastly, an attempt is made to evaluate the individual contribution of inertial and kinematic loads for the seismic design of pile foundations considering soil-pile-structure interaction effects

Centrifuge Modelling of Earthquake Effects in Uniform Deposits of Saturated Sand

Centrifuge models representing level uniform saturated deposits of relatively loose and dense sand were tested at Cambridge University\u27s Schofield Centre to clarify the behaviour of these deposits under earthquake loading. The excess pore pressure, vertical propagation of the accelerations and ground surface settlements resulting from a model earthquake are presented and discussed. The results show that, for similar dynamic loading, the models undergo large shear stiffness degradation resulting from significant pore pressure build up, this taking place at a slower rate in the dense sand. As a result of the cyclic loading, the models suffer settlements, occurring mostly during the event, that are noticeably smaller in the dense model. The upwards propagation of the accelerations through the model depends on the relative density of the sand and changes during the seismic event, following degradation of sand mechanical properties. Large short-duration acceleration spikes are observed near the surface of the dense model, corresponding to large amplification of input acceleration. The results presented and discussed contribute to the understanding of the basic mechanisms of earthquake-induced liquefaction and the use of densification as a measure to mitigate its effects