Spg Madabhushi

Mechanical considerations in impaction bone grafting

In impaction grafting of contained bone defects after revision joint arthroplasty the graft behaves as a friable aggregate and its resistance to complex forces depends on grading, normal load and compaction. Bone mills in current use produce a distribution of particle sizes more uniform than is desirable for maximising resistance to shear stresses. We have performed experiments in vitro using morsellised allograft bone from the femoral head which have shown that its mechanical properties improve with increasing normal load and with increasing shear strains (strain hardening). The mechanical strength also increases with increasing compaction energy, and with the addition of bioglass particles to make good the deficiency in small and very small fragments. Donor femoral heads may be milled while frozen without affecting the profile of the particle size. Osteoporotic femoral heads provide a similar grading of sizes, although fewer particles are obtained from each specimen. Our findings have implications for current practice and for the future development of materials and techniques.

Effectiveness of vertical drains in mitigation of liquefaction

Abstract One method of mitigating the damaging effects of earthquake-induced liquefaction is to provide rapid dissipation of excess pore pressures by the use of vertical drains through the liquefiable material. Drain systems are currently designed using a chart-based approach. Field experience suggests that the performance of these installations cannot yet be accurately predicted. In this paper, high quality centrifuge testing is used to help clarify drain behaviour. It will be established, supported by centrifuge test data, that the pore water from a radially expanding zone of soil contributing to drainage through the drains is developed. Naturally, the geometry of this expanding zone changes with time. It will be shown that fluid from deeper strata is drained first, reducing the effectiveness of the drain for near-surface soil layers. It is concluded that these zones are useful in analysing more complicated drain system geometries.

Vibration-assisted bone-graft compaction in impaction bone grafting of the femur

The complications of impaction bone grafting in revision hip replacement includes fracture of the femur and subsidence of the prosthesis. In this in vitro study we aimed to investigate whether the use of vibration, combined with a perforated tamp during the compaction of morsellised allograft would reduce peak loads and hoop strains in the femur as a surrogate marker of the risk of fracture and whether it would also improve graft compaction and prosthetic stability. We found that the peak loads and hoop strains transmitted to the femoral cortex during graft compaction and subsidence of the stem in subsequent mechanical testing were reduced. This innovative technique has the potential to reduce the risk of intra-operative fracture and to improve graft compaction and therefore prosthetic stability.

Applications of wavelet analysis to the investigation of the dynamic behaviour of geotechnical structures

Abstract Harmonic wavelet analysis is a tool that has been developed over the past 15 years for the analysis of non-stationary signals in the time–frequency domain. This paper discusses the use of this technique and its application to the problems of soil dynamics and earthquake engineering. Specific reference is made to the use of this technique in investigating the performance of liquefiable slopes in centrifuge model earthquakes and the investigation of earthquake accelerograms. Harmonic wavelet analysis will be shown to be a versatile tool that can reveal information unavailable traditional time or frequency domain analysis.

Novel Computer-Controlled Saturation of Dynamic Centrifuge Models Using High Viscosity Fluids

The practice of saturating centrifuge models with a highly viscous pore fluid is widely used. As the fluid saturates the model, care must be taken to ensure that the hydraulic gradients are not too high to cause excessive disturbance to the model. A new computer-controlled saturation system (CAM-Sat) has been developed and implemented at the University of Cambridge to aid the saturation of soil models with a high viscosity pore fluid. Models are saturated under vacuum and pore fluid enters through ports in the model’s base. The system controls the ingress of pore fluid into the model by monitoring the fluid mass flux and then altering the pressure drop between a reservoir of pore fluid and the model. The system was found to be stable across a wide range of fluid viscosities and sand types. A further test showed that the system was able to respond to changes in mass flux demanded by the researcher.

Seismic response of flexible cantilever retaining walls with dry backfill

Failure of retaining walls during earthquakes has occurred many times in the past. Although significant progress has been made in analysing the seismic response of rigid gravity type retaining walls, considerable difficulties still exist in the seismic-resistant design of the flexible cantilever type of retaining walls because of the complex nature of the dynamic soil–structure interaction. In this paper the seismic response of cantilever retaining walls with dry backfill is simulated using centrifuge modelling and numerical modelling. It is found that bending moments on the wall increased significantly during an earthquake. After the end of base shaking, the residual moment on the wall was significantly higher than the moment under static loading. The numerical simulation is able to model quite accurately the main characteristics of acceleration, bending moment, and displacement recorded in the centrifuge test.

The effects of pile flexibility on pile-loading in laterally spreading slopes

Piles passing through laterally spreading slopes can be subjected to considerable loads by the soil flowing past them. Many case histories have been documented of piles which suffered failure as a result of horizontal loads exerted by the flowing soil. This paper details the results of a series of dynamic centrifuge tests carried out at Cambridge University Engineering Department, to investigate the transfer of load from the spreading soil to the piles passing through it, with particular emphasis on the effective stress state of soil elements immediately upslope and downslope of the pile. This soil stress state can be calculated by virtue of instrumentation measuring both horizontal total stress and pore pressures at locations close to the upslope and downslope faces of the piles. By comparison of results obtained for both rigid and flexible piles, conclusions will be drawn as to the effects of pile flexibility on modifying the behavior of the soil-pile system.

RESPONSE OF CONCRETE DAMS ON RIGID AND SOIL FOUNDATIONS UNDER EARTHQUAKE LOADING

Dynamic response of dams under earthquake loading depends on their foundation stiffness. Additional hydrodynamic pressures are generated not only by the ground motions but also due to the dynamic response of the dam to the ground motions. The magnitude and distribution of hydrodynamic pressures vary and these effect the deformation of dam which in turn influences the pressure. This paper aims at investigating the effect of dam–foundation interaction on the dynamic response and consequent development of hydrodynamic pressure on dam face using dynamic centrifuge modelling technique. From a series of centrifuge tests it was found that the inclusion of a flexible foundation significantly reduces the dynamic response of the dam. Significant correlation was also observed between the dynamic response of dams and the hydrodynamic pressures developed on their faces. Comparisons with theoretical hydrodynamic pressures show that Westergaards equation gives a conservative estimation of hydrodynamic pressures during most nonresonant vibrations. Comparison with Chopras method revealed that it severely underpredicts hydrodynamic pressures for low reservoir depths. However, it yields a reasonably good approximation of the hydrodynamic pressures for higher depth of reservoirs during nonresonant vibration.

Amplification of seismic accelerations at slope crests

Earthquake accelerations can cause many problems in sloping ground. One such problem is that accelerations are greatly amplified at the crest of slopes. This topographic amplification can lead to acceleration gradients along the ground surface, which could create tensile forces in long surface structures that extend between areas of different amplifications. This paper uses centrifuge modelling to demonstrate and quantify this as a problem for a particular slope configuration. A special brittle structure has been constructed to undergo damage in the presence of large differential accelerations. The structure is seen to connect the crest to the level ground behind the crest during an earthquake, reducing the amplitude of the crest motion at the expense of structural tension. Topographic amplification is shown to be a clear function of frequency, and is especially serious for loading frequencies above the natural frequency of the soil layer.

Measurement of Coefficient of Consolidation During Reconsolidation of Liquefied Sand

Saturated sands particularly at low relative density commonly exhibit rises in excess pore pressure when subjected to earthquake loading. The excess pore pressure can approach a maximum value, limited by the initial vertical effective stress. After the completion of earthquake shaking, these excess pore pressures dissipate according to the consolidation equation, which can be solved to produce a Fourier series solution. It will be shown by manipulation of this Fourier series that excess pore pressure traces provide a method for back-calculation of coefficient of consolidation C v . This method is validated against dissipation curves generated using known values of C v and seen to be more accurate in the middle of the layer. The method is then applied to data recorded in centrifuge tests to evaluate C v throughout the reconsolidation process following liquefaction conditions. C v is seen to fit better as a function of excess pore pressure ratio than effective stress for the stress levels considered. For the soil investigated, C v is about three times smaller at excess pore pressure ratio of 0.9 compared to excess pore pressure ratio of 0.