Aj Brennan

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.

Consolidation of Lumpy Clay Backfill Over Buried Pipelines

Offshore pipelines are usually buried to protect th e pipe from external loads. When trenching is achieved by jetting or ploughing, some clayey soils can be cut into distin ct lumps and this lumpy soil is then used as the backfill materi al under which the pipe is buried. To counter the effects of uphea val buckling, the resistance of the soil to pipe uplift must be k nown. There is still uncertainty about the performance of lumpy ba ckfill in this regard. A series of centrifuge tests were performed with such soils as backfill, utilising a specially designed p ore-pressure measuring pipe, to determine the influence of lump size, lump shape and pullout rate on uplift resistance Backfil l comprising larger lumps consolidates quicker than if the backf ill lumps are smaller. It is also observed that backfill comprisi ng larger lumps provides greater resistance to pipe uplift after co nsolidation.