Pore pressure consolidation settlements

A tailings disposal facility is provided with an underdrain to enhance consolidation and settlement. To reclaim the land it is proposed to place free draining material on top of the tailings at a rate of 1 m/yr (15 kPa/yr) for a period of 4 years. Use the spreadsheet to explore the consequences on the pore pressure distribution and settlement of the under-drain clogging up at various times after the land reclamation starts. The tailings deposit is 10 m thick and has Cy = 2 m /yr and my = 0.0002 m /kN. 

Assuming that consolidation and settlement are one-dimensional, use the numerical method to investigate the effect of the magnitude of the maximum pre-load stress, Q, on the excess pore pressures and the settlement, time response of the road. 

The primary consolidation settlement (sj occurs due to the slow expulsion of the pore water and hence the dissipation of the excess pore water pressure generated by the application of the static service loads at the rates normally expected of permanent loads. As the excess pore water pressure dissipates, the net effective vertical stress on the soil increases as the primary consolidation continues to occur. 

The secondary consolidation settlement (s ) occurs after the completion of the primary settlement (i.e., after the complete dissipation of the generated pore water pressure) due mainly to the reorientation of the soil particles under essentially constant effective vertical stress. 

A surcharge usually consists of a temporary load of sand or other material, which is placed on top of the reclaimed area that needs to be consolidated. Once sufficient consolidation (usually expressed in terms of settlement or dissipation of excess pore pressure) has taken place, the surcharge can be removed and construction activities may start. The thickness of a surcharge may vary considerably, but will generally be in the range of 2-10 m (i.e., a surcharge of approximately 35 kPa 180 kPa when placed above the water table). 

If the organic content of a soil exceeds 20% by weight, consolidation becomes increasingly dominated by the behaviour of the organic material (Berry and Poskitt, 1972). For example, on loading, peat undergoes a decrease in permeability of several orders of magnitude. Moreover, residual pore water pressure affects primary consolidation, and considerable secondary consolidation further complicates settlement prediction. 

Settlement can present a problem in clayey soils, so that the amount that is likely to take place when they are loaded needs to be determined. Settlement invariably continues after the construction period, often for several years. Immediate or elastic settlement is that which occurs under constant-volume (undrained) conditions when clay deforms to accommodate the imposed shear stresses. Primary consolidation in clay takes place due to the void space being gradually reduced as the pore water and/or air are expelled therefrom on loading. The rate at which this occurs depends on the rate at which the excess pore water pressure, induced by a structural load, is dissipated, thereby allowing the structure to be supported entirely by the soil skeleton. Consequently, the permeability of the clay is all important. After sufficient time has elapsed, excess pore water pressures approach zero, but a deposit of clay may continue to decrease in volume. This is referred to as secondary consolidation and involves compression of the soil fabric. 

The FEM is in the first place a method to calculate deformations (horizontally and vertically). The analysis of time-dependent settlements is possible as well, although the calculation of the degree of consolidation will require the input of the horizontal and vertical permeability of the soil and the computation of groundwater flow (dissipation of excess pore-water pressures) in the FE mesh. 

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