8. Interpretation of pore water pressure profiles in underdrained strata

This Paper considers the pore water pressure distribution in underdrained clay strata, and shows that the permeability variations with depth can have a significant effect on the long-term, steady seepage conditions. Both geological conditions and the engineering behaviour of soils can lead to a systematic decrease of permeability with depth, and the impact of this variation on the pore water pressure distribution leads to a bowed pore-pressure depth plot. Such a plot is nearly identical to the isochrone for a partially consolidated state in a soil with uniform permeability and compressibility, and may be misinterpreted as incomplete consolidation due to underdrainage or, in the case where few piezometer readings are available, to instrument malfunction or misreading. The observed distribution of pore water pressures with depth in a number of cases are shown to match this theory. The principal effect of the permeability variation with depth on pore pressures, and hence also on effective stresses, is to ‘insulate’ the upper part of an underdrained clay stratum from the underdrainage effect. This reduces the settlement from that computed for the underdrainage effect using homogeneous properties, and does not give the same increase in strength. It also means that the effect of groundwater recovery in the underdraining layer will be less severe in respect of the bearing capacity of foundations located at high level than might otherwise be feared. Some observations on the behaviour of tunnel linings in clay strata subject to changing pore water pressures in the surrounding soil are also given.

• Introduction

• Effect of variable permeability on the pore pressures in the long term

• Field variations of permeability

• Implications to foundations at shallow depth in an underdrained stratum

• Implications to foundations at great depth in an underdrained stratum

• Tunnels

• Conclusion

• References