A.7 Monitoring

The effect of vertical drainage should be monitored by both settlement and pore pressure measurements. The measured values are used to check the actual rate of consolidation and the assumptions made in the design. It is important that the monitoring system is installed in due time before the installation of the drains, both with regard to the effect of drain installation itself (excess pore pressure due to disturbance caused by drain installation and its possible negative influence on stability) and with regard to the interpretation of the results of observation subsequently achieved.

The aim of soil improvement by vertical drainage is generally to prevent unacceptable settlement from taking place. Therefore, settlement observations are a necessary ingredient in the monitoring system.

Excess pore pressure observations by means of piezometers installed at different depths is doubtless the most appropriate way of checking that the degree of consolidation has reached the set level according to the design. The piezometers should be placed in the centre between the drains where the rate of consolidation is a minimum. However, the interpretation of the results of pore pressure measurements can be quite intricate. The results will depend on the position of the piezometer in relation to the drain (which may differ from intended position), the piezometer (the filter tip) will move downwards in the course of consolidation, the results may be affected by pore back pressure from the surroundings, gas evolution may give erroneous results etc. Moreover, the pore pressure situation after completed consolidation may not revert to its original equilibrium condition. In spite of the problems, pore pressure measurements are an important part of the monitoring system and the conclusion to be drawn about the result of soil improvement achieved should be based on both settlement and pore pressure observations.

Typical locations for observations of settlement and pore pressures for a case with homogeneous ground of limited thickness are shown in Figure A.15 and for a case with stratified ground in Figure A.16. The number of measurement profiles depends on the extent of the site and the thickness and layering of the compressible layers that are treated by vertical drainage.

Key

Key

In practice, one needs to consider the degree of consolidation achieved in the soil layers having the lowest coefficient of consolidation (usually having also the most unfavourable compression characteristics). In homogeneous soil condition, the lowest degree of consolidation is achieved where the effect of vertical one- dimensional consolidation is minimal, i.e. in the middle of the clay layer. If the discharge capacity of the drains is too low this will strongly influence the degree of consolidation achieved with increasing depth of installation. Using only surface settlement observations as a means of checking the degree of consolidation achieved throughout the soil layer may consequently lead to wrong conclusions.