Report No. : GEO Report No. 218

Report Title : Study on the Potential Effect of Blockage of Subsurface Drainage by Soil Nailing Works (2007), 104 p.

Author : Halcrow China Limited

Abstract

An investigation has been made into the potential effect of blockage of subsurface drainage by soil nailing works. Numerical models have been set up in both 2D and 3D for various geological settings, subjected to infiltration. Infiltration events modeled have included applying an average daily wet-season rainfall in Hong Kong until a steady state is achieved and applying intense rainfall over a 4-day period to simulate one of the major rainstorms in 1982. Various run off conditions were also modelled.

Models were initially run without nails to determine base conditions. Models were then conducted including nails represented as:

1. discrete, low hydraulic conductivity elements (in 3D),
2. discrete, low hydraulic conductivity elements (in 2D). "Pseudo-K" values were assigned to the nail elements as an average for the nail and surrounding ungrouted soil,
3. grouted soil nail "horizons" to simulate lateral migration of grout so that the whole horizon had reduced hydraulic conductivity, and
4. a general zone with hydraulic conductivity reduced by an order of magnitude (3D model).

In general it is concluded that under typical conditions, where there is little grout loss during the grouting operation, there should be no significant blockage of the drainage paths and therefore little or no adverse effect on slope stability.

It is found that only if excessive grout loss were to occur and this is responsible for reducing hydraulic conductivity over extensive zones or horizons would there be significant adverse consequences for slope stability.

It is recommended therefore that where excessive grout loss occurs during installation of soil nails, the cause should be investigated and, if necessary, measures taken to monitor rises in hydraulic head and to take action to drain the ground upstream of the nails.

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