Determination of land subsidence value under permanent drainage conditions at the site of highway tunnel in Moscow

Abstract

The paper discuses one of the most complicated cases in tunnel construction practice in the water-saturated fine-grained deposits in Moscow. Due to the imperfectness of enclosing engineering structures, groundwater and water-saturated soils broke through to the tunnel at the time of its construction. This resulted in the formation of decompaction zones in the enclosing ground massif as well as in deformation of buildings and land surface subsidence. After the end of construction, settling of land surface near tunnel has almost stopped; however, groundwater seepage to the tunnel still continues despite numerous percolation-control measures. As a result, the decision was made to arrange the permanently operating drainage, which could have sustain the groundwater level at the required depth. The possible settling of the land surface has been calculated for the area near the tunnel due to the soil compaction upon the water drawdown. The calculation is based on the principle of linear deformability of soils and on the Terzaghi law about the stresses of two kinds in a ground massif. The results obtained prove that the settlement values will not exceed the critical values adopted by standard regulatory documents. It appears virtually impossible to take account in calculations the transformation of ground massif parameters at the sites of soil breakthrough to the tunnel. Therefore, it is necessary to keep continuous observations over the deformation of buildings and the land subsidence in the tunnel zone. Monitoring and permanent drainage should ensure the safe operation of the tunnel.