Examination of the underground construction site, Moscow, Russia
March 25, 2014
Construction site.
In the course of soils jet grouting at an underground construction site at depths of 21-26 meters, soils, different from the ones given in the drilling report, as well as a constant flow of water, were found.
The grouting compound, chosen in advance in accordance with the topographic plan, wouldn’t curdle, so the works were paused till the problem would be solved.
The objective of Timer CJSC was to compare the geological section, based on the drilling data, with GPR data to verify the structure of the soil.
To obtain maximum information about the structure of the soils to a depth of 50 meters, our specialists used a GROT 12 low-frequency georadar and a GROT 12H high-frequency georadar with 2 and 4 meters long antennas accordingly with the frequencies of 75MHz and 37MHz respectively.
Ground-penetrating radar at work.
It took us two days to cover 20 tracks with a total length of 436.5 meters and to carry out measurements at 1504 points to a depth of 50 meters.
Measurements near the construction site confirmed that the soils lithological structure beyond the construction site matches the topographical plan data.
The GPR survey says that the first waterproof clay layer is situated at depths of 6-13 meters and lies below the water-saturated one.
Fig. 3D model based on two radargrams.
The three-dimensional model of the site shows that the deformation of the soils structure and zones with high conductivity are present.
GPR data proved that the deformation was caused by a recent construction of a grillage at the construction site. The grillage’s stilts reach the depth of 18 meters, so they damaged the horizontal structure of adjacent soils, where zones of high conductivity at depths of 10 meters and greater were detected. The water-saturated upper clay soils lowered to depths of 21-26 meters, where the works took place.
The geophysical studies indicated that the construction of the grillage resulted into the deformation of the water-resistant loam layer, and that led to:
1 – the relocation of the clay fraction to the area under development;
2 – the formation of the water flow, caused by seasonal rains and melting snow.
Early GPR research allowed reworking the underground developing plan with respect to the changes of the soils structure and their hydrogeological characteristics in general.