Following the collapse of a sewer system tunnel collector in Estonia, Fugro was called in to carry out detailed geotechnical investigations and evaluate the surrounding ground conditions to help determine why the tunnel collector had sunk.
We conducted the geotechnical investigation in two stages over a two month period. Stage one involved Cone Penetration Tests being carried out with pore pressure measurements (CPTU). Stage two identified target depths and involved drilling and sampling procedures, along with field vane shear testing.
The CPTU tests were performed at 16 locations, reaching depths of 27 to 35 metres. At certain locations and depths, where sandy soil was present, dissipation tests were carried out. In the case of soft soils, field vane shear tests were performed at certain intervals, where the limit of the test was 2 MPa.
All the CPTU, dissipation and field vane shear tests were performed by one of Fugro’s Iveco REO41 hydraulic trucks, which are equipped with an electronic piezocone and Geonor vane shear equipment, and which have a maximum pushing force of 200 kN.
Following the CPTU tests, three boreholes were drilled in the soft strata and undisturbed samples retrieved from a depth of up to 15 metres. Disturbed samples were also taken from the same depth at sandy soil locations.
The drilling procedures were undertaken by subcontracted drilling rig PBU-2A2, under the supervision of a Fugro engineer. Undisturbed samples were recovered using a pitcher sampler with Shelby tubes. The soil was described according to BS/ISO standards.
Drilling and sampling procedures were carried out using a single core barrel of 100 mm diameter, which resulted in a large amount of sand being recovered from 9 to 12 metres depth. This sand - mixed with cement - was used to perform compression strength tests.
Finally, water samples from monitoring wells were taken and laboratory tests of recovered soil and water samples were carried out. These tests were performed at accredited laboratories in Lithuania and The Netherlands, and processed data, together with laboratory tests results, were sent to the client for further studies and reporting.