The study and monitoring of the condition of the soil massif on oil terminal shore

The work was carried out with the aim of preventive measures designing to strengthen the upper part of the soil mass, primarily at the sites of transfer oil pipeline overpass supports installation. The objective of the work was to determine the distribution of longitudinal wave propagation velocities in soils of the overpass foundation in the band of 3-4 m to a depth of 4-5 m below the distributing pipeline pillars foundation level .

The scheme of seismic-acoustic scanning of the soil massif and an example of seismic record

The value of soil massif surface subsidence under the slabs of concrete structures was established visually by drilling of holes in concrete with a diameter of 150 mm. Holes were drilled by the customer with an interval of 4-6 m along the shore of oil terminal. In greater part of holes at the sites of ground subsidence below the slabs the voids between the lower surface of concrete slabs and gravel dumping on the surface of a soil massif were identified.
On the areas of greatest subsidence of concrete slabs at the top of the bank protection wells were drilled to a depth of 8-10 m. For the estimation of the spatial distribution of velocity heterogeneities of the soil massif acoustic scanning was performed with the excitation of oscillations in the holes on the shore slope and receiving in the wells cased with plastic pipes.
Distances from the axis of well to the holes at the oscillations excitation points varied from 6 to 40 m. However, as a rule, at distances over 30 m it was rarely possible to obtain clear records of longitudinal waves in the first breaks due to the high level of noise generated by the machines of tankers and compressor stations, as well as due to insufficient energy of the impact during excitation of elastic vibrations. Thus, for each of wells it was possible to perform the scanning on the bank slope section up to 60-70 m long.

 

Vertical section of longitudinal wave velocities in one of the areas of coastal strengthening plates sagging

Vertical velocity sections are obtained along vertical planes parallel to the axis of the transfer oil pipeline overpass. The planes of the vertical cross-sections located at intervals of 1 m. Axis of the pipeline supports corresponds to a section along the plane passing through the ordinate 301,0 m. For each polygon, the resulting cross-sections for the planes with the ordinates 299, 300, 301, and 302 m were calculated. Because of the characteristics of seismic rays distribution and spatial arrangement of their refraction points velocity sections in different planes have different limits on the depth and on coordinate X. At the series of sections low-velocity anomalous zone to depths over 3-4 m are discovered, expanding towards the sea.

Monitoring of longitudinal wave velocity distribution at one of the sites of repair-and-prophylactic works

Some drilled holes in the concrete were used for injection of plasticized cement solute into voids and at intervals of soil decompaction. The other part of the holes was preserved and used to re-measure of elastic waves propagation velocity in the soil body of the shore strengthening in next two years after the repair. Below is a map of the voids under the slabs at the time of repair work start and vertical cross sections of the velocities prior to injection (a), 2-3 days after the injection of plasticized concrete (b) and 15 months after repair (b). The sections clearly show the compaction of the subsurface layer of the soil massif. The residual zone of reduced velocities near the observation well projection is associated with the soil massif decompaction in the well bottom space, which occurred during its drilling.

Manifestations of suffusion focus in soils of upper part of the coastal slope on the electric field tomography section and on P-wave velocity section

Low values of electrical resistance and longitudinal wave velocity correspond to local areas of possible manifestation of suffosion processes.