Investigations of the soil condition of the coastal slope and the port water area

Geophysical studies were performed after the large-scale dredging works in the port aquatory. As a result of the bottom deepening along the piers, the steepness of the shore slope and the angles of the bottom slope along the boundaries of the deepening works increased. With the increase of accepted tankers tonnage, lithodynamic processes on the bottom of backwaters and, to a large extent, on the coastal slope were sharply intensified. The soils of the coastal slope under a layer of loose sands are represented by weak marine sands, lake-sea loams and lake-glacial deposits, easily subjected to suffocation and mechanical effects of turbulent flows from the working screws of large tankers. One of the negative results of this was the removal of fine fractions of sand from the ground base of shore protection, the formation of extensive zones of decompaction and voids under concrete slabs. During heavy storms and water level rising due to the water surge, cases of concrete slabs subsidence became more frequent.

Deformation of backwater seabed and coastal slope

To assess the deformations of the seabed echosounding measurements data were used and high-frequency channel of the side-scan sonar survey. Soundings were performed along profiles oriented mainly along the quays. The profiles along the shore were made as linking ones. The velocity of acoustic waves in the water was taken from the calibration tables taking into account the temperature of the water and its salinity. During the measuring work the water temperature was + 6 degrees, so the velocity of sound in the water was taken 1436 m/s.
The accuracy of the depth determination was of ±0.1 m. The distance between the profiles was 20 m. Depth determination points interval along the profile in average 4 m. The detail of the measurements matches the scale of 1:500.
The obtained data were compared with the data of measuring works performed a year earlier, immediately after dredging in the backwaters. The most noticeable deformations of the opposite sign were revealed along the sides of the soil excavations and along the bank slope. As can be seen from the map below deformations of the bottom topography along the borders of the excavations slumping of their sides took place and raising the elevations of the bottom inside of excavations. The height of sediment accumulated along the sides in some places exceeded 1 m.
Along the foot of the bank slope, mainly, the lowering of the bottom marks was observed due to sediment leaching during tanker screw working. In the southern backwater at the initial part of the second pier a large positive deformation was revealed – accumulation of a soil layer up to 1.5 m high taken out of the coastal slope. Later, according to the drilling of a set of 3 wells on the adjacent section of the shore and non-londitudinal VSP performance in them, a significant anisotropy of elastic wave velocities in the soils massif was revealed with a sharp velocity decrease in direction to the coastal slope. In subsequent years, studies of the coastal slope under the plates of shore protection in this area confirmed the high speed of suffosion processes.

Subbottom profiling in backwaters and shallow seismic exploration on the shore

Works for soil properties investigations included the side-scan sonar survey and subbottom profiling in the water area. Cone penetration testing, drilling of geotechnical wells, vertical seismic profiling and sallow seismic profiling by the method of reflected waves (CDP) were performed on the shore. Despite the significant difference in the frequency range of elastic waves when working on land and in the water area, it was possible to link the main reflecting horizons in the depth range up to 85 m.

Cone penetration testing and vertical seismoacoustic profiling

The records of non-longitudinal VSP in wells, cased with plastic pipe, along with the first arrivals of the direct longitudinal wave has a distinctly intense outgoing and upgoing hydro-waves which sources the harmonics of low-frequency surface wave are coming from the shock source to a wellhead at the surface of a soil massif. Hydrowave velocity closely connects with the transverse wave velocity in space of well trunk and can be used to calculate elastic modules of the soil massif.

The anisotropy of surface wave velocities in the layer of bulk soils and marine sands underlying them is clearly manifested in the records of non-longitudinalt VSP from two remote sources located perpendicular in the direction from the well with receivers along the coastline at a distance of about 25 m from the beginning of the slope and in the direction from the well towards the coastal slope. The energy attenuation and delay of the surface wave modes arrival to the wellhead in the direction perpendicular to the shore are well manifested on the sounding records.

Stratigraphic control of seismic and acoustic sections

Cone penetration test data and VSP was used for stratigraphic control of reflections from the boundary layers, singled on the subbottom profiling sections in the backwaters. The layers of Quaternary soils and moraine are uplifted towards the open sea with the exit on the bottom of dense moraine soils in remote parts of the water area.

Seismogeological section of the soil massif in the backwater waters

Reconstruction of the bottom relief and its reflectivity on the backwaters area

A map of the reflectivity of the seabed soils, depending on the magnitude of their acoustic impedance and, accordingly, the density imposed on the volumetric reconstruction of the bottom relief, gives an idea that on the coastal slope and on the greater coastal part of the water area the bottom is represented by weak Quaternary soils. In these areas of the backwaters it is necessary to expect significant deformation of the bottom relief associated with erosion and re-deposition of bottom sediments. It may occur in shallowing of bottom areas along piers and erosion of the foot of the coastal slope in zones of intensive influence of turbulent flows from operating screws of tankers.