Engineering Geological Surveys: Guba, Gusar

Engineering geological conditions in Guba and Gusar

The Guba and Gusar districts lie on the north-eastern slope of the Greater Caucasus, on a broad inclined plain descending from the mountains to the Caspian lowland. The land falls from south-west to north-east: higher up is the mountain and foothill zone cut by deep valleys; lower down are flat interfluves dissected by the valleys of the Gudiyalchay, Gusarchay, Velvelechay and other rivers. River terraces at several levels are clearly expressed in the valleys.

The inclined plain is built mainly of alluvial and proluvial river deposits: gravel, coarse clastic material and sand, with loess-like loams and sandy loams between and above them. On the interfluves the loam cover can be several metres thick, thinning on the river terraces. In the mountains the slopes consist of older rocks — indurated clays, siltstones, sandstones, limestones — and their weathering products, i.e. slope deposits.

Because of this variety, a plot on the flat part of Guba with sound gravel at shallow depth and a hillside plot in a mountain village call for completely different approaches.

Key construction risks in Guba and Gusar

• Mudflows. In mountain river catchments, heavy rain and snowmelt can generate mudflows. Near channels, alluvial fans and the mouths of dry gullies, the zones a mudflow could pass through and the protective measures must be defined before design begins.
• Landslides. In the mountain and foothill zone, clayey slope deposits become saturated after rain and start to move. Typical triggers are undercutting the slope, excavating for a road or building pad, and uncontrolled discharge of water onto the slope.
• River terraces and erosion. At terrace edges and on riverbanks, lateral erosion gradually wears the bank away. For structures near a terrace edge, a safe setback and the need for bank protection are assessed.
• Loess-like loams. The loess-like loams common on the interfluves may undergo additional settlement when wetted. Their collapsibility is checked in the laboratory; if the result is positive, special requirements apply to the foundation and to draining water away from the building.
• Seismicity. The slopes of the Greater Caucasus are a seismically active zone. The design seismic intensity is taken from the applicable standards and codes, and on slopes an earthquake can trigger landslides.

Gravels usually have good bearing capacity, but the loam cover above them and clayey infill between the pebbles can change how the foundation soil behaves. Whether to take the foundation down to the gravel is decided once the boreholes have established the depth and composition of that layer. Near rivers, the high permeability of gravels can flood an excavation quickly, which is taken into account in the earthworks method.

Surveys typically required

Construction in the region mostly involves private houses, leisure facilities, hotels, roads and agricultural buildings. Depending on the plot location, the following are needed:

• a topographic survey to capture the terrain accurately and, over large or hard-to-reach areas, a UAV (drone) aerial survey;
• on slopes, riverbanks and alluvial fans, a geohazard assessment;
• borehole drilling and sampling — the drilling method is matched to gravelly strata; for shallow foundations, trial pits can be more informative;
• an electrical resistivity survey to trace clay and gravel boundaries and estimate the thickness of a landslide body;
• for loess-like loams, physical and mechanical properties, including collapsibility;
• for structures founded on rock, rock strength testing;
• for a private house, a small-scope geotechnical survey for a private house.

How the work is organised

In mountain areas the site visit is especially important: we check in advance whether the rig can reach the plot, the width and gradient of the access road and the load capacity of bridges. On slopes without roads, compact or lightweight rigs are used, sometimes hand-dug trial pits. Field work is scheduled around the rainy season and the condition of mountain roads.

Samples are packed on site, the moisture of undisturbed samples is preserved, and they are delivered to the laboratory as quickly as possible. Distance has a direct effect on the price: transporting equipment, accommodating the crew and shipping samples are separate cost items. Combining several sites in one trip, or completing all work in a single mobilisation, reduces them. To get a price, send the plot coordinates or its position on a map via a quote request.