Geotechnical survey for a wind farm: standard field and laboratory tests

In general, the geotechnical study of a wind farm should describe the terrain in order to provide the necessary information to develop the following works:

• Turbine foundation project: definition of the allowable soil bearing capacity and the most appropriate type of foundation (e.g. shallow, semi-deep, foundation on piles, etc..).
• Wind farm access roads and hardstands project: determination of the characteristics of subgrade and its possible improvement if necessary.
• Substation Project: Design of building foundations, along with the design of the earthing network (analysis of the electrical resistivity of the soil).
• Determination of the suitability of the material from excavation for use in embankments, determination of the angle of stability for slopes.

The company responsible for the geotechnical study should collect all available geological information about the area (geological maps, photo geology, and visual inspection). The purpose of this phase is to provide a first approximation of the type of materials to be able to find and confirm the type of field and laboratory tests more suitable, and if necessary to propose other type of tests or additional studies.

With this initial information, it must be prepared an exhaustive proposal for the geotechnical survey detailing all the work to be executed and the time schedule.

The type of field tests to be performed will depend on the geological nature of the materials in the area.

 Field tests

In soft soils:

Boreholes drilling to a depth of 30 meters, including standard penetration tests (SPT) every 2 to 3 meters in not cohesive layers, extraction of undisturbed samples in cohesive layers, determination of the water table and graph with the geological and geotechnical profile, photographical report of the samples and of the boreholes, diameter and type of drilling at each depth, rate of recovery of specimens, presence of water.

The number of boreholes depends on the type of materials, being at least:

• In favourable terrains, where is possible to predict homogeneity and continuity of the layers: boreholes in 40% of the positions of the wind turbines.
• In geologically complex areas: boreholes in 100% of the positions of the wind turbines.

Trial pits using a backhoe. Includes sampling for posterior laboratory test, photographic report, carrying out of the appropriate in situ tests and classification of soils for engineering purposes using unified soil classification system. Also shall be indicated the position of the water table and a subjective estimate of the consistency and permeability.

The trial pits to be done are the following:

• In all the WTG position where hasn’t been done a borehole: a trial pit in every position, up to the foundation depth or the bedrock.
• In the access roads: a trial pit every Km, or where it is considered to be representative. The required depth is from 2 to 3 meters.
• In the substation area: a trial pit in the building area, with an additional pit in the switchgear area if it is considered necessary. The required depth is from 2 to 3 meters.

In hard soils and rock:

Drilling up to a depth of 30 meters with continuous rotary drilling. Alternatives geophysical methods are acceptable (e.g. Seismic refraction profiling) but they must be technically justified. This is the case in situation where the area is clearly homogenous and made of solid rock.

In karst areas or when the presence of underground cavity is suspected an exploration of the ground using geophysical methods (micro gravimetric georadar, etc.) must be performed in order to determine the position of the cavities.

Resistivity tests. In order to provide information for the design of the grounding system of the substation electrical resistivity tests of the subsoil shall be conducted in the whole area of the substation.

The tests will be normally realized using Wenner four-electrode method arrangement or equivalent array as per ASTM D6431 – 99 (2010).

Laboratory tests

With the specimens obtained during the field works at least the following test will be done:

Classification tests:

• Determination of the particle size distribution by sieving.
• Determination of the Atterberg limits (liquid limit, plastic limit, plastic index).
• Determination of the natural water content.
• Modified Proctor test to determine the relationship between water content and dry unit weight of soils (compaction curve).
• Soil classification.

Mechanical tests:

• Direct shear test.
• California Bearing Ratio (CBR).
• In plastic, expansive or poorly consolidated clays: triaxial test.
• In rock: geological classification of the sample as per ASTM D5878 – 08 using a suitable system of classification for Engineering Purposes. Rock quality designation, rock mass rating.

Chemical tests:

• Determination of environmental aggressiveness and corrosion risk for concrete: pH and sulphate content in topsoil and water.