A geotechnical investigation assesses the ground properties at a site where buildings or other construction is planned, to establish whether the ground conditions are suitable for the intended land use.
It is achieved by inspecting and testing soils, rocks and other materials (e.g., possible fill), below the Earth’s surface. It is a necessity in all building and construction projects and provides information on the ground nature, its strength, contamination, groundwater conditions and other characteristics.
Soil investigations can also reveal potential problems with the soil, which may impact the design and construction of the project and allow to accordingly adjust the budget of the proposed development.
How these tests are conducted
Douglas Partners’ sister company, Ground Test, assists geotechnical investigations with the use of geotechnical drilling rigs and other specialised testing equipment.
A geotechnical investigation employs a variety of testing methods, used in the field to assess the ground conditions, and to retrieve soil and rock samples for subsequent testing and interpretation. These include:
Test Pits and Test Trenches
Test pits and test trenches are excavated on site. The test holes are commonly up to 3 m depth and are often dug during the design phase of the project, providing engineers the opportunity to assess the soil and rock conditions to inform on suitable design and construction operations. They are more cost effective than drilling work for shallow testing, and can help identify issues early in a project, which will avoid unexpected costs and delays to the project timeline.
Geotechnical drilling requires a drill rig and a drill crew to test, sample, and inspect the soil and rock to great depths (i.e., if tall buildings are proposed). The resulting borehole can be equipped with a piezometer to monitor groundwater seasonal fluctuations.
In-situ tests are conducted at ground surface, within shallow test pits and deep boreholes, or on the collected samples in a specialised soil and rock laboratory. Testing, both in the field and in laboratory, allows a more accurate assessment of subsurface conditions and subsequent better data analyses and an informed geotechnical design. Examples of typical field-testing methods include Standard Penetration Tests (SPT) and Cone Penetrometer Testing (CPT).
Five key geotechnical tests
SPT is an in-situ test carried out at regular interval depths during the drilling of a borehole. Each SPT involves repeatedly dropping a heavy hammer on a tube sampler that penetrates the bottom of the borehole. The number of hammer ‘blows’ are recorded to provide an indication of the ground strength at the base of the borehole, and soil at the base of the borehole is sampled at the same time into the hollow tube sampler for subsequent inspection and possible laboratory testing. SPT results are used to determine the soil type and its various characteristics such as bearing capacity, density, and angle of shearing resistance.
CPT is another in-situ testing technique used to determine the geotechnical engineering properties of soils and is well suited to delineate soil stratigraphy because it returns a continuous soil profile (unlike the SPT that is performed at discrete depths). It is performed by pushing a specialised cone-shaped probe at a constant rate along the soil profile, using a ballasted rig. Gauges at the tip and along the shaft of the probe measure the tip and shaft resistance of the soil against the probe. The recovered CPT data are useful to identify soil types and can be corelated to numerous geotechnical properties. CPT is considered more cost effective than SPT in suitable ground conditions, although it is a blind test where no samples are recovered for inspection.
Soil compaction testing is carried out in field (generally using a nuclear gauge) and in a geotechnical testing laboratory, to verify that the soil has been compacted in accordance with the project specifications. Soil compaction is simply when soil particles are pressed closer to each other, reducing voids between them. Soil compaction prevents soil settlement and frost damage, increases ground stability, reduces soil permeability, and mitigates undesirable settlement of structures. In the construction of earth structures such as dams, paved roadways, and for construction projects that rely on the stability of embankments, soil compaction is used to increase soil strength.
Rock testing determines the strength of rock and is used to set design parameters for new developments such as tunnels, an excavation for a deep basement, or a cutting for a road or railway. Laboratory tests can inform on the following properties of rock: Young’s Modulus, Poisson’s Ratio, unconfined compressive strength, point load strength index, rock tensile strength (Brazil Splitting), slake durability, moisture content and resistance to salt attack.
Concrete testing is used as a quality indicator to measure factors such as strength, consistency, unit weight, air content, and temperature, and confirm how it compares to standard concrete specification.
Performed in a laboratory, it ensures that the concrete of suitable quality is placed at the site so that concrete structural members of targeted strength are obtained. Concrete testing is used in various applications, including pavements, bridges, drainage culverts and headwalls, industrial, commercial, and residential buildings, precast walls, retaining walls and driveways.
NATA accredited laboratories
Most of Douglas Partners’ 20 locations across Australia are equipped with NATA accredited laboratories. This means our testing is compliant with current Australian Standards and International Standards such as British Standards (BS), American Society of Testing and Materials (ASTM), and various State and Territory test methods.
With an in-house laboratory capability, we can ensure our clients receive reliable service with fast and accurate turnaround of test results, as well as a fully integrated LIMS software that allows clients to access their test reports on-line.
Each laboratory is staffed with highly qualified technicians who work with our clients to provide unique and innovative solutions that are technically correct, easy to understand and practical to implement.