What is a Geotechnical Investigation?

Most projects require us to have an understanding of the subsurface conditions of a given site. In order to understand the subsurface conditions a geotechnical investigation is necessary. A geotechnical investigation comprises intrusive or non-intrusive methods to observe and test the ground.

The most common intrusive geotechnical investigation method in New Zealand is the hand auger borehole. A hand auger is a manual soil sampling tool used by geotechnical engineers or engineering geologists to drill into the ground and extract soil samples. It consists of a handle, a series of extendable rods, and a cutting head (the "auger") that twists into the soil when turned. The tool is lightweight, portable, and ideal for shallow subsurface investigations — usually down to 3–5 meters deep, depending on the soil conditions.

Hand augers are especially useful in areas where machinery access is limited, such as narrow or environmentally sensitive sites. This is also the most economical method and allows for in-situ shear vane testing.

Other intrusive investigation methods include the machine borehole which is typically rotary cored and the Cone Penetration Test (CPT). The machine borehole, as the name suggests, consists of a machine which drills into the ground and retrieves samples. This method allows for significant depths reached including in excess of 50m below ground level.

The Cone Penetration Test (CPT) is a highly accurate and efficient in-situ testing method used in geotechnical investigations to assess subsurface soil conditions without needing to drill or remove soil.

In a CPT, a cone-shaped probe is pushed into the ground at a constant rate using hydraulic equipment. As the cone penetrates the soil, sensors measure:

• Tip resistance (how hard the soil pushes back against the cone)

• Sleeve friction (friction along the sides of the probe)

• Pore water pressure (in CPTu, or piezocone tests)

Ground Penetrating Radar (GPR) and Multichannel Analysis of Surface Waves (MASW) are two widely used non-intrusive geotechnical investigation methods that provide valuable insights into subsurface conditions without disturbing the ground. GPR uses radar pulses to detect changes in material properties, making it ideal for identifying shallow utilities, voids, and soil layering, especially in dry or sandy soils. MASW, on the other hand, analyzes seismic surface waves to determine soil stiffness and shear wave velocity, which is critical for assessing ground response to seismic events and evaluating liquefaction potential. Both methods are efficient, cost-effective, and particularly useful in urban or environmentally sensitive areas where intrusive testing may not be feasible.