Geotechnical Investigations & Testing Methods

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What are Geotechnical Investigations?

Geotechnical investigations are performed to obtain data on physical characteristics of soil/rock around a site to design earthworks & proposed structures, or to support the repair of distressed earthworks/structures caused by subsurface issues.

Geotechnical Investigations Available From Geomechanics

Geomechanics 25 yearsGeomechanics cc is industry leader in world-class geotechnical investigations in Southern Africa. We have 25 years’ experience carrying out successful geotechnical investigations to the minerals exploration, geotechnical drilling & civil industries. With that in mind, here is the list of available geotechnical investigations available from GeoMechanics. If you are looking for a suitable partner look no further than Geomechanics.

1. Vane Shear Testing

The vane shear test is an in-situ geotechnical investigation used to estimate the undrained shear strength of fully saturated fine grained clays and silts and other fine geomaterials. It cannot be used on unsaturated or highly permeable soils.

Procedure

The test is conducted using a four-blade stainless steel vane attached to a steel rod that is pushed into the ground.  The vane is pushed vertically into the soft soil and is then rotated at a slow rate of 60° per minute. The torque is measured at regular time intervals (15-30s) and the test continues until a maximum torque is reached and the vane rotates rapidly for several revolutions. This is when the soil fails in shear on a cylindrical surface around the vane. The rotation is usually continued after shearing and the torque is measured to estimate the remoulded shear strength.

The undrained shear strength of the saturated soil is proportional to the applied torque and the dimensions of the vane. The results provide a simple and convenient index of shear strength and guidelines for foundation construction.

2. Permeability Testing

Soil permeability is a measurement of the rate water moves through soil. Soil permeability is governed by the makeup of the soil. The rate of soil permeability affects engineering and planning for structures and helps determine footings in buildings to prevent settling.

Applications

  • Estimation of quantity of underground seepage water under various hydraulic
  • conditions
  • Quantification of water during pumping for underground construction
  • Stability analysis of slopes, earth dams, and earth retaining structures

There are several methods used to measure and quantify soil permeability. Perculation, or perc, and Lugeon Testing are field tests. 

Percolation Testing

In a percolation test, holes are dug, usually with a post hole digger, in the test area. Test holes of at least 6 inches in diameter and at least as deep as the planned construction are required.

The hole is filled with water and left overnight. If there is still water in the test holes the next day the hole is bailed out until 6 inches of water remains. If all water has drained away water is added until there is 6 inches of water in the hole. The drop in the water level is measured every half hour until three consecutive readings vary by less than 10 percent. The drainage times are used in a formula to calculate the percolation rate.

Lugeon Testing

A Lugeon test is a single-hole in situ test of formation permeability performed by measuring the volume of water taken in a section of test hole when the interval is pressurized at 10 bars (150 psi). Used primarily in variably permeable formations under evaluation for grouting. 1 lugeon is 1 litre per minute per meter (l/min/m) at a pressure of 10 bars.

3. Piezometer Installations

Standpipe piezometers are used for long term monitoring of ground water tables in proposed cut and fill areas. They are also useful in determining whether there is an upward or downward gradient of water.  

Procedure

The height to which the water rises in the hose is a direct measure of the water pressure of the soil at the depth the piezometer tip is installed. There are two types of standpipe piezometers  - slotted (open) and porous stone. The latter is smaller and is used in smaller specific zones or in material containing a high silt content.

4. Pressure Meter Testing (PMT)

Pressuremeter Geotechnical Tests can be done in soft clay or loose sands, but are particularly valuable in dense sands, hard clays and weathered rock.

  • The test can be used to design shallow foundations using bearing capacity and settlement criteria,
  • the pressuremeter limit pressure can be used to design for vertical capacity of deep foundations

Procedure

The PMT is typically performed by inserting a cylindrical probe into an open borehole, supporting it at the test depth, and then inflating a flexible membrane in the lateral direction to a radial strain of as much as 40% depending on the probe design.  The PMT operator may expand the pressuremeter probe in equal pressure increments (stress controlled test) or in equal volume increments (strain controlled test) typically stopping the test when the initial volume of the probe has doubled or when reaching the maximum allowable pressure. 

The PMT results include the at-rest horizontal earth pressure, the pressuremeter elastic modulus, the reload modulus, and the pressuremeter limit pressure (plastic failure).

Good test results begin require a high quality borehole with minimal disturbance to its side walls, typically requiring mud wash rotary techniques. 

The PMT remains one of the most labor‑intensive and time‑consuming in-situ tests. 

5. Standard Penetration Testing

The standard penetration test (SPT) is an in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil. It is simple and inexpensive method for getting an indication of the relative density of the ground in gravels, sands, silts, clay containing sand or gravel and weak rock from which it is difficult to get undisturbed samples. 

Procedure

A thick-walled sample tube, with an outside diameter of 50 mm and an inside diameter of 35 mm, and a length of around 650 mm  is driven into the ground at the bottom of a borehole by blows from a slide hammer with a weight of 63.5 kg falling through a distance of 760 mm. The sample tube is driven 150 mm into the ground and then the number of blows needed for the tube to penetrate each 150 mm up to a depth of 450 mm is recorded. The sum of the number of blows required for the second and third 6 in. of penetration is termed the "standard penetration resistance" or the "N-value". In cases where 50 blows are insufficient to advance it through a 150 mm (6 in) interval the penetration after 50 blows is recorded. The blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae.

6. Penetrometer Testing

The cone penetration test (CPT) is a cpt method used to determine the geotechnical engineering properties of soils and delineating soil stratigraphy.

Procedure

The test method consists of pushing an instrumented cone, with the tip facing down, into the ground at a controlled rate (controlled between 1.5 -2.5cm/s). The resolution of the CPT in delineating stratigraphic layers is related to the size of the cone tip, with typical cone tips having a cross-sectional area of either 10 or 15 cm², corresponding to diameters of 3.6 and 4.4 cm.

CPT is more popular compared to SPT as a method of geotechnical soil investigation because of its increased accuracy, speed of deployment, more continuous soil profile and reduced cost.