Why do I need an in-situ test?

We carry out this dynamic test in boreholes during the site investigation process. The test provides a means of estimating the relative density of granular soils and shear strength characteristics of cohesive soils.

We drive a standard 50mm diameter split barrel sampler (cohesive soils) or a 60° cone (granular soils) into the ground at the base of the borehole through repeated blows from a drop hammer of mass 65kg, falling a distance of 760mm. The test is driven a total of 450mm into the soil and the number of blows recorded for the final 300mm penetration is recorded as the ‘N’ value.

We can assess the undrained shear strength of cohesive soils using handheld instruments such as a hand shear vane, which records the resistance to shear against a rotational force applied by hand equal to one revolution per minute. We can also assess the shear strength by Pocket Penetrometer, a device which measures the resistance of a piston of 0.32sq/cm being inserted into the soil under test.

We can obtain concrete cores at various diameters, typically 100mm, 150mm or 200mm using rig mounted and handheld coring equipment, to allow an assessment of an existing floor slab to be made.  The core allows a visual assessment to be undertaken, identifying the size and nature of clasts, the matrix and whether a reinforcing bar is present.  The recovered core can then be tested for its unconfined compressive strength.  Cores can also be obtained by horizontal coring through concrete walls.

Soakaway Testing allows us to assess the drainage properties of the site. We dig a 2-metre pit, fill it with gravel and pour in water. Once filled, the water level will be measured and monitored to ascertain the rate at which it soaks away; hence the name ‘Soakaway’. These results will determine the drainage strategy and whether additional new drainage is required, including the installation of Sustainable Drainage Systems (SuDS).

The Plate Bearing Test can be undertaken using a 300mm, 450mm or 600mm diameter plate to obtain the in-situ deformation characteristics of soil, the results can be provided as an equivalent California Bearing Ratio (CBR), Modulus of Subgrade reaction (k₇₆₂) or ultimate bearing capacity (kN/m²).

The California Bearing Ratio (CBR) test is an empirical measure of soil strength. It is most often used on subgrade for road design and construction to determine the thicknesses of materials to be used. The stronger the material being tested, the higher the CBR value, which can lead to thinner layers of material being used, which in turn can reduce construction cost.

The CBR test often forms part of the overall site investigation and is useful for determining the thickness of materials needed for the proposed construction.

We can obtain the CBR value of a soil by several means. This could be through Plate Load Testing, when information for pile mat design or for mobile crane operations is required. We can also assess the CBR value of the soil using more rapid means such as a Dynamic Cone Penetrometer (DCP), which allows a profile of the shallow soils to be obtained to a depth of c. 1mbgl. We can also assess CBR values of the ground at discrete depths using the Clegg Hammer.

The LWD test measures the bearing capacity of soils placed in earthworks. The results obtained can be converted to an equivalent CBR value, confirming the suitability of the placed material.

The LWD utilizes a 300mm diameter plate and the zone of influence extends to between 1 and 1½ times the plate diameter making it ideal for testing layers upto 450mm thickness.  The LWD allows rapid testing of placed and compacted material.