Insitu testing of soils

[edit] Introduction

Insitu testing can be used to determine the density or shear strength of soils, particularly sands, gravels, saturated clays and silts. The main advantage of testing in this way is that it can be undertaken without undue disturbance of the soil.

As well as soil property information, insitu soil testing allows for groundwater pressure to be measured and moisture content data to be gathered, which can be safety-critical factors.

There are four main tests are:

• Standard penetration test.
• Vane test.
• Plate bearing test.
• California bearing ratio (CBR).

[edit] Standard penetration test

This test involves measuring the soil’s resistance to penetration under static or dynamic loading. It is commonly used for testing sands and gravels, and can establish the relative density of soils.

A 35 mm (internal diameter) split-barrel sampler (a central tube that splits in two when unscrewed) is driven into the soil at the base of a bore hole. It is first driven 150 mm into the soil, or given 25 blows, by a standard weight. This is usually 65 kg falling 760 mm. The sample is driven another 300 mm and a record is kept of the number of blows required to achieve each further 75 mm of penetration.

The table below gives some sample data results (for illustrative purposes only).

Sands:

Clays:

Data adapted from ‘Introduction to Civil Engineering Construction’ (Holmes, R.)

[edit] Vane test

This test is used to estimate the shear strength of soft cohesive soils. It is common used in saturated clays, and will be yield unreliable results if the clay contains silt or sand.

The apparatus consists of a four-blade stainless steel vane attached to the end of a steel rod. This is inserted into the soft clay and hand-rotated at a constant rate. The amount of torque (how much force is required to enable rotation) is measured and from this the shear strength calculated. The advantage of this test is that it is time and cost efficient and also does not disturb the soil.

[edit] Plate bearing test

This test is used as a means of determining the ultimate bearing capacity of the ground and the likely behaviour of soils under a given load.

A pit is first excavated to the depth of the proposed foundations or other structure and a steel plate, ranging from 300 – 1,000 mm, is lowered into place. The plate is then loaded in incrementally, either by applying kentledge (concrete blocks or steel billets) or by means of a hydraulic jack, until the plate starts to settle at rapidly. The settlements corresponding to each load increment are recorded.

The value of the soil’s ultimate bearing capacity is calculated by dividing the total value of load on the plate by the area of the steel plate. A safety factor is also applied, which is usually taken as one-third of that load which causes failure.

Whilst this is a commonly used test, care should be taken since the pressure exerted during the test is often much smaller than from the foundation itself which could lead to settlement occurring.

[edit] California bearing ratio test (CBR)

As a result of pavement failures in the late-1920s, the California Division of Highways designed the CBR test as a means of determining the thickness of materials required for road and pavement construction. Particle sizes of a maximum of 20 mm can be tested in this way; the plate bearing test being more suitable for larger particle sizes.

The test is carried out by recording the required pressure for penetrating a soil sample. This is done by driving a small cylindrical plunger of a known cross-sectional area into soil at a given rate. Tests are carried out in 20-30 m intervals along the centre line of the proposed construction, and usually on soil at least 1 m below ground level. The measured pressure is divided by the pressure required to achieve an equal penetration on a standard crushed stone sample.

The CBR value will be higher depending on the hardness of the surface being tested. Typical values are as follows:

• Clay (2%).
• Sands (10%).
• High quality sub-base (80-100%).

[edit] Related articles on Designing Buildings Wiki

• Alluvium.
• Bearing capacity.
• Condition survey.
• Desk study.
• Geophysical survey.
• Ground conditions.
• Ground improvement techniques.
• Ground investigation.
• Pile integrity test.
• Pre-construction information.
• Road construction.
• Site appraisal.
• Site investigation.
• Slaking.
• Soil report.
• Soil survey.
• Subsoil.
• Testing construction materials.
• Testing pile foundations.
• Topsoil.
• Trial pit.
• Types of soil.
• What does in situ mean?

[edit] External references

• ‘Introduction to Civil Engineering Construction’ (3rd ed.), HOLMES, R. (1995), The College of Estate Management
• Southern Testing
• STS Construction