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Testing the ground

Accurate testing and analysis of soil as an engineering material is crucial to the success of a wide range of construction projects. Adeel Hassan, Product Manager at ELE International, a leading supplier of construction materials testing and environmental instrumentation, explains how the latest technology is complementing traditional testing methods to offer benefits to both ground engineers and construction companies.

Whether soil is present at a potential construction site or is to be used as a construction material itself, it is essential to understand the way in which it is likely to react to factors such as pressure, moisture and shear, to minimise risk and costs. In addition, soils can vary greatly between sites, requiring samples to be taken and subjected to a number of tests in order for engineers to analyse foundations and structures effectively. Testing in this way enables costs to be minimised both through economies in design made possible with sound knowledge of the environment, and by minimising the risk of unplanned reactive design and engineering in a finished structure. Similarly, sites that may not previously have been perceived as suitable for construction can potentially be reconsidered.

In-situ

Combining traditional in-situ testing methods and the use of high quality equipment in a laboratory environment, allows the properties of soil to be more accurately and reliably understood than ever before. Through on-site information gathering, using simple hand tools, engineers can obtain considerable information regarding the sub-surface structure of a site, while also collecting samples for further testing in the laboratory.

Soil index properties can be determined on site, allowing engineers to differentiate between broad categories of soil types. Classification tests to determine index properties provide valuable information when compared with further test results from an individual sample. Soil index classification takes into account moisture content, shrinkage, density, and particle size and involves the use of equipment such as drying ovens, linear and volumetric shrinkage apparatus, pycnometers and standard sieve sets.

In the laboratory

After samples have been taken from a site, they can be analysed using the latest generation of soil testing equipment in the laboratory, which is able to give engineers an accurate indication of a soil’s characteristics. Permeability, consolidation, compaction, strength and shear can all be tested in this way.

Permeability

It is often beneficial to measure the permeability of a soil sample to evaluate drainage characteristics, particularly in the design and assessment of landfill sites and in the investigation of contaminated ground. The use of a flow-net analysis, together with permeability data, enables engineers to estimate the rate of seepage of water through or under a structure, and seepage pressures to be calculated. 

The permeability of granular soils (sand and gravels) can be tested in the laboratory using constant head permeability apparatus, where water passing through the sample is collected and measured for a specific quantity or time period. Clays and silts are tested in a similar way using the falling head technique, where the flow of water through a saturated sample is observed by monitoring the rate of the fall of water in a connected tube. This test can now be performed in a modern triaxial cell providing saturation and consolidation prior to determining a soil’s permeability.

Consolidation

Consolidation testing of clays and other compressible soils is important to avoid long term problems when a structural foundation has been put in place, and the resulting load applied. Traditionally, consolidation has been measured using an oedometer, in a process known as the one dimensional consolidation test. Tests are carried out on samples prepared from undisturbed soils of low permeability and the resulting data is used, together with classification information and knowledge of the soil’s loading history, to estimate the behaviour of the foundations under load.

Alternative consolidation test equipment uses hydraulics technology to allow tests to be carried out on samples of a much larger diameter. The advantage of these systems is their ability to control drainage and measure pore water pressure during testing, making several drainage conditions possible and allowing back-pressure to be applied to the sample.

Compaction

Soil is often required as a fill material, to refill an excavation or void, to provide make-up ground to support a structure, as a sub-base for a road, railway or runway, or as a structure such as an earth dam. In order for the required properties to be achieved, soil is typically compacted, through rolling, ramming or vibrating, making it more stable, while reducing its compressibility, permeability, and susceptibility to frost.

The latest generation of automatic soil test compactors, from manufacturers such as ELE, incorporate a rotating mould table and vertically mounted rammer to compact a sample to the required specification. The rammer can be adjusted for drop height and weight, while the turntable is automatically rotated after each blow as the rammer rises.

These new machines are designed to improve the accuracy and repeatability of soil compaction tests, while increasing efficiency. Unlike hand compaction methods, the latest automatic soil compactors can be left operating unattended for pre-determined periods and are capable of offering far more consistent results as each compaction stroke is identical in downward force and angle of alignment, with engineers being able to set the number of blows via a simple front mounted control panel.

Strength (triaxial)  

The strength of a soil sample can also be accurately and efficiently tested using the latest technology. Strength can be defined as total stress and effective stress; the latter being both time and permeability dependent and offering a more accurate measurement of strength. 

Both total and effective stresses can be measured in a triaxial cell where the sample is subjected to an all round confining pressure, enclosed in a rubber membrane preventing drainage in or out, and with a load applied through a piston onto a pressure pad acting on the top of the sample. When measuring effective stresses, additional parameters can be recorded including back-pressure, pore water pressure and volume change, and from these values various engineering properties can be calculated. Effective stress tests are usually referred to as consolidated drained (CD) or consolidated undrained (CU), with the CD test generally being applicable to sands; either the CU or CD test is applicable to clays.

Triaxial tests can now be carried out with greater efficiency and accuracy thanks to new testing equipment incorporating the latest microprocessor technology, such as ELE’s Digital Tritest 50 triaxial load frame. The load frame provides the uniform rate of axial loading required in triaxial tests to determine the strength and stress-strain relationships on a cylindrical specimen of soil. 

This new generation of triaxial testing equipment is sufficiently versatile for a variety of research environments, while the use of microprocessor-controlled digital stepper motor drive systems allows variable speed control with no gear changes or maintenance required. Ease of use is also a high priority, with the equipment incorporating backlit LCD displays and sealed membrane keypads.

Shear

The stresses that building a structure imposes on the soil below it can cause deformation of the soil with particle slippage leading to the sliding of one body of soil relative to the surrounding mass. Shear failure occurs when the shear stresses set up in a soil mass exceed the soil’s shear strength, making it important to test this maximum resistance level. Like triaxial testing, modern direct shear testing apparatus incorporates microprocessor technology, to enable more accurate results to be achieved and for much of the testing to be carried out automatically, without constant supervision.

Accurate analysis

For test results to be analysed quickly, accurately and displayed in a way that can be easily distributed and communicated, data acquisition software programmes are now available, including ELE’s DS7 suite of geotechnical testing software. These new software programmes act as a link between soil testing equipment, including triaxial and shear testing apparatus, intelligent data logger and a computer to provide consistently accurate results, with no need for programming. Reports can be automatically generated to eliminate time-consuming calculation of test data, and printed out, simplifying the distribution of test information.

This latest testing and analysis equipment, from manufacturers such as ELE, is making soil testing faster, easier and more accurate. Through achieving a greater understanding of potential construction sites, ground engineers and construction companies are able to make significant savings in time, resource and costs.

For further information contact Adeel Hassan, Product Manager, ELE International, Chartmoor Road, Chartwell Business Park, LEIGHTON BUZZARD, Bedfordshire, LU7 4WG.  Tel: 01525 249 240. Fax: 01525 249 249.  Email: ele@eleint.co.uk.  Web: www.ele.com.