Test the ground before you build

ELE International explains the importance of accurately testing the soil at a potential construction site to avoid costly repercussions once building has begun.

Thorough investigation and testing prior to construction is essential to ensure that a completed building is both safe and structurally sound. In particular, obtaining an understanding of the properties of soil at a construction site enables the level of uncertainty associated with a build to be minimised. In addition to minimising risk, information obtained through extensive testing can enable engineers to build in problematic areas, some of which may not otherwise have been considered to be suitable construction sites, and reduce costs, both through economies in design made possible with extended knowledge of the environment, and by achieving a sound structure with minimised risk of unplanned reactive design and engineering. 

This knowledge can most efficiently be achieved through a combination of traditional on-site analysis and more advanced laboratory tests using the latest generation of testing equipment, which are together able to provide an accurate understanding of how the soil will react to factors such as moisture, pressure and shear forces. 

Initial on-site information gathering, using traditional methods and simple hand tools, enables engineers to obtain a considerable amount of valuable information regarding the sub-surface structure of a site. Soil index properties can be applied in-situ to differentiate between broad categories of soil types. Soil index classification involves the use of equipment such as drying ovens, linear and volumetric shrinkage apparatus, pycnometers and standard sieve sets, and takes into account a number of factors including moisture content, shrinkage, density, and particle size. Soil samples of a suitable quality for advanced laboratory testing can also be taken at this point, using equipment such as hand augers and sample tubes. Careful sampling, description and preparation of soil and soil mixtures are essential to the success of laboratory tests, in order to achieve test results are representative and meaningful. 

In addition to traditional on-site analysis, a number of more sophisticated tests can be carried out in the laboratory to determine accurately the properties of collected soil samples. One such test, to record the permeability of granular soils (sand and gravels), involves 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 flow of water through a saturated sample is observed by monitoring the rate of the fall of water in a connected tube. These tests can now be performed in a triaxial cell providing significant advantages, including saturation and consolidation prior to determining the permeability characteristics of the soil. Testing the permeability of soil is necessary to evaluate drainage characteristics before construction begins.

When a structural foundation is put in place, and the resulting load applied, some settlement will occur even if the applied pressure is well within the safe bearing capacity of the soil; this is known as consolidation. Traditionally, consolidation is 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. An advantage of these systems is their ability to control drainage and measure pore water pressure during testing, making several drainage conditions possible.

Soil is occasionally required as a fill material, to refill an excavation or void, or to support a structure. In these instances, soil is commonly compacted, making it more stable, while reducing its compressibility, permeability, and susceptibility to frost. The latest generation of automatic soil compactors, from manufacturers such as ELE, have been developed to improve the accuracy and repeatability of soil compaction tests, while increasing efficiency. These automatic soil compactors can be left operating unattended for pre-determined periods and, perhaps as importantly, produce far more consistent results as each compaction stroke is identical in downward force and angle of alignment.

New technology is also making it easier to test accurately the strength of a soil sample, using standard triaxial tests such as total stress and effective stress; effective stress being both time and permeability dependent and offering a more accurate measurement of strength. These tests can now be carried out with greater efficiency and accuracy thanks to new testing systems incorporating the latest microprocessor technology, such as ELE’s Digital Tritest 50 triaxial load frame. Featuring microprocessor-controlled digital stepper motor drive systems, this new generation of triaxial testing equipment allows variable speed control with no gear changes or maintenance required.

As well as soil strength, the maximum shear resistance that the soil can offer, known as its shear strength, may also need to be determined. Like triaxial testing, the latest range of direct shear testing systems incorporate microprocessor technology for ease of use, and more accurate results than previously achievable. This new technology allows much of the testing to be carried out automatically, without constant supervision, saving both time and money.

With each of these testing methods, it is essential that the analysis of results is carried out quickly, accurately and displayed in a way that can be easily distributed and communicated. Cutting edge data acquisition and analysis software programmes, such as ELE’s DS7 geotechnical testing software, are now available to assist in collecting and evaluating test results, able to generate reports automatically and, therefore, eliminating lengthy calculation of test data. 

When building on soil, it is essential that its properties as an engineering material are properly understood, and testing for characteristics such as strength and permeability is by far the best way to achieve this. Along with traditional techniques, the latest technology from manufacturers, such as ELE, is now making testing and analysis faster, easier and more accurate, enabling a greater understanding of potential construction sites, and, ultimately, minimising risk and reducing costs.

For further information contact us at Tel: +44 (0) 20 7193 6027  Email: ele@eleint.co.uk.  Web: www.ele.com.