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Putting soil to the test

Adeel Hassan, Product Manager at ELE International, a leading supplier of construction materials testing and environmental instrumentation, looks at the importance of consistently accurate soil testing and analysis in construction, and discusses how the latest technology is offering benefits to ground engineers and construction companies alike.

The accurate and detailed testing of soil at a potential construction site, or to be used as a construction material itself, is essential to ensure that its properties as an engineering material can be properly understood. Soil samples should be subjected to a number of tests in order for engineers to reduce significantly the level of uncertainty in the analysis of foundations or structures. 

Information obtained through extensive testing can enable engineers to build in, particularly problematic areas, some of which may not previously have been considered suitable construction sites, and to reduce costs, both through economies in design made possible with sound knowledge of the environment, and by achieving a sound structure with minimised risk of unplanned reactive design and engineering. 

While soil testing as a practice within civil engineering is more than 70 years old, the technology used to test and analyse soil on-site and in a laboratory environment has been constantly evolving, with the latest generation of equipment able to provide ground engineers with consistently accurate and detailed information. 

In-situ or laboratory testing?

Both testing in-situ and in the laboratory are necessary in order for the properties of soil to be accurately and reliably understood. On-site information gathering, using simple hand tools, enables engineers to obtain considerable information regarding the sub-surface structure of a site. Likewise, sampling can be carried out in-situ, using equipment such as hand augers and sample tubes to assist in obtaining samples of a suitable quality for laboratory testing.

Careful sampling, description and preparation of soil and soil mixtures are essential to the success of laboratory tests. In order to ensure test results are representative and meaningful, national and international standards should be adhered to; these specify a range of equipment and procedures to be used. A number of more sophisticated tests can be carried out in the laboratory to accurately determine the properties of collected soil samples.

Soil index properties

Engineers can use soil index properties to discriminate between broad categories of soil types. Classification tests to determine index properties can provide valuable information when the results are compared with 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.

The moisture content of soil has a strong effect on the way it behaves, and therefore changing the moisture content can alter its properties significantly. The liquid limit is the moisture content at which a soil sample passes from the plastic to the liquid state, and knowledge of this limit can allow an engineer to correlate the engineering properties of the soil.

Permeability

In addition to identifying how a soil reacts to moisture, it can be necessary to measure its permeability to evaluate drainage characteristics before construction begins. 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. Permeability is particularly relevant in the design and assessment of landfill sites and the investigation of contaminated ground. 

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 flow of water through a saturated sample is observed by monitoring the rate of the fall of water in a connected tube. A modern alternative method is to perform this test in a standard triaxial cell providing significant advantages, including saturation and consolidation prior to determining the permeability characteristics of the soil.

Consolidation

The problems associated with long term consolidation of clays and other compressible soils are well known. 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. 

Consolidation has been traditionally measured using an oedometer, in a process known as the one-dimensional consolidation test. Tests are carried out on specimens prepared from undisturbed samples of 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, offering more reliable results. 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

In civil engineering projects 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 these instances, soil is commonly compacted, through rolling, ramming or vibrating, to a dense state in order for satisfactory properties to be achieved. Compacting soil makes it more stable, while reducing its compressibility, permeability, and susceptibility to frost, and ultimately making it more suitable for use as a fill material

The California Bearing Ratio test (CBR) is a widely used empirical penetration test that can be applied to the design of granular base courses. Tests can be carried out in the laboratory or in-situ using alternative types of equipment. The CBR value enables a suitable thickness of sub-base construction to be determined to withstand anticipated traffic conditions. The use of in-situ CBR apparatus on road construction contracts enables the bearing capacity of soils to be determined quickly and efficiently.

Being able to control the degree of compaction enables these desired properties to be achieved at a reasonable cost, which makes laboratory compaction crucial. The latest generation of automatic soil 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.

The new machines are designed to improve the accuracy and repeatability of soil compaction tests, while increasing efficiency. By comparison with hand compaction methods, the latest 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, with engineers able to set the number of blows via a simple front, mounted control panel.

Strength (triaxial)  

New technology is also making it easier to accurately test the strength of a soil sample, using standard tests such as total stress and effective stress; effective stress being both time and permeability dependent and offering a more accurate measurement of strength. Total stresses are normally 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 a load is then applied through a piston onto a pressure pad acting on the top of the sample. 

Effective stresses can also be measured in a triaxial cell but are more complex as numerous parameters may be measured including back-pressure, pore water pressure and volume change. 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 applicable to sands and either the CU or CD test 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. Using a 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 flexible enough 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

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. Every structure that is founded in or on the earth imposes loads on the soil that supports the foundations and these stresses can cause deformation of the soil with slippage in the soil particles 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.

Like triaxial testing, the latest range of direct shear testing apparatus incorporates 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 engineers both time and money.

The importance of analysis

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 software programmes, such as ELE’s DS7 geotechnical testing software, are now available to assist engineers in collecting and evaluating test results.

The new DS7 software programme acts as a link between soil testing equipment, including triaxial and shear testing apparatus, intelligent data logger and a computer to provide consistently accurate results. The system provides fully integrated support and control of data acquisition with no need for programming. A test monitoring screen displays real time graphs of the parameters most critical to a particular test stage, with the option for hard copy print outs. The automatic generation of reports effectively eliminates lengthy calculation of test data. 

Conclusion

When building on or with soil as an engineering material, it is crucial that factors such as strength and permeability are accurately determined. The latest technology from manufacturers, such as ELE, is making testing and analysis faster, easier and more accurate, allowing ground engineers and construction companies alike to achieve a greater understanding of potential construction sites, and saving time and money in the process.

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.