Ele International group marketing manager John Sutton looks at new testing technology.
Information obtained through extensive testing has allowed engineers to build in areas previously not considered suitable for construction. This will become even more common as the latest generation of equipment to test and analyse soil provides ground engineers with more accurate and detailed information.
Careful sampling, description and preparation of soil and soil mixtures are essential to the success of laboratory tests.
Consolidation, for example, has been traditionally measured using an oedometer in a one-dimensional consolidation test. Tests are carried out on specimens prepared from undisturbed samples of soils of low permeability and the data is used, together with classification information and knowledge of the soil's loading history, to estimate the behavior of the foundations under load.
Alternative consolidation test equipment uses hydraulics technology to test samples of a much larger diameter, offering more reliable results. The advantage of this system is the 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.
The CBR (California Bearing Ratio) test is a widely used empirical penetration assessment that can be applied to the design of granular base courses. The CBR value enables the determination of a suitable thickness of sub-base construction to withstand anticipated traffi c conditions. The use of in-situ CBR apparatus on road construction contracts enables the bearing capacity of soils to be determined quickly and efficiently.
The latest generation of automatic soil compactors incorporates 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.
Compared with hand compaction, today's automatic soil compactors can be left operating unattended and produce far more consistent results as each compaction stroke is identical in downward force and angle of alignment, and engineers are able to set the number of blows via a front-mounted control panel.
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 allround 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.
Triaxial tests now have 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.
The use of microprocessorcontrolled digital stepper motor drive systems allows variable speed control with no gear changes or maintenance required.
With any testing method, it is essential that results are analysed quickly, accurately and displayed in a way that can be easily distributed and communicated. Modern 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 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 printouts. The automatic generation of reports effectively eliminates lengthy calculation of test data.
When building on soil, or using it as an engineering material, it is crucial that factors such as strength and permeability are accurately determined. The latest technology from manufacturers means ground engineers and construction companies can gain a greater understanding of potential construction sites, saving time and money in the process.