Ground Investigation for Slope Stabilisation
Slope stability is an issue impacting the construction and maintenance of roads, railways, commercial and residential projects and there are many stabilisation techniques available to address the problem including soil nailing, slope drainage, facing systems, re-grading, retaining structures or Electrokinetic Geosynthetics (EKG). Alone or in combination, these slope stabilisation methods are effective, but for these long-term, or permanent solutions, the choice of remediation must be directed and assessed by Ground Investigation (GI) techniques. A thorough GI of the slope is not only effective in directing the method of stabilisation, it also provides an assessment of how safe the slope / ground conditions are for temporary works and construction and can provide a more efficient design.
Slopes can fail for many reasons and whilst the term ‘instability’ is a good catch-all for the result, the causes can be quite varied. Within the UK, the most common causes are extreme weather events, poor drainage or lack of drainage, the removal of anchoring vegetation, cutting a slope too steeply or it may simply be due to the localised presence of natural planes of weakness. In most cases however, water is involved in some way. Instability can also be caused by excavation work being carried out at the base of a slope without recognising the loading at the top and thereby reducing the factor of safety. The complexity of ground conditions and the ‘hidden geology’ of a slope has to be assessed. With ageing infrastructure and slope failures becoming increasingly common, good ground and site investigations are important, to characterise the ground conditions in these areas of instability and produce a sound ground model. This then enables geotechnical engineers to design appropriate remedial measures to arrest any further slope deterioration and prevent any recurrence.
Cone Penetration Testing (CPT) is an under-used technique, providing high quality in-situ data rapidly; a long steel cone is driven vertically into the ground using at a constant rate with measurements recorded of the cone resistance and the side friction against the shaft. Where a piezocone test is undertaken, the pore water pressure generated by the penetration of the cone into the ground is also assessed. Measurements are recorded using an electric current with the readings displayed graphically as an interpretation of the variation of the measured parameters as the depth increases. CPT is useful as it can define the sub-surface stratigraphy, which enables the materials encountered to be determined too. It is these results that can then be further developed to obtain parameters for the design of a geotechnical solution to stabilising the slope. Confirmed and calibrated by boreholes, CPTu data can be very valuable to aid design.
By their very nature, slopes are difficult areas to access, which often makes the process of GI difficult. Traditionally, these sites typically required the design and installation of platforms using scaffolding to service a cut down cable percussion or rotary rig. It was also more common for clients to accept a downgrade in captured data, typically acquired using hand held window sampling and other equipment. On infrastructure, areas to be tested may also be in remote locations away from access points, so possessions of the railway and traffic management on highways are often required, without the use of bespoke equipment. Moving and establishing equipment on and off site can also be a lengthy process or even prohibitively expensive, which also means having to accept lower quality data.
However, many GI companies have now developed bespoke drilling rigs, which are better able to access restricted sites and provide the high-quality geotechnical data that is required by design engineers to provide an effective slope stabilisation solution, that is not over-designed and is cost effective. These ‘slope-climbing and modular rigs are able to provide true ‘geotechnical investigation’ without compromise, including CPT, CPTu, direct push sampling, undisturbed sampling, dynamic sampling, rotary drilling and Standard Penetration Testing (SPT). In addition, the availability of long-reach and telehandler mounted platform rigs with dual dynamic sampling and rotary masts are available, which also help with acquiring data in restricted access areas, typical of embankments and cuttings on highways and railways.
Bespoke rigs enable ground investigation data to be acquired at the same location, orientation and angle as a proposed remedial solution. For example, by using a bespoke CPT slope climbing rig, inclined CPTu’s can be undertaken at the same angle, location and orientation as a proposed soil nailing solution, whilst also acquiring data on pore water pressure, thereby minimising the risk of unforeseen ground conditions.
Slope failure can occur through natural causes, but those that have been ‘stabilised’ fail only due to poor engineering design, which can often be traced back to the use of poor or compromised original data sets. Modern technology with bespoke rigs can avoid having to accept this sub-optimal data, which means no matter how difficult or remote the slope, site or location, high quality GI is possible and given how essential it is in effective slope stabilisation design, today there can be few reasons not to use it.