Slope failures are a significant geotechnical hazard, with the influence of groundwater, insitu water pressures and external influences such a precipitation being poorly understood (Ng et al, 1998).
Highway embankments and cuttings in clay soils are susceptible to landslips, particularly during periods of heavy rainfall. So while remedial works due to landslips cost up to £15M a year (Highways Agency 2005), determining the engineering properties of soil embankments, slopes or earthworks is critical for the safe construction and operational life such structures.
Typically, there are five main properties of primary significance:
Soil classification l Water or moisture content l Groundwater level l Angle of internal friction and shear strength l Plasticity Index (cohesive) and fines content (granular) Other factors that may influence the soil slope stability include:
Saturated zones l Artesian water pressures l Interbedded permeable soil units allowing drainage (lenses, beds and laminae) l Organic or other soft soils, which would further jeopardise stability, particularly in embankments.
Water content Water content of the soil is critical in the assessment of slope stability.
An essential part of any investigation concerns groundwater conditions on the site and the accurate measurement of water pressures in the ground. Great care is needed to obtain reliable data, as important in assessing the stability of a slope as determining shear strength properties (Highways Agency 2005).
However, conventional sampling and laboratory testing methods are subject to error. Insitu water content can differ from that obtained in the laboratory because of:
l Sampling method (disturbance) l Poor quality field preparation and handling of samples l Length of temporary storage and transportation of samples from site l Extrusion and preparation of samples for laboratory testing.
Significantly, the time delay from sampling to testing can be weeks.
Soil moisture probe The soil moisture probe (SMP) was developed for insitu measurement of the soil moisture content, electrical conductivity, and temperature.
The SMP system comprises an SMP module containing four measuring electrodes. It measures directly the dielectricity (s), the electrical conductivity (EC), and the temperature (T).A fourth parameter, the permittivity (e), is measured indirectly.
As with all CPT data, measurements or data points are obtained at intervals of 10mm over the entire test depth. Therefore a near-continuous data profile is achievable.
Use of 'sensitive' cones Typically, 10t to 15t (10cm 2to 15cm 2) cones are used for ground investigation purposes. A range of low range calibrated cones may be adopted for use in sensitive soils (ie pre- and post-failure slopes). Typically, the strain gauges in these cones are calibrated at multiple-point intervals between: 0 and 1t, 0 and 3t, and 0 and 5t.
A range of cone sizes are available (2cm 2to15cm 2). But, in accordance with the International Reference Test Procedure (2001), the use of 10cm 2cones is recommended. This enables consistency in test data interpretation and estimation of soil type, and greater resolution and data quality.
Sensitive cones should be considered where very soft to soft 'sensitive' soils are expected.
Typically, within post-failure slopes the horizon or plane of failure will be typified by a soil unit(s) of lower cohesion/stiffness having been remoulded as a result of failure.
Geotechnical parameters Derived parameters that can be obtained from the CPT and piezocone penetration tests for use in slope stability assessment include:
l U o=insitu hydrostatic pore pressure l f9= effective angle of internal friction l S u= undrained shear strength l K = coefficient of permeability l C v= coefficient of consolidation l M = constrained modulus.
It is generally considered that an approximation of remoulded shear strength is equal to the sleeve friction recorded by the cone.
Alternatively, push-in vanes systems (eg Norwegian Geotechnical Institute Geonor Penetration Vane) can be used with the CPT plant to determine insitu S uand remoulded S uparameters.
MOSTAP sampling Using CPT equipment, Mostap soil samplers can be used in most soils, either at discrete horizons and contiguous with CPTs, or continuous to provide an uninterrupted sample profile of the soil.
The system obtains samples for laboratory tests (moisture/water content, plasticity index, hand shear vane, bulk/dry density etc).
Depending upon soil type, samples produced would be defined as Class 2 under BS5930:1999.
The sampler consists of a cone and cutting shoe at the base of the sampling tube. A UPVC liner and stocking liner is inserted, before the system is connected to standard CPT rods and pushed into the ground.
Once at the required depth, a fishing tool is lowered through the hollow rods to release the cone face, which exposes the cutting shoe. The rams advance the sampler while the cone face is withdrawn to the top.
CPT plant and equipment Given the access problems when working on inclined surfaces, easily adaptable and flexible equipment and techniques are preferred. CPTs can offer this. Typical plant units include 1.5t to 2.5t (reaction weight) mini-crawlers, with inclined rams and stage mounted units that are elevated and positioned using telescopic or forklift equipment. Both systems have up to 20t thrust capacity and are able to perform vertical or inclined tests through slopes.
Case study 1 CPT firm Lankelma carried out trials in May 2003 to compare the performance and test results of the soil moisture probe (SMP) and CPT against conventional open tube sampling and moisture content determination by laboratory testing.
The test site was in Appledore, East Sussex.
Eight tests were performed to depths of up to 8m. Continuous Mostap soil samples were obtained to a depth of 5.5m for subsequent laboratory testing.
Ground conditions consisted of various soft to firm clays, sandy clays, soft silt and very loose silty sand.
All Mostap samples were delivered and tested within 18 hours of leaving site. Selected samples for determining water content were taken at intervals of 200mm over the depth of the Mostap samples.
A comparison of the results show a marked similarity between the profiles from the SMP against the laboratory test results. But some of the tests performed showed false results. These were attributed to intrusion from the adjacent marine environment. This caused high saline content of the soils at certain locations.
Case study 2 The second case study involves a Cambridge site using the SMP in conjunction with the CPT.
Seven SMP tests were performed to depths up to 21m. Ground conditions were firm to very stiff locally soft clay and silt units.
Data was used to determine high moisture contents of soils in a large embankment to determine potential slip planes for an overall slope stability appraisal.
Conclusions Cone penetration testing provides a rapid and efficient platform for obtaining geotechnical data on soils in embankments and slopes. Data quality is exceptionally high, with data being obtained every 10mm over the depth of the test.
Standard cone friction CPT and piezocone CPTs can be used to derive geotechnical parameters required for slope stability assessment and design. The critical parameter of moisture/water content of soils within a slope or embankment (pre- or post-failure) can be accurately determined using the soil moisture content probe.
CPT plant is highly flexible and enables difficult access for safe and efficient working on embankments and slopes. Ancillary instrumentation and equipment can provide additional parameters (Geonor vane), provide samples for visual inspection and/or subsequent laboratory testing (Mostap) and installations. This is again for pre- or post-failure performance monitoring (eg vibrating wire piezometers, inclinometers, extensometers and vibration sensors).
There needs to be continued use and promotion of the soil moisture probe combined with 'sensitive' cones for analysis and monitoring of slope and embankments.
In essence, slope failure is the result of soil liquefaction. Seismic cones are used to determine potential liquefaction of sensitive soils as a result of earthquake activity.
Further research on using seismic shear wave velocity data to determine the potential liquefaction of slope soils is required.
Rob Gardiner is new business manager Lankelma.
References Highways Agency Website (2005).
ISSMFE (2001) Appendix A. International Reference Test Procedure for cone penetration test (CPT ). International Society for Soil Mechanics and foundation Engineering.
Ng CWW, Shi Q (1998). Influence of rainfall intensity and duration on slope stability in unsaturated soils. Quarterly Journal of Engineering Geology, 31, 105-113.