A new CPT, sampling and instrumentation system offers a solution to investigating slopes.
Rob Gardiner reports.
Slope failures are a significant geotechnical hazard in which the influence of groundwater, insitu water pressures and external influences (such as precipitation) are poorly understood.
Determination of the engineering properties of soil embankments, slopes or earthworks is critical for their safe construction and operation. Routine investigation and monitoring of slopes is becoming more commonplace as owners need to qualify and quantify the risk associated with failure of their 'asset' - such as road and rail embankments or cuttings, rivers, canals, pipelines, quarries, landfils, reservoirs and building developments.
Management of such assets includes detailed investigation and subsequent monitoring of slope behaviour and performance.
Typically, there are five main elements to consider:
Soil type classification (granular or cohesive).
Water or moisture content.
Angle of internal friction and shear strength.
Plasticity index (cohesive) and fines content (granular).
Other factors, which may inflence the soil slope stability, include the presence of:
Artesian water pressures.
Interbedded permeable soil units allowing drainage (lenses, beds and laminae).
Organic or other soft soils, which would further jeopardise the soil slope stability, particularly in the case of embankments.
All these factors generally interact to cause a soil embankment or slope to be stable or unstable under external inflences such as loading, vibration and increased rainfall intensity and duration.
Access to the slope is usually difficult because it is overgrown or because of steepness or instability.
So specialist equipment is required that enables high quality data to be obtained safely and efficiently.
Traditional methods of investigation include window sampling, dynamic probing, hand auger borings and more recently CPTs, which can be used to derive a number of geotechnical parameters for slope stability assessment (see box). Conventional equipment can be used by mounting the investigation system onto crawler mounted units which incorporate either mast, ram or caterpillar tracks that can adjust to the slope angle to enable vertical testing or sampling.
However, systems without anchoring have some limitations regarding slope angle, stability and slope surface materials (Lankelma's slope crawler is limited to 30° slopes, for example).
Lankelma has developed a new CPT, sampling and instrumentation deployment system. This is essentially a 10t capacity frame with 20t capacity hydraulic CPT rams mounted at the centre.
The frame is attached to the arm of a conventional 360° excavator to allow safe positioning of the frame parallel with the slope face. The excavator can be positioned either at the toe or apex of the slope and then pushed into the face to provide stability for the frame and excavator arm. Once in position, the gantry and rod-handling cradle unfold to provide the operating platform.
A variety of conventional and 'sensitive' CPT penetrometers (friction, piezocone, soil moisture, seismic and enviro-cones) can be used.
A range of sampling (Mostap and Shelby) and instrumentation (BAT piezometers - see box - inclinometers and extensiometers) can also be installed on the system, which can be used both on- and off-rail.
Lankelma recently completed a rail embankment site investigation at Pound Green near Reading, on behalf of consultant Arup and client Network Rail. Works included 12 soil moisture piezocone penetration tests (SMCPTUs - see box) up to 8m below ground level and installation of three BAT sensors to 3m.
Site workers installed both the SMCPTs and BAT sensors using the boom-mounted CPT system, completing works in two days.
Cone penetration testing provides a rapid and ef. cient platform for obtaining geotechnical data on soils making up embankments and slopes, in particular:
Data quality is exceptionally high, with data obtained every 10mm over the depth of the test.
Standard cone friction CPTs and piezocone CPTs can be used to derive geotechnical parameters required for slope stability assessment and design.
The critical parameter of moisture or water content of soils in a slope or embankment (pre- or post-failure) can be accurately determined using the soil moisture content probe.
Existing and new CPT plant is highly flexible, enabling safe and efficient work in difficult access areas when working on embankments and slopes.
State of the art instrumentation for pre- or post-failure performance monitoring is vital in addition to intrusive investigation techniques (eg BAT piezometers, inclinometers, extensometers and vibration sensors).
Rob Gardiner is business development manager with Lankelma.
Geotechnical parameters that can be derived from CPT and piezocone penetration tests (PCPTs) for slope stability assessment include:
U o - insitu hydrostatic pore pressure.
' - effective angle of internal friction.
Su - undrained shear strength.
K - coefficient of permeability.
Ch/M v - coefficcient of consolidation parameters.
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 vane systems (eg Geonor penetration vane) can be used with the CPT plant to determine insitu Su and sensitivity Su parameters.
With the BAT-system, accurate measurements of soil pore pressure fluctuations can be measured and monitored.
Measurement results are digitally stored in the internal memory of the BAT-sensor and later downloaded to a PC for further analysis.
Accurate assessments of the permeability of the soil and/or to collect accurate groundwater samples can be performed.
The system consists of a filter tip and a sensor.
For slopes, the filter tip is pushed into the ground using a standard gas standpipe. The BAT sensor is lowered through the standpipe and as soon as the contact with the filter tip is made, pore pressure measurement begins. The BAT sensors can either be extracted and reused for interval measurements, or remain in place for continuous monitoring via internal data logger as at this particular site.
At the Pound Green site the BAT system was chosen over conventional piezometer instrumentation (such as standpipe or vibrating wire piezometers) as it measures absolute pressure, which guarantees stability, especially during long-term measurements.
Soil moisture content probe
The soil moisture probe was developed to enable insitu measurement of the soil moisture content, electrical conductivity and temperature.
The complete SMP system comprises an SMP module containing four measuring electrodes. It measures directly the dielectricity ( ), the electrical conductivity (EC), and the temperature (T). A fourth parameter, the permitivity ( ), is measured indirectly.
As with all CPT data, measurements or data points are obtained at intervals of 10mm over the entire depth of the test, which means a near-continuous data pro. le can be achieved.
The digital output of the module is transmitted to the surface by means of a conventional 16 pin umbilical cable into a standard CPT logging box and from the logging box via an SMP converter module and RS 232 signal to the second serial port of the CPT computer.