Cone Penetration Testing (CPT) Services
Cone penetration testing (CPT), also known as static cone testing, provides a continuous profile of in-situ soil properties. Emerson Moore offers an innovative form of down-hole CPT testing (‘Memocone’) allowing tests to be carried out within boreholes, combined with window sampling of the strata tested. This maximises the soil data available in soft ground sections of the borehole, while allowing the recovery of physical soil samples to confirm CPT data and for laboratory testing.
Fig 1: Example continuous CPT log
The test is carried out by pushing an instrumented cone into the ground at a constant rate with the parameters being recorded against depth. The piezocone allows the recording of the cone end resistance (qc), sleeve friction (fs) and pore water pressure (u). Extensive research and experience exists with the test being proven as a valuable tool for providing ground data in many soil types. CPT data is commonly used for the following:
- Identification of soil type.
- Identification of potential contaminant migration pathways.
- Correlation with geotechnical parameter.
- Direct use in foundation design.
- Identification of drainage horizons for settlement analysis.
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Soil Type Interpretation Soil Type Interpretation is carried out using soil identification charts that relate the physically measured soil properties recorded with soil type. Various charts have been produced that allow the use of the available parameters. The correlation charts generally allow an accurate description of the soil types encountered. However, to provide maximum confidence in the interpretation confirmatory borehole data is often useful, especially in very soft sensitive soils. |
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| Fig 1: Eslami-Fellenius profiling chart |
Geotechnical Parameters
Geotechnical Parameters can be determined directly from results of the CPT test, providing a continuous profile rather than isolated results and avoiding the delays commonly associated with laboratory testing. However, the accuracy of the correlation depends on the parameter and soil type with site specific laboratory testing improving confidence in correlations. CPT data can be used directly for design in some applications although, again, the applicability is dependant on application and soil type. The following table provides an indication of the likely applicability of CPT data for several common design uses and soil types:
Fig 2: Predicted applicability of CPT for direct design (from Lunne, 1997)| Pile design | Bearing capacity | Settlement | Compaction control | Liquefaction | |
| Sand | 1-2 | 1-2 | 2-3 | 1-2 | 1-2 |
| Clay | 1-2 | 1-2 | 2-3 | 3-4 | |
| Intermediate soils | 1-2 | 2-3 | 3-4 | 2-3 |
Reliability rating: 1 High; 2 High to moderate; 3 Moderate; 4 Moderate to low; 5 Low.
The Memocone
Emerson Moore has adopted the latest in Swedish wireless piezocone
testing technology that allows static cone testing to be carried out
downhole, using the rigs standard drive mechanism. The cone stores
data in its internal memory, which is downloaded at the surface. This
allows reliable, repeatable in-situ soil data to be collected where
soft overburden soils are present.
The Memocone’s accuracy exceeds the ISSMGE (1998) test procedure specification. Obviously, this system cannot replace the traditional ‘cone truck’ for high output applications and testing of dense soils. Its application with our drilling system does, however, allow testing of soft horizons at varying depths. Physical samples of these soils can also be retrieved for laboratory testing.
The Memocone, when combined with the Commachio 205 rig, is able to push up to approximately 2.5 tonnes with the cone being pushed using 32mm diameter dynamic probe rods and the rigs hydraulic feed system to provide the down force. This gives, with a 10cm2 cone area, a maximum possible cone resistance of 25MN. Due to the limited maximum push tests will be carried out in short (<5m) sections to reduce rod friction. Each test section will be drilled out after testing, allowing disturbed samples to be recovered for laboratory testing if required.
During the test data is recorded using internal memory within the cone. On completion of the test the cone data is downloaded onto computer, allowing processing and interpretation. The data is then presented in the normal graphical format and combined with other borehole information on the final logs.
Summary of Advantages of the Memocone system
- Complete shear strength profile in soft soils – even where sampling is not possible.
- In-situ test data to corroborate laboratory data.
- Instantly available, repeatable test data to allow preliminary design to commence well in advance of receipt of laboratory test results.
- Data can be used directly for design in some circumstances.
- Identification of thin permeable horizons (to millimetre scale) in alluvial soils, allowing the accurate delineation of drainage paths for settlement analysis as well as assessment of contaminant migration pathways.
- Unlike stand-alone cone testing, the tested strata can be sampled after the test is complete.