Benefits of the Cone Penetrometer Test
- Accurately profiles the geological strata
- Measures the low strain shear wave velocity for seismic evaluations satisfying the new International Building Code (IBC) requirements
- Can be used to predict vertical pile capacity
- In normally consolidated or recently aged cohesionless soils, provides good estimates of settlement for shallow foundations
In-Situ Soil Testing often performs CPT with the direct push truck that Dr. Schmertmann used for his pioneering research (Good Karma). To assure high quality control, all tests are performed by a registered professional engineer.
Cone Penetrometer Test (CPT), ASTM D 3441 and D 5778: The mechanical cone penetrometer probe, invented in The Netherlands in 1932 by P. Barentsen, measures the quasi-static thrust required to push a solid, conical tip having a 60 degree apex angle and a cross‑sectional area of 10 cm2 into the foundation soil. The operator advances the cone using a nested, dual‑rod system, the outer rods providing strength to penetrate the cone in a collapsed configuration, and the inner rods allowing him or her to advance only the cone tip at each test depth (generally at 20-cm intervals) while measuring the hydraulic thrust pressure at the top of the rods. In 1953, Begemann modified the probe to include a friction sleeve just behind the tip. For the friction cone test, the inner rods initially advance only the tip for a short distance, and then engage both the tip and a friction sleeve together. The center of the friction sleeve is located 20 cm above the tip, and the value of unit soil adhesion acting on it is computed by subtracting the tip-only thrust force (from the previous test depth) and dividing by the sleeve area of 150 cm2. The engineer then divides the unit tip bearing from the previous test depth by the unit adhesion to determine the friction ratio (both readings then apply to the same depth), and uses an empirical chart to identify the type of soil. Depth plots of unit bearing and friction ratio also provide a relative profile of the site stratigraphy.
The improvement of electronics and computers in the 1980s led to the development of stainless steel electrical cone penetrometer probes that obtain and record more reliable test measurements and eliminate the dual-rod system (Figure 1). Strain gauges are used to measure the tip and friction values and a pressure transducer measures the pore water pressures generated during penetration. With the electric cone, data are collected at penetration increments of 0.5 cm to 5 cm depending on the computer acquisition system (ours uses 1 cm), such as the one shown in Figure 1. Engineers prefer the electronic cone’s accuracy and productivity, relegating the mechanical cone to profiles containing strong materials that might damage the more expensive electrical cone. The cone penetrometer can be pushed with a direct push rig into soil with an N60-value of about 50 blows per foot or with a heavy drill rig into soil with an N60-value of about 40 blows per foot.
Figure 1: CPT and Data Acquisition Computer
Engineers have obtained reasonable accuracy in correlations between the CPT unit bearing and soil strength parameters, such as friction angle and undrained cohesion (see Lunne, et al., 1997). More indirect correlations with at rest coefficient of lateral earth pressure, modulus, and overconsolidation ratio are much less reliable due to the significant effects of stress history and the in-situ state of stress. The addition of pore pressure measurements, generally made just behind the tip, to the electrical cone (CPTU) improves stratigraphy profiling and various indirect correlations. By collecting data at close depth intervals, thin layers are detected. Two correlation charts are used to identify the soil type: Friction Ratio (Rf) vs. Corrected Cone Bearing (qT) and Pore Pressure Ratio (Bq) vs. Corrected Cone Bearing (qT). (Campanella et al., 1988 and Robertson et al., 1989) Generally, the pore pressure ratio correlation chart is more sensitive to thinner layers, while the friction ratio chart is better for cohesionless soils. When there is a discrepancy in soil type between the two charts, either pore pressure dissipation tests or sampling can be used to identify the correct soil type.
Figure 2: Typical SCPTU Shear Wave Measurements
Recently updated International Building Code (IBC) requires structural engineers to evaluate the geotechnical seismic classification. They must either measure the low strain shear wave velocity or use conservative correlations. Conservative correlations can be quite costly to the owner. Fortunately, it is relatively easy to measure the shear wave velocity with a seismic cone penetrometer (SCPT), which contains a geophone located about 15 cm above the tip. The SCPT is similar to downhole seismic tests, but the cone is used as a convenient method of advancing a geophone (Robertson, Campanella, Gillespie and Rice, 1985). Tests are performed at 1-meter intervals (rod breaks). The SCPT’s data acquisition computer includes an oscilloscope, which records and stores the shear wave. The shear wave is generated by striking a horizontal plate with a sledge hammer that contains a triggering device (Figure 2). Shear wave measurements made with the SCPT usually enable the structural engineer to improve the IBC site classification by one grade over the conservative correlations.
Vertical Pile Capacity
The CPT data can be used to predict pile capacity (the cone models a pile). Many prediction methods are commonly used, but the three best methods as documented by Robertson, Campanella, Davies, and Sy (1988) are:
- Schmertmann and Nottingham (1978) — qc and fs used
- de Ruiter and Beringen (1979) — qc and fs used
- Laboratoire Central des Ponts et Chaussées (1982) — qc only used
With these methods, their study showed that the predicted pile capacity averaged within 10% of the measured capacity with a standard deviation of less than 25%. Caution should be used when predicting pile capacities with CPT in cemented soils. The cone detects the cementation (high qc values), while pile driving may destroy the cementation. The LCPC method filters out data that are either 30% higher or lower than an average value within 1.5 of the pile diameter or width zone. We provide our clients with an Excel spreadsheet that computes vertical pile capacity using the LCPC method. We plot the results using Grapher as shown in Figure 3.
Figure 3: Typical LCPC Vertical Capacity Prediction
Figure 4: Range of Es/qcRatios in Sand
Settlement of Shallow Foundations
Schmertmann’s method (1970 & 1978) provides good estimates of settlement for shallow foundations in normally consolidated or recently aged cohesionless soils. In overconsolidated sands the E/qc ratio can be significantly higher but difficult to predict as shown in Figure 4. Additionally, for cohesive soils, the cone data do not accurately measure deformation moduli, and therefore we recommend using dilatometer or pressuremeter tests for prediction of settlements for shallow foundations in those soils.
Time Rate of Consolidation (Dissipation Tests)
In cohesive soils, excess pore water pressure is developed when the CPT probe is pushed into them. When the penetration stops, those pressures decrease. A dissipation test consists of recording the pore water pressure and elapsed time as it decays. Like laboratory dissipation tests, the time for 50% dissipation to occur is computed. This value is needed to compute the cost of the coefficient of consolidation in the horizontal direction, Ch. Typical dissipation test results are shown in Figure 5.
Figure 5: Typical Dissipation Test Results