CPT cones are available in multiple sizes, but the 10 cm2 cone is the industry standard. Other sizes, the most common of which is the 15 cm2 cone, are essentially scale models of the 10 cm2 cone, having the same proportions as specified by the ASTM Standard for CPT testing.
What factors determine what cone size you should use?
Most CPT cones range from 5 cm2 to 15 cm2 in cross-sectional area, though smaller cones (down to 1 cm2) are used in specialized lab or research applications.
Different cone sizes have different advantages depending on the testing situation:
- The larger 15 cm2 size is more robust and gives more accurate cone resistance values in very soft soils. Additionally, it has more room inside for additional sensors.
- Smaller piezocones have faster pore pressure sensor response and thus are better suited for characterizing very thin layers of soil.
- The 10 cm2 cone is suitable for most applications. It is the industry standard and considered the reference penetrometer for field testing.
Cones in the 5 cm2 to 15 cm2 range have been shown to produce consistent data in most soils, so corrections for different sizes are generally not needed. When using a cone outside this size range, corrections may be necessary to ensure that results are consistent with the body of CPT data: for example, very small cones tend to produce higher cone resistances than standard-size cones. If there are questions as to the effect of scaling the penetrometer to either larger or smaller size, a 10 cm2 penetrometer should be used in the same soils so that the results can be compared.
Penetrometers are made of high strength steel and designed to resist abrasion by soil, but over time, normal wear and tear may blunt the cone and effect the accuracy of the data. Since the proportions of all penetrometer sizes are held to an exact standard, it is important to measure them daily to ensure that they remain within the ASTM standard tolerances. An excessively blunted or asymmetrically worn cone tip or sleeve should be replaced.
It’s imperative to field check with calipers and document the diameter of the peizocone tip and sleeve before each sounding to validate that the diameter of the tip is smaller than the sleeve. Vertek can provide electronic documentation to help track your daily readings that can be stored on your field PC as well as easy-to-use field gauges, calipers and load cells for more stringent field verifications.
Minimum and Maximum Cone Measurements
|Cone Size (Cross-Sectional Area)||10 cm2||15 cm2|
|Cone Diameter (dc)||35.3 mm||35.7 mm||36.0 mm||35.3 mm||43.7 mm||44.0 mm|
|Cone Tip Height (hc)||24.0 mm||31.2 mm||31.2 mm||29.4 mm||38.2 mm||38.2 mm|
|Cone Tip Angle||55°||60°||65°||55°||60°||65°|
|Lip Thickness Before Porous Filter (he)||3 mm||N/A||7 mm||3 mm||N/A||7 mm|
|Friction Sleeve Diameter||35.7 mm||35.7 mm||36.05 mm||43.7 mm||43.7 mm||44.05 mm|
|Friction Sleeve Surface Area||147 cm2||150 cm2||153 cm2||220.5 cm2||225 cm2||229.5 cm2|