What Is DCP Testing?
The Dynamic Cone Penetrometer (DCP) is a portable field instrument used to measure the in-situ strength of subgrade soils. It consists of a steel rod with a hardened conical tip, and a standard-weight hammer that is dropped from a fixed height against an anvil to drive the cone into the ground.
After each hammer blow, the operator records how far the cone penetrates into the soil. This measurement — expressed as millimeters per blow (mm/blow) — is called the DCP Penetration Index (DPI). A soil that requires many blows per millimeter of penetration is strong. A soil where the cone advances easily is weak.
DCP testing follows ASTM D6951/D6951M — “Standard Test Method for Use of the Dynamic Cone Penetrometer in Shallow Pavement Applications.” This standard defines the equipment, procedure, and provides correlation equations to convert the penetration index to California Bearing Ratio (CBR) values, which are the standard measure of soil strength used in pavement and geogrid design.
How DCP Results Relate to CBR
The California Bearing Ratio (CBR) is a widely used measure of subgrade strength. A CBR of 3 or below indicates very weak soil that will likely require stabilization. A CBR of 10 or above generally indicates adequate bearing capacity for most pavement applications.
ASTM D6951 provides correlation equations to convert DCP penetration index values directly to CBR. This allows engineers to assess subgrade strength on-site in minutes rather than collecting samples and waiting for laboratory CBR test results. The DCP also measures strength at multiple depths in a single test, showing how soil conditions vary through the subgrade profile — information that a single-depth lab test cannot provide.
What Is Tensar Plus?
Tensar Plus (Tensar+) is Tensar's design software for engineering mechanically stabilized layers (MSLs) using geogrid. It takes project-specific inputs — including subgrade CBR values, traffic loads, and design life — and calculates the optimal geogrid product and aggregate thickness required to meet performance criteria.
Tensar Plus incorporates data from Tensar's geogrid product line, including the InterAx series with its multi-directional triangular aperture geometry. The software accounts for the confinement effect that geogrids provide to aggregate, which creates a composite mechanically stabilized layer with greater bearing capacity than aggregate alone.
You can download Tensar+ from Tensar's website, or contact us — we run Tensar+ designs for our customers at no charge as part of our free DCP testing service.
How They Work Together
The workflow is straightforward:
- Field testing: DCP tests are performed at representative locations across the project site. The penetration index is recorded at each depth interval.
- CBR conversion: The DCP penetration index values are converted to CBR using the ASTM D6951 correlation equations. This produces a soil strength profile for the site.
- Design input: The CBR values are entered into Tensar Plus along with the project's traffic requirements (axle loads, number of passes) and design life.
- Geogrid selection: Tensar Plus calculates the required geogrid product and aggregate thickness to create a mechanically stabilized layer that meets the design criteria.
- Comparison: The software can compare the geogrid-stabilized section against an unstabilized section, showing the reduction in aggregate thickness and cost savings.
What Is a Mechanically Stabilized Layer?
When geogrid is placed within an aggregate layer, the aggregate particles interlock with the geogrid's apertures. Provided the geogrid ribs are stiff enough and the junctions strong enough, this confines the aggregate and prevents lateral movement under load. The result is a geogrid-aggregate composite — called a mechanically stabilized layer (MSL) — that distributes loads more effectively than aggregate alone.
The MSL increases bearing capacity over weak subgrade soils, controls differential settlement, and can significantly reduce the amount of aggregate required for a given application. This is the engineering principle that Tensar Plus quantifies using the DCP-derived CBR data.
Why This Matters for Your Project
Without field testing, geogrid design is based on assumptions about soil strength. Those assumptions can lead to over-engineering (wasting money on unnecessary aggregate) or under-engineering (premature failure). DCP testing removes the guesswork by providing actual subgrade data that feeds directly into the design software.
As an authorized Tensar distributor, Asphalt Fabrics & Supply performs on-site DCP testing and runs the results through Tensar Plus to provide engineered geogrid recommendations. This means you get a solution sized to your actual site conditions — not a generic specification.
Need DCP Testing for Your Project?
We offer on-site DCP testing and Tensar Plus design support throughout Ohio. Contact us to schedule a site assessment.