Subgrade Evaluation Methods: From DCP Testing to Resilient Modulus

A pavement is only as good as the ground it rests on. Accurately characterizing subgrade strength and stiffness is therefore one of the most consequential steps in pavement design. From rapid field tools like the Dynamic Cone Penetrometer (DCP) to sophisticated laboratory measurement of resilient modulus (Mr), engineers have a suite of methods, each with its own cost, data quality, and design relevance. This article walks through the most common techniques, explains how their results feed pavement design equations (AASHTO 1993 and Mechanistic-Empirical), and offers guidance on when to use which test.

 

Why Subgrade Evaluation Matters

• Design thickness - Weak subgrades require thicker (and costlier) pavement structures.

• Performance predictions - Accurate moduli improve rutting and fatigue predictions in M-E software.

• Risk management - Identifying soft spots early prevents surprises during construction.

Rapid Field Reconnaissance

Before formal testing, inspectors often perform:

• Visual soil classification (USCS/AASHTO)

• Hand auger sampling to check moisture, color, and texture

• Proof-rolling with a loaded truck to locate pumping or deflection zones

These low-cost steps help target heavier testing budgets where they will matter most.

Dynamic Cone Penetrometer (DCP) Testing

Item	Details
Principle	Count blows required for a standard cone to penetrate set depths (usually per 20 mm).
Key Output	DCP Index (mm/blow), easily correlated to CBR, k-value, and even Mr.
Advantages	One-person operation, immediate results, ideal for rehab projects needing many points.
Limitations	Sensitive to large particles; correlations introduce uncertainty.

Typical Correlation (for fine-grained soils)
CBR ≈ 292 / (DCP1.12)
Mr (MPa) ≈ 10 × CBR0.64

California Bearing Ratio (CBR) Tests

• Laboratory CBR - Remolded sample compacted to optimum moisture & density; soaked for four days to simulate worst-case.

• In-situ CBR - Penetration test executed in the field; faster but more variable.

CBR remains the backbone of many highway agency design catalogs, especially for low-volume roads. ASTM D1883 (lab) and D4429 (field) govern procedures.

Plate Load Test (Modulus of Subgrade Reaction, k)

A circular plate (300-760 mm) is loaded incrementally; surface deflection is measured under each load level. The slope of the pressure-deflection curve over a specified range gives k (kN/m³). Because loading plates mimic tire footprints, k is popular for rigid-pavement (concrete) design. ASTM D1195/1196 standardize the method.

Falling Weight Deflectometer (FWD) Back-Calculation

FWD drops a known mass onto an instrumented plate, then measures surface deflection at radial offsets. Specialized software fits a layered-elastic model to derive moduli for the asphalt, base, and subgrade.

• Pros - Non-destructive, captures in-service moisture and stress states.

• Cons - Requires calibration cores and experienced analysts to resolve non-uniqueness in solutions.

Laboratory Resilient Modulus (Mr)

Mechanistic-Empirical Pavement Design Guide (MEPDG) uses Mr as its fundamental soil input.

• Test Setup - Repeated axial haversine loads in a triaxial cell (AASHTO T 307).

• Stress Dependency - Mr increases with confining pressure and decreases with deviator stress; thus tests run across multiple stress states to calibrate constitutive models.

• Typical Values - 25 MPa (soft clay at high moisture) to 200 MPa (dense sand/gravel).

• Moisture-and-Freeze Adjustments - Seasonal factors (k1, k2) modify the design modulus over the year.

Converting Between Methods

Because not every project can afford triaxial testing, agencies publish empirical equations such as:

• AASHTO 1993:
   Mr (psi) = 1500 × CBR (for 3 ≤ CBR ≤ 10)

• TRB Circular 703:
   Mr (MPa) = 0.24 × k (kPa/mm)

• Minnesota DOT:
   Mr (MPa) ≈ 17.6 × (DCP penetration rate)-0.65

Use correlations cautiously; confirm with at least one direct measurement on critical subgrade sections.

Choosing the Right Method

Project Scenario	Recommended Test(s)	Rationale
Rural low-volume road rebuild	DCP + limited lab CBR	Low cost, adequate for thickness charts
Urban arterial reconstruction	FWD + lab Mr on representative soils	Needed for M-E calibration and staging traffic loads
New interstate alignment	Lab Mr + supplemental DCP grid	High reliability justifies higher testing budget
Emergency rehab / thin overlay	FWD only	Fast, non-destructive snapshot of remaining life

Best Practices for Reliable Data

1. Control moisture during sampling and transport.

2. Match field density in lab compaction to capture stress-strain behavior accurately.

3. Calibrate correlations with local soil databases; generic equations can mislead.

4. Integrate GIS so test points map directly to design segments and construction change orders.

 

An informed, multi-level approach to subgrade evaluation, beginning with quick DCP probes and, when warranted, culminating in resilient modulus testing, helps pavement designers balance risk, performance, and cost. Selecting the right method for each project size and traffic level ensures that pavement thicknesses are neither over- nor under-designed, ultimately delivering longer-lasting roads and better stewardship of construction dollars.