Methods to properly evaluate subgrade strength

by Nathan Young, P.E.

Accurately evaluating subgrade strength is critical for almost any job site. The strength may be acceptable in a few areas; however, assuming that same strength spans the entire site can prove to be very costly if that assumption is wrong. Fortunately, a few test methods exist to quickly gather the required data in the field: proofrolling, a standard penetration test (SPT), Probe Rod Evaluation and Dynamic Cone Pentrometer (DCP) testing. For this discussion, an accurate evaluation of subgrade modulus and California Bearing Ratio (CBR) for pavement support and design are desired. Let’s review these methods, as well as the pros and cons of each.

Proofrolling: A fully loaded tandem axle dump/water truck drives over the site

Pros:

• Quick and practical method for a generalized evaluation of subgrade stability for a relatively large area of exposed subgrade.
• During earthwork construction, fully loaded trucks are readily available.

Cons:

• Does not provide a subgrade modulus or CBR value
• Exposed soils may have hardened resulting in a “crust” that supports the loaded truck. This prevents any soft, underlying soils from exhibiting their lack of strength that can be realized later as the “crust” breaks down because of environmental conditions (freeze/thaw, moisture, etc).

SPT: Performed by a drilling rig during the due diligence phase of a project

Pros:

• Allows for the physical evaluation of underlying soils through additional laboratory testing
• Ascertain soil strength at various depths through N-values (blow counts) by a weighted hammer that can be translated to estimated values for subgrade modulus and CBR values

Cons:

• Involves more time to mobilize the drilling rig, drill and obtain samples, perform laboratory testing and generate a geotechnical engineer’s report
• The soil strength parameters used for pavement support are based on an engineer’s previous experience with soils having similar physical characteristics.
• Information is only accurate at the time of subsurface exploration. Soil conditions can vary substantially depending on the timing of construction and weather conditions.
• Evaluates a very small surface area and not practical to accurately evaluate a large area

Probe Rod Evaluation: Handheld T-handle metal rod

Pros:

• Quick method of determining the depth to firm, underlying soils
• Readily available method because most materials testing firms’ field representatives are equipped with probe rods

Cons:

• Evaluates a very small surface area and not practical to accurately evaluate a large area
• Does not result in a subgrade modulus or CBR value
• Variables regarding operator use and experience

DCP (ASTM D6951): An instrument consisting of a standard diameter steel rod, a hardened conical tip and a standard weight hammer that is dropped from the top of the rod against an anvil to advance the tip into the ground 

Pros:

• Quick evaluation of subgrade conditions
• Obtain accurate subgrade modulus and CBR values in the field during construction. Performed with calibrated equipment in accordance with ASTM testing procedures.

Cons:

• Evaluates a very small surface area and not practical to accurately evaluate a large area
• Most materials testing firms have DCP equipment; however, they do not regularly use it.

Now that we’ve reviewed the typical tools and methods for evaluating soil strengths, how should subgrade strength be properly evaluated? These following steps are suggested:

1. Gather SPT data during the planning/due diligence phase of the project. Use the N-values to establish a conservative subgrade CBR value. This value should be slightly conservative to allow for any changes in subsurface conditions that may occur between the time that drilling operations are performed and when construction commences.
2. Perform a proofroll during the initial phase of construction to evaluate the stability of the entire area.
3. Supplement the above with DCP testing to provide accurate and timely value(s) of subgrade strength.

This article is republished from the Tensar Americas Blog, https://info.tensarcorp.com.