Suitable Pavement Subsoils

• Subsoil supports pavement, whether under a road or under your driveway. Road construction experts divide subsoil into levels 1, 2 and 3. Level 1 is the most important zone when it comes to support, since it is closest to the pavement. Ideally, the subsoil should be fairly rigid and able to spring back into its original shape after repeated compression stresses from vehicles passing over it.
  
  

Resilient Modulus Testing

• Resilient modulus testing takes place in a laboratory, using soil cores taken at the construction site. The procedure uses the same equipment as another type of testing called conventional triaxial compression (CTC) testing. Both tests compress a cylinder of soil, surrounded by a flexible membrane and pressure sensors, with a piston. Unlike CTC tests, however, resilient modulus testing compresses the soil hundreds of time in quick succession, with each compression and recovery period lasting only one second.
  
  

On-Site Soil Testing

• Because of the hassle and logistical issues involved with removing numerous soil core samples at the construction site and transporting them to a laboratory, an on-site test for resilient modulus is something of a holy grail in the construction industry. Several models can relate small strain modulus, a test using low-level soil vibration that can be performed on-site, with resilient modulus. These models are used with Level 2 and 3 subsoil, but laboratory analysis is used with Level 1 subsoil.
  
  

Test Protocols

• Several standardized protocols for resilient modulus testing exist, since different testing protocols produce different results that make it impossible to accurately compare soils. The first widely used protocol, called Long Term Pavement Performance Protocol P46, was implemented by the Federal Highway Administration in 1996. Many roads in use today were tested under this protocol, which was replaced in 2002 by National Cooperative Highway Research Program (NCHRP) 1-28A protocol. Some states also use the American Association of State Highway and Transportation Officials (AASHTO) T-294 protocol.