Geotechnical Services: TRL Dynamic Cone Penetrometer Testing

by | Apr 26, 2024 | Last Articles | 0 comments

Introduction in Transport Research Laboratory Dynamic Cone Penetrometer Testing

Building Compliance Testing Ltd. (BCT) is a specialist company dedicated to ensuring building regulation compliance. In this blog, we delve into the world of geotechnical services, focusing on the TRL Dynamic Cone Penetrometer (DCP) test. This quick and effective method allows us to estimate the strength and thickness of unbound pavement layers. Let’s explore the equipment, test procedure, and its significance for road and pavement design.

The TRL Dynamic Cone Penetrometer (DCP)

1. The Equipment Details

The TRL DCP is a portable device designed to assess the in situ strength of existing flexible pavements constructed with unbound materials. Here are the key components:

  • Hammer Weight: The heart of the DCP, a calibrated weight is dropped or blown onto the pavement surface during testing.
  • Penetration Rod: A sturdy rod that carries the hammer weight and penetrates the pavement layers.
  • Cone Tip: The cone-shaped 60-degree tip at the end of the rod facilitates penetration.
  • Depth Measurement Scale: Indicates the depth of penetration.
  • Extension Rod (Optional): Allows measurements down to a maximum depth of 1500 mm.

2. DCP Test Procedure

  • Preparation:
    • Set up the DCP on the pavement surface.
    • Ensure the cone tip is clean and free from debris.
    • Calibrate the depth measurement scale.
  • Testing Process:
    • Drop or blow the hammer weight onto the pavement.
    • Record the depth of penetration (mm/blow).
    • Repeat the process at multiple locations along the pavement section.
  • Data Collection:
    • Create a depth profile by plotting penetration depth against the number of blows.
    • Identify weak layers based on anomalous penetration rates.
  • Interpretation and Reporting: 

    Back at the office these numbers are then converted into CBR values. These are California Bearing Ratios (CBR) which evaluates the strength of the subsoil and underlying strata, for pavement and roadway design. We tend to use NHBC guidance – Standards 10.2.6

3. TRL Use and Significance

  • Pavement Assessment: The DCP provides valuable data for assessing pavement layers.
  • Unbound Layer Strength: By correlating mm/blow with the Californian Bearing Ratio (CBR), we estimate layer strength.
  • Road and Pavement Design:
    • Thickness Design: Determine the required thickness of unbound layers.
    • Rehabilitation: Identify weak layers for targeted rehabilitation.
    • Construction Quality Control: Verify pavement construction quality.

4. DCP Requirements for Road and Pavement Design

  • Design Manual for Roads and Bridges (CS 229): The TRL DCP test aligns with CS 229 requirements.
  • Soft Verges and Central Reservations: Test in these areas to assess pavement strength.
  • Trial Pits and Cores: Conduct DCP testing at the base of cores or trial pits.


The TRL Dynamic Cone Penetrometer plays a crucial role in understanding pavement behavior. Whether you’re designing new roads or maintaining existing ones, the DCP provides essential insights. At BCT, we pride ourselves on accurate testing and compliance. For further details or a quotation, contact our Service Desk directly via email 1.