A contractor on the M18 motorway widening near Ennis once faced a pavement subgrade that looked solid but failed under compaction. The material had a high silt content, a common feature of the drumlin-derived glacial tills spread across County Clare. They needed a quantifiable bearing value, not a visual guess. That is where the laboratory CBR test comes in. We determine the exact resistance of your subgrade or granular layer to penetration at controlled moisture and density, following the soaked protocol that simulates the most critical condition for Irish roads: a wet winter. Before running the CBR, it is often useful to check the particle size distribution with a grain size analysis to identify the fines fraction, which directly controls the soaked strength loss.
A soaked CBR value of 2% in an Ennis silt subgrade requires a capping layer of at least 350 mm just to reach a foundation class suitable for light traffic.
Methodology applied in Ennis
Critical ground factors in Ennis
The most frequent mistake we see in contracts around Ennis is using an unsoaked CBR value for design. A dry glacial till can show a CBR of 15% or more. That same material, after one Irish winter with a high water table, drops to 3%. Designing a pavement on the unsoaked figure means the bituminous layers crack within two years. The soaked CBR test exists precisely to prevent this. Another error is testing the bulk sample without removing the cobbles and boulders that are common in the limestone till. A single stone under the plunger gives a false high reading. We scalped and test the matrix fraction—the fines and sands—because that is what controls the long-term bearing capacity when the water rises in the drumlin landscape surrounding the town.
Our services
Our laboratory in Ireland provides the CBR test integrated with the full geotechnical investigation workflow. We do not just send a number; we deliver the compaction curve, the moisture-density relationship, and the soaked CBR on the same sample, giving the design engineer the complete input for a TII-compliant pavement foundation.
Soaked CBR with Moisture-Density Relationship
Determines the CBR at optimum moisture content and after 96 hours of submersion. Includes the Proctor curve and dry density. Essential for capping and subbase design under Irish conditions.
CBR Swell Potential Measurement
Measures the percentage of swell of the specimen during the 4-day soaking period. Critical for silty and clayey subgrades in Ennis where volume change can heave the pavement.
CBR at Specified Density (End-Product Specification)
We compact the material to a client-specified density and moisture content, then test the CBR. Used to verify that compacted fill meets the design bearing value before paving.
Questions and answers
What does a laboratory CBR test cost in Ennis?
The laboratory CBR test, including the Proctor compaction curve and the 96-hour soaked CBR value, ranges from €130 to €180 per point. The final cost depends on whether we test one or three compaction points and the need for swell measurements. We recommend three points for a reliable CBR versus density curve.
Why is the 96-hour soaking period mandatory for Ennis subgrades?
The 96-hour submersion replicates the worst-case saturation scenario for a pavement foundation. In Ennis, the combination of high rainfall—over 1,100 mm annually—and low-permeability glacial till means the subgrade remains saturated for long periods. The soaked CBR is the only value accepted by TII for design; the unsoaked value is not representative of the long-term condition.
Can you test granular material with large stones?
We test the matrix fraction only. The CBR mold is 152 mm in diameter, so particles larger than 37.5 mm are removed by wet sieving and replaced with an equivalent mass of 20–37.5 mm material. This prevents a single cobble from biasing the result and gives a conservative value that represents the fines and sands controlling the bearing capacity.
What CBR value is required for a residential road in County Clare?
For a lightly trafficked residential road, TII standards require a minimum soaked CBR of 2% for the subgrade without a capping layer. However, if the CBR is below 5%, a capping layer of 350 mm to 600 mm of Class 6F1 or 6F2 material is specified. Most Ennis tills achieve 2–5% soaked CBR and require a capping layer.
How long does it take to get the CBR results?
The full process takes 5 working days. Compaction and sample preparation take one day, the 96-hour soaking is four days, and the penetration test and report are completed on day five. We can provide the Proctor data on day one for urgent compaction control, with the CBR value following.