Atterberg Limits Testing in New Westminster: Plasticity & Soil Behavior

For a single sample tested for liquid limit and plastic limit, the cost ranges from CA$80 to CA$140 depending on the number of points in the liquid limit determination and the sample preparation method. A multi-point liquid limit test takes more technician time but gives a more reliable flow curve. We typically recommend budgeting for at least three samples per borehole to capture the vertical variability we see in the city.

The Casagrande cup method, which we use per ASTM D4318, measures the number of blows to close a groove over a standard distance. The fall cone test, more common in European practice, measures penetration depth. For the sensitive marine clays found in New Westminster, the Casagrande method has a large database of correlations to undrained shear strength that local engineers rely on, so we default to it unless the project specifications require otherwise.

The test requires material passing the No. 40 sieve. If the sample has significant sand, we wet-sieve it first and then dry the fines fraction. For typical New Westminster soils, the marine silts process easily, but the glacial till from the upland areas can contain gravel and cobbles that require careful splitting in the lab to obtain a representative fines portion.

The plastic limit is the moisture content at which the soil transitions from a plastic to a semisolid state. For compaction, you want the moisture content near the optimum, which is typically a few percent below the plastic limit for clayey soils. If the contractor is working with Queensborough silts that have a plastic limit of 22, we would recommend targeting a moisture content around 19 to 20 percent to achieve maximum dry density without creating a pumping failure under the roller.