When we receive samples from the Papamoa coastal strip or the Te Puke hinterland, the first thing we check is the fines fraction. Tauranga’s geology is dominated by the Pliocene-age Matua Subgroup, producing thick sequences of silts and clays interbedded with volcanic ash layers. Our laboratory team runs the Atterberg limits test not as a routine checkbox, but as the critical link between soil classification and the engineering behaviour expected on site. The liquid limit, plastic limit, and plasticity index are fundamental parameters for assessing the workability of these materials, which often arrive at the lab still carrying the moisture signature of the Bay of Plenty’s high-rainfall environment. For deep excavations in the Te Papa peninsula, we often pair these results with a triaxial test to capture the undrained shear strength of the saturated cohesive layers.
A plasticity index above 25% in Tauranga’s volcanic-derived soils signals potential volume change issues that standard bearing capacity calculations will miss.
