Austin’s hot summers and heavy spring rains create a tough cycle for road subgrades. The soil here is mostly clay, and it shrinks and swells with every change in moisture. That movement directly affects pavement performance. We run tests on soil samples to determine the subgrade’s strength and stiffness. Our lab follows ASTM D1883 for CBR and AASHTO T-193 for soaked conditions. Before we start any road subgrade design, we check the natural moisture content and compaction curves. The goal is to give you numbers you can use for structural section thickness. For weak layers, we often recommend adding geotextiles to separate the subgrade from the base course. This simple step reduces long-term deformation.
A 2% swell in a soaked CBR test means the subgrade needs stabilization before placing pavement layers.