Australia Model Code of Practice: Excavation Work 6.1a: 6.1 Assess ground conditions and factors that weaken soil

Soils vary in cohesion and friction; dry or saturated sand and gravels low in clay have almost no cohesion. Ground met in excavations falls broadly into three types: loose or running material, soil that tends to crack or crumble, and hard compact soil. Hard compact soil can be the most troublesome because a good-looking face invites risk-taking, while loose material makes the need for precautions plain; crumbly soil is doubtful and needs careful thought, and local authorities can often help. Non-cohesive faces can look solid at the right moisture but crumble on drying or wetting. A face stays stable only while soil strength exceeds the stresses on it; stresses rise with deep cuts and steep slopes, surface loads near the top, shock and vibration from piling, blasting, passing loads or plant, groundwater pressure, and saturation. Strength falls with excess water pressure in sand, drying of sandy or organic soils, prolonged stress, and long idle periods, after which the soil should be evaluated before work restarts.