Definition - What does Cobbled Soil mean?
Engineers use geotechnical surveys to reveal the soil types of the ground through, which a trenchless project will run. Cobble soil contains small rocks of various sizes (from a pebble to a basketball size) that are mixed in with the soil. Some trenchless methods and equipment are not suitable for cobbled soil as they can be damaged by the presence of cobbles and the project can be delayed for repairs.
Trenchlesspedia explains Cobbled Soil
Directional drilling through cobbled soil is not recommended because cobble can damage or break the drill bit blades. Cobble tends to move around in the soil and thus is not easy to penetrate with the drill bit. They also fall back into the tunnel during drilling and reaming which blocks up the bore. Using the right drilling fluid is essential, otherwise the cobble will not be brought out of the hole. Auger boring is a more suitable trenchless method for cobbled soils as the auger more easily penetrates the soil and extracts the cobble.