Step one of a Subsurface Investigation: The Desktop Study.

Each site has an underlying geologic unit, which is the first thing I look up. The easiest way to look up local geology is Rock’d. This App overlies your phone's GPS with mapped geologies. ‘Check-ins,’ allow you to document observations of geologic units where they are/were observed. You may even be able to see pictures of your site.

What can you figure out from the geology alone? Consider the types of rock in our area. Sedimentary and metamorphic formations dominate our region. Typically, the parent rock determines the residual soils found underground along with possible subsurface conditions.

Sedimentary mapped rocks mean we expect those rocky conditions to be fairly shallow. The caveat to shallow sedimentary rock? The rock is fairly soft and broken up. It also means the soil is going to be made of what the rock was originally: mud and silt, which turns out reddish purple. The rock durability increases with abrasiveness as the grain size increases. Once the grains are close to gravel size, they can be ‘buff’ or akosic and light purple. Weak layers of weathered rock will be observed in repeating patterns, usually fining upwards.

Sites underlain by metamorphic rock, phyllite, schist, or gneiss, will often have zones of weathered and decomposed parent material. These zones can include different degrees of soil decomposition. These zones are not planar but more like 3D features. Picture the hills you see in a landscape, existing below the surface. Removing this rock depends heavily on the grade of metamorphism, foliation and weathering patterns. Most soils produced are a sandy silt, with even distributions of silty sands.

The last most important geology to understand is karst. Karst is sedimentary but chemically bonded rock. Limestone and dolomite are dissolvable in rainwater, which creates underground drain patterns, erodes rock into voids, and creates water reservoirs that feed dissolving rock. Ultimately, a jumbled bedrock cap holds up the soil mantle. Occasionally, rapidly draining water into a void carries soil down, producing a sinkhole.

With karst landscapes, ‘rock surface’ is an underground variable which does not necessarily match the surface topography. Where solid rock is found, or RQD of 90% or more, removal can be difficult. Soils may also be weak and variable.

Geology under a site is not something you can change, but you can prepare by studying and understanding it. Then, with any necessary adjustments, we can plan cost-effective ways to address the conditions we expect to find below the surface.