Bedrock

Cols

Summary

Bedrock is the solid rock that underlies looser surface material, with the top of the rock called rockhead. 🟠Locating rockhead by digging, drilling, or geophysical methods is an important task in most civil engineering projects, because it sets foundation type and excavation cost.

Cols

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Terminology

Rockhead: the top of the bedrock
Regolith: bedrock plus weathered rock and loose superficial deposits
Saprolite: weathered rock formed in subsurface weathering at the upper boundary
Crystalline bedrock / sedimentary and foliated rock: the two bedrock classes in the code presumptive bearing table

Composition and production

Raw materials

Solid rock; exposed bedrock weathers physically or chemically, which alters its structure and makes it more susceptible to erosion
Where superficial deposits are thick, bedrock lies hundreds of meters below the surface

Uses in buildings

Building elements

Spread and Continuous Footings: footings bearing directly on sound rock at the highest presumptive capacity

Pros and cons

Pros

Strongest bearing material in the code presumptive table (crystalline bedrock 12,000 psf)

Cons

Depth is unknown until investigated
Weathering at the rockhead can alter quality
Excavation in rock is slow (sedimentary and foliated rock presumptive 4,000 psf)

Sources

Wikipedia: Bedrock, https://en.wikipedia.org/wiki/Bedrock
UpCodes: 2024 IBC Chapter 18 soils and foundations, https://up.codes/viewer/department-of-defense/ibc-2024/chapter/18/soils-and-foundations