The AASHTO LRFD code allows the use of IGM for foundations. The WVDOH uses the definition as presented in C 10.8.2.2.3 with the modifications as follows:
- Both Cohesive and Cohesionless IGM’s are determined at the depth where a set of blows per 6 inches is 25 or greater, and any successive blow counts must be at least this value or greater.
- The sample must have rock-like structure or rock grain.
- Inflated blow count due to the presence of harder rock fragments in a soil matrix does not constitute IGM. We do not have till.
- Use the blow counts directly from autohammers without the N160 adjustment.
- The upper limit of IGM is where the sample cannot be peel by a pocket knife.
When logging core samples, state the rock type (e.g., claystone, mudstone, siltstone, etc.) followed by “(IGM)” when the above conditions are met. The depth at which a given IGM is shown on the boring logs is where a set of blows is 25 or greater. An IGM may be encountered beneath harder rock.
Refer to the WVDOH Core Boring Document for further discussion as to standard penetration sampling at 2.5-foot intervals within the IGM until coring should first be attempted at 50 per 6 inches. The intent is to better define the bedrock surface where our piles and footing are sometimes supported.
For bearing resistance in IGM, use the same resistance factors of 0.45 as discussed further in the Spread Footings on Rock
section of this webpage. Generally, skin friction is not used in IGM.