Structural engineers require several specific types of soil data and investigations to accurately design a helical pile foundation. These tests allow the engineer to determine the soil’s ultimate bearing capacity and identify the most suitable bearing strata for the piles.
Key soil tests and data sources referenced by structural engineers include:
- Standard Penetration Tests (SPT): Engineers analyze the SPT N-value to correlate soil strength parameters for both cohesive and granular soils.
- Cone Penetration Tests (CPT): These tests help determine tip resistance, which is used to calculate the end-bearing capacity of the helical plates.
- Laboratory Soil Analyses: Comprehensive laboratory testing provides further detail on soil behavior, such as undrained shear strength for cohesive soils (clays and silts) and friction angles for cohesionless soils (sand and gravel).
- Geotechnical Analysis and Stratigraphy: Accurate layering data is essential for determining the depth of the bearing strata and the placement of the helix plates. This includes identifying whether the soil is cohesive, cohesionless, or stratified.
- Site-Specific Investigation: According to the ISSMGE, site-specific investigation is necessary to accurately determine capacity, as local stratigraphy directly influences both end-bearing and shaft friction contributions.
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