FOUNDATION DRILLING
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berkel & company feature
excavation. Berkel was selected by the design team for this unique project, in part, due to its background in underpinning and earth-retention required for the lowering of similar below-grade structures.
Site Geology
The project sits at the intersection of the Coastal Plain and Piedmont Physiographic provinces with alluvial soils overlying residual soil and rock. The subsurface profile can be generalized into five strata: (I) fill, (II) natural alluvial soils, (III) natural residual soils, (IV) weathered rock, and (V) less-weathered rock. The weathered (IV) and less-weathered rock (V) were generally the target bearing strata for deep foundations for the new construction. f Stratum I: Fill soils at the site ranged from 4.5 to 22 feet (1.37 to 6.7 m) below existing grades with varying Standard Penetration Test (SPT) N values from 2 to 58 blows per 1 foot (300 mm), however the higher SPT N values were attributed to the presence of gravel and other debris. f Stratum II: Natural alluvial soils were encountered from 22 to 47 feet (6.7 to 14.3 m) below the existing grade. These soils consisted of various sands, gravel, and clay deposits. SPT N values ranged from 2 to 48 blows per foot with the higher blow counts again being attributed generally attributed to the presence of gravel and not indicative of in situ density or consistency. f Stratum III: Natural residual soils were encountered below Stratum II or directly beneath Stratum I where some deeper fills were present. These soils were generally sand and silt with some clay. SPT N values ranged from 3 blows per foot to 50 blows over three inches (75 mm). Rock fragments found in core samples were the likely cause of the high SPT N values in this layer. f Stratum IV: Weathered rock was encountered below Stratum III to depths of 50 to 72 feet (15.2 to 21.9 m) below the existing grade. SPT N values typically exceeded 50 blows per foot. f Stratum V: Boring refusal occurred at multiple boring locations on less weathered rock that ranged in depth from 52.5 to 72 feet (16 to 21.9 m) below existing grade.
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Generalized subsurface profile
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