Driven Piles in Fine-Grained Soil-Based Intermediate Geomaterials

Piles driven in Intermediate GeoMaterials (IGM) have possessed many design and construction challenges due to high uncertainty in the engineering properties of IGM, absence of geomaterial classification and static analysis (SA) methods, inadequate Load and Resistance Factor Design (LRFD) recommendations, and lacking knowledge on the change in pile resistances with time. This paper presents our recent development and recommendations for piles driven in fine-grained soil-based IGM (FG-IGM) based on 51 test piles from 25 bridge projects completed in four US states. FG-IGM is categorized into clay-IGM and silt-IGM based on grain size and plasticity. A classification boundary between fine-grained soil and FG-IGM is established at undrained shear strength (su) of 0.129 MPa (2.7 ksf). SA methods for unit shaft resistance (qs) are recommended based on su while SA methods for unit end bearing (qb) are established using the combination of su, pile size (D), and total pile penetration (DB). The proposed SA methods are compared against the existing α-methods developed for soil and validated using 33 independent test pile data. Our statistical assessment concludes that the proposed SA methods provide a more accurate estimation of qs and qb than the α-method. Higher LRFD resistance factors and efficiency factors are determined for the proposed SA methods than for those developed for fine-grained soils. For 1 day after the end of driving (EOD), an average 77% and 44% increase in qs can be considered for steel H-piles in clay-IGM and silt-IGM, respectively, while a higher average 132% increase in qs can be considered for 360-mm close-ended pipe piles in clay-IGM. However, pile setup should be neglected in the qb prediction as H-piles experienced both pile setup and relaxation with the percent change in qb varying from −23% to 345% in clay-IGM and −33%–22% in silt-IGM, and 360-mm close-ended pipe piles experienced −51%–202% change in qb.