The Effect of Grain Shape on Pile Capacity in Sand
Authors: Stevens Robert 
Published: April 2012

A case history is presented to better understand the effect of grain shape onpile capacity in sand. Results are presented for 20-in.-square precastprestressed concrete piles driven with a Delmag D46-23 single acting dieselhammer in Escambia Bay, Pensacola, Florida. Soil conditions consisted of verysoft to soft clay to a depth of 40 ft, underlain by medium dense to dense fineto medium sand to a depth of 78 ft, and dense to very dense silty fine sand. The driving of 18 indicator piles was monitored using a Pile Driving Analyzer. The Case method was used to estimate the pile capacity. CAPWAP analyses werealso performed for 11 piles. Static load tests were performed on four piles. The piles drove harder than expected but the load tests indicated lowercapacities than expected. The sands did not have high carbonate content, cementation, or high mica content. The angular to subangular shape of the sandgrains was the only unusual feature. Large rebounds were observed during theinstallation of the indicator piles. The CAPWAP analyses also indicated thatthe sands have soil quake and damping values generally larger thanexpected.

Introduction

Wave equation analyses were performed for a 20-in.-square prestressed concretepile driven with a Delmag D46-23 single-acting diesel hammer operated at fuelsetting #4. The hammer cushion consisted of a micarta disc-aluminum platesandwich. The pile cushion consisted of 6 inches of laminated green oak planksor 4 inches of laminated 0.75-in.-thick plywood sheets. The piles were designedwith a prestress of 1250 psi using 20 strands of 0.5-in.-diameter Grade 270prestressing wire. The concrete had a 28-day compressive strength of 5500 psi. The soil quake and damping parameters recommended by Roussel (1979) were usedin the wave equation analyses. The side and point quake is 0.10 inch. Sidedamping of about 0.08 sec/ft and point damping of 0.15 sec/ft, recommended forsand, were used in the wave equation analyses. Wave equation analysesdetermined that an ultimate axial compressive capacity of 400 tons could beachieved with a 20-in.-square prestressed concrete pile driven with a DelmagD46-23 single-acting diesel hammer operated at fuel setting #4 to a final blow count of 56 to 72 bpf, for oak and plywood pile cushions, respectively. Indicator piles IN-1 through IN- 12 were generally installed tothis criterion. The pile capacity determined using the Case method wasgenerally greater than 400 tons using a damping factor, J, of 0.15, a valuenormally used for clean to silty sands. Pier, soil boring, and static load testlocations are presented in Fig. 1.

Definitions

Pile driving is monitored using strain transducers and accelerometers attachednear the pile top. The energy transmitted to the pile is obtained byintegrating the product of the measured pile top force and velocity. The soilresistance to driving is determined from the measured force and velocity and adamping that is a function of soil type. The Case damping coefficient is equalto 0.10 to 0.15 for clean sand, 0.15 to 0.20 for silty sand, 0.20 to 0.40 forsilt, 0.40 to 0.70 for silty clay, and 0.70 to 1.00 for clay.