UC Davis

The Standard Penetration Test (SPT) N60 value is one of the most common measurements used to estimate in-situ soil strength and density of sands. During SPT testing, a sampler is attached to a series of rods and driven through the bottom of a borehole. A hammer strike at the surface generates energy that propagates through the rods and into the soil at a sampler head. The amount of energy driving the sampler through the soil is dependent on the hammer type, drill rig, and wave propagation along the rod. However, the magnitude of energy loss as a function of the rod length has not been well characterized at depths greater than 15 meters. This study presents the results of four field studies that attempt to quantify the amount of energy loss that can be attributed to increasing rod length. Two SPT rods were instrumented with a series of strain gauges and accelerometers which yielded force and velocity measurements. Measurements acquired from these sensors were integrated to determine the energy loss along the sampler rod. Drilling was performed to depths of up to 85 meters across various sites and soil types to develop the relationship between rod length and energy loss. The study revealed that approximately 1.4% of the theoretical hammer energy is lost for each 3 m of rod added to the sampler rod.