Effect of SPT hammer energy efficiency in the bearing capacity evaluation in sands

Abstract

Geotechnical investigation plays a vital role in all projects as the subsurface soil conditions at the project site determine the required site preparation and foundation sizes. Data from Standard Penetration Testing (SPT), the most common form of field testing in geotechnical investigations, is often correlated to soil properties for the evaluation of Net Allowable Bearing Capacity (NABC). The NABC of the project site is critical for the project as this value determines foundation sizes and whether soil improvement is required. The energy efficiency of an SPT hammer is fundamental to obtaining the correct SPT test data. While older SPT hammers, which are manually operated, may have an energy efficiency close to 60%, newer SPT hammers are often automatic and have much higher energy efficiencies. As the majority of correlations between SPT data and soil properties, which lead to NABC in geotechnical engineering, were developed based on SPT data from manual SPT hammers with energy efficiencies close to 60%, it is mandatory to normalise SPT data from higher energy efficiency hammers to that from 60% energy efficient hammers. The practice of using SPT test data without proper energy efficiency correction still exits around the world. Such use of SPT test data leads to unreliable soil bearing capacity and, in case of automatic SPT hammers, this can lead to increased project cost due to larger foundations and unnecessary soil improvement costs. This paper presents insight into such issues and how to ensure correct use of SPT test data and evaluation of soil bearing capacity.