NZ Geotechnical Society (NZGS) 22nd Symposium 2025 Auckland
In gravelly soils, the evaluation of the bearing capacity and settlement required for foundation design is challenging. The dynamic cone penetrometer test (DCP), standard penetration test (SPT) and cone penetration test (CPT) are generally not suitable because, due to the presence of large particles, their penetration resistance increases considerably and may reach refusal at shallow depths even when the soil is not particularly dense. This limitation makes it difficult to obtain consistent and reliable correlations between the penetration resistance and basic gravelly soil properties. However, since the 1970’s, the penetration resistance derived from the extra-heavy dynamic penetration test (DPT) has been widely used in China in geotechnical practices for foundation design in gravelly soils. The traditional extra-heavy DPT consists of a 74 mm diameter cone tip continuously driven by a 120 kg hammer with a 1000 mm free fall. Recently, DPT was introduced in New Zealand. Field investigations conducted in Blenheim indicate that it can be economically and effectively driven through alluvial gravelly profiles using a standard 63.5 kg SPT-hammer with a free-fall of 760 mm (heavy DPT). Following a comprehensive literature review undertaken by the authors, in this paper, a DPT-based procedure to evaluate the bearing capacity and settlements of typical shallow foundations in gravelly soils is first presented in detail and then used to characterise an alluvial gravel site in Blenheim. This procedure can assist engineers in designing foundations in gravelly soils based on heavy DPT penetration resistance.
