The in situ density and shearing resistance of the Hampshire Basin Plateau Gravels

Abstract One of the principal Quaternary deposits in Central Southern England are extensive spreads of sandy fine to coarse, mainly flint gravels: those at the higher elevations being referred to as “Plateau” Gravels to distinguish them from the “Valley” Gravels. The most widely accepted view of the origin of the Plateau Gravels is that they represent fluvial terraces deposited under periglacial conditions. Although they have provided excellent conditions for foundations, their actual in-situ density and shear strength characteristics were virtually unknown. To remedy this deficiency, a series of in-situ density tests using the sand replacement technique and direct shear tests using 300 mm square samples have been carried out. Samples of sandy gravel from Highcliffe were compacted by hand tamping to minimise particle breakdown and tested in a saturated condition with a dry density approximately 97% of the in-situ value. The results show a curved failure envelope which in the usual form of power law notation gives a best fit result of:- τ = 1.475 σ n ′ 0-955 Over the range of normal Stresses from 80 to 370 kPa used in the tests, this corresponds to secant φ′ values from 51°. to 48°. Tests carried out on dry samples gave φ′ values up to 4° higher. Tests on other samples with different grain size distributions show that the shearing resistance increases with the percentage gravel (as opposed to sand) content. These results have been combined with published work on other well graded, mainly flint gravels to give a preliminary quantitative evaluation of the influence of percentage gravel content on shearing resistance.