The significance of bubble structures in borehole samples of fine sand : Chaplow, R Imperial Coll. Sci. Technol. London, GB 7F, 21R Geotechnique, V24, N3, Sept. 1974, P333–344

Abstract

G. E. Rawlings, Binnie and Partners, London I wish to add to Chaplow’s Paper on bubble structures as a recent site investigation in North Wales has provided excellent examples of the same phenomenon. However, in this case they were developed in much finer grained deposits and it is considered that they were probably caused by the pressure of gas coming out of solution on stress relief during drilling. During the investigation for the Central Electricity Generating Board’s Dinorwic pumped storage scheme at Llanberis in 1972-3 diamond drilling and percussive boring were carried out by Soil Mechanics Limited through slate tips in the Dinorwic Quarry to sample the underlying materials (Fig. 1). The investigation was required in order to assess the stability of the tips during the rapid fluctuations in the level of the planned lower reservoir formed from the existing Llyn Peris. From previous work at either end of the lake it was anticipated that the tips would be underlain by peats, organic silts, lacustrine silts and silty clays with some localized gravels. Therefore, any method of sampling had to cope with the recovery of such sediments from depths in excess of 35 m. Moreover it was considered desirable to obtain a continuous sequence of samples through the deposits. In practice the optimum method of recovering samples was found to be by using a conventional double tube core barrel to drill through the slate tips to the tip/sediment interface and then, as at Lar, by using a Mazier core barrel to obtain continuous undisturbed samples in the underlying sediments. These methods proved to be remarkably successful although towards