Records of settlement of the M5 motorway over the Somerset Levels

Abstract

An account is given of the secondary consolidation characteristics of recent alluvial deposits under the line of the M5 motor-way where it crosses the Somerset levels. It uses a group of 161 long term settlement records between East Clevedon and St George which were maintained after contract completion in 1973 as the instrumentation installed during construction had .predicted continued settlement. The records were combined with a data file of soils information from the same area compiled by the first author in 1984. The key features of secondary consolidation are reviewed as a guide to the analysis of the records, which are examined for statistical relationships between the measured rates of secondary compression and the operative field variables namely, the embankment loading, the depth of consolidating material and its light preconsolidation. Because of the complex deposition sequence it was only found possible to follow trends. These showed that secondary consolidation was most sensitive to the loading relative to the preconsolidation, or thresh-hold pressure. They also gave some indication of decreasing overall rates of secondary strain with increasing thickness of deposit. Laboratory tests significantly underestimated the field rates of secondary consolidation in all cases. It was reckoned that persistent low level vibrations from heavy traffie contribute to the variations, affecting the shallower layers by a greater amount.