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Session 1b Engineering Implications of Glacially Derived Deposits |
Babtie Geotechnical, 95 Bothwell Street, Glasgow G2 7HX, U.K.
Improvements to the trunk road network around Perth during the early 1980–s involved construction of two new routes crossing the River Earn. Embankments were constructed across lower Strathearn to carry the M90 Edinburgh to Perth motorway and the A9 Stirling to Perth trunk road. Thick Quaternary deposits laid down under conditions of varying climate and sea level occupy the Earn Valley in this area. The nature and extent of these deposits dictated the Engineering Solutions which were adopted for the highway construction.
In this area the Earn Valley is deeply incised into rockhead. Investigations indicated that late glacial "arctic" clays and post glacial organic "carse" clay infilled the buried Valley to depths in excess of 30m and 60m at the A9 and M90 sites respectively. Efforts were made at the site investigation stage to detErmine the anticipated behaviour of these weak and compressible subsoils under embankment loading. Particular attention was given to assessing design parameters for embankment stability and settlement in relation to soil structure, in situ permeability, effective shear strength and pore water response.
The greater problems were encountered at the M90 where embankments of up to 13m in height were designed to prevent shear failure in the foundation subsoils and accommodate Settlements of up to 1.5m within the construction programme. Ground Engineering techniques employed on the various sections of both schemes included the use of stability berms, stage construction, controlled rate of filling and installation of band drains. Extensive use was made of soils Instrumentation to monitor pore pressures, horizontal deflections and Settlements and to control the rate of earthmoving operations to a predetermined construction procedure.
