Geotechnical and Ground Improvement Aspects of Motorway Embankments in Soft Clay, Southeast Queensland

Abstract

This dissertation is the first Geotechnical Engineering doctoral thesis in Griffith University, and a detail study of the soft clay as encountered in Southeast Queensland is carried out. In the study process, due to the insufficient laboratory equipments and access to Geotechnical softwares, the dissertation has to be presented in a practical format. In addition, laboratory tests were conducted to investigate the application of chemical (cement and lime) treatments. Three case histories (Sunshine Motorway, Port of Brisbane Motorway, and Gold Coast Highway) are presented in this thesis. The main focuses are on the following aspects: i) soil parameters needed in engineering design from laboratory tests and field measurement, ii) the behaviour of constructed embankment on soft ground with and without ground improvement, iii) the performance of ground improvement techniques. The methods which have been employed to achieve the main aims were conventional methods. Laboratory test data and field measurement data, which were utilised for back-analyses and prediction of constructed embankment of soft ground behaviours, were obtained from QDMR. Thick layer of soft sensitive marine clay were found in the studied areas with up to 13m depth. The performance behaviour of constructed embankment on Southeast Queensland soft clay deposit has been evaluated based on the interpretation of test data, the theoretical analyses and conventional methods for settlement, lateral displacement, and excess pore pressure dissipation. Detail study of the estuarine soft clay as encountered in the Sunshine Motorway is carried out. 33 borehole data were examined to delineate the soft clay profile, which is about 10.5m thickness and varies substantially along the longitudinal section of the motorway. The laboratory value of the coefficient of volume decrease ranged from 1 to 5 MN/m2 and the laboratory values of the coefficient of consolidation are in the range of 0.25 to 0.5 m2/year. The Port of Brisbane Motorway embankments are installed with vertical drains and consist of three sections. Embankment A had drains at 3 meter spacing, and Embankments B and C had drains at 1.5 meter spacings. The maximum settlement obtained after 226 days of monitoring is shown. It can be seen that vertical drain treatment significantly increased final settlement. This increase varied from 70 to 80%. It can be concluded from the settlement results that vertical drains would have increased rate of consolidation. A trial embankment was constructed along the Gold Coast Highway. This embankment was divided into three sections, one section contained no ground improvement, and the other two sections had stone columns at 2m spacing and 3m spacing. For embankment with 3m spacing, the maximum settlement was 490 mm. For embankment with 2m spacing, the maximum settlement was 386 mm. For embankment without stone column, the maximum settlement was 522 mm. Based on the laboratory tests, for cement treated samples with 5 percent to 15 percent cement content, the maximum unconfined compressive strength increased from 132 kPa to 370 kPa for 7 days curing period; these values for 28days curing increased from 170 kPa to 405 kPa. For lime treated samples with lime contents from 2 percent to 15 percent, the maximum unconfined compressive strength increases from 47 kPa to 199 kPa (for 7 days curing period). Results indicated that, 2 percent lime has little effect on peak unconfined compressive strength. This thesis summarises some ground improvement techniques used in Southeast Queensland, and demonstrated the applicable of chemical stabilisation. Overall it was concluded that the addition of cement and lime has favourable effects on the strength characteristics of Southeast Queensland soft clays.