Geotechnical properties of sandstone fill
University of New Brunswick
Sandstone fill is used in the construction of the Miramichi waste water treatment lagoon. One of the consultants for this project has requested a study of the geotechnical properties of this material. The knowledge of the geotechnical properties is an essential component for designing earth retaining structures. A 100 ton floating ring consolidometer is used to measure the settlement, percentage of wall friction and the angle of wall friction between the sandstone fill and the test cylinder. The angle of wall friction decreases from 39° to 34° with an increase in the average vertical stress from 178 kPa to 976 kPa. Approximately 21% of the load applied to the sample is lost to wall friction. This value is used in a correction factor for the vertical stress in the split ring consolidometer. For an average vertical stress of 976 kPa, there is 4.1% settlement in the sandstone fill. The vertical rebound is only 0.4%. The angle of internal friction (oJ>) was measured from direct shear tests on the sandstone fill. The Mohr failure envelope between 73 and 320 kPa is x = 34 + on tan 46° with an angle of internal friction of 46°. The coefficient of earth pressure at rest (Ko) and coefficient of active earth pressure (Ka) are measured in the split ring consolidometer. The decrease in Ko as the vertical load is increased is caused by the mobilized friction angle. At a vertical stress of 200 kPa, Ko is 0.16. Ka is measured by allowing the wall of the consolidometer to yield at a constant vertical stress of 200 kPa. There is a decrease in Ka from 0.16 at 0% horizontal strain to 0.02 at 0.8% horizontal strain. The earth pressure coefficients are calculated from Jaky's (Ko) and Rankine's (Ka) relationships. The calculated value of Ko and Ka is 0.28 and 0.16 respectively. There is a difference of 0.12 - 0.14 between the measured and calculated earth pressure coefficients caused by the "clast effect". The irregular and elongated shaped sandstone clasts act as fibres which laterally reinforce the sandstone fill.