Question

Question 4 A sample of soft silty clay from the Thames Estuary is taken from the ground, trimmed to a cylindrical specimen and then placed into a triaxial cell. The sample is compressed isotropically under drained conditions by increasing the cell pressure in the triaxial cell to 450 kN/m², with a back (pore) pressure of 200 kN/m2. At the end of the compression stage the sample is calculated to have dimensions of length = 74.03 mm and diameter = 37.17 mm. The drainage tap is now closed and the specimen is loaded by increasing the axial load whilst maintaining the cell pressure at 450 kN/m2. The sample reaches a critical state and fails at an axial load of 127 N. It has decreased in height by an additional 12.12 mm. Calculate the deviator stress at failure and hence state the undrained shear strength of the soil, Su. [8 marks] b) If the critical state angle of friction of the sample was previously found to be 24º use a sketch giving the Mohr's circles of total and effective stress at failure to find the pore pressure in the sample at failure. [7 marks]

Answer 1

0 Given. Cell pressure cca 450 kN/m": pore pressure U = 200 MNO diameter (d): 37.17 mm - 37.17 x 10'm? Length [4]: 74.03 mm · Ascial Load (X)=127N : Change in Length [42]: 12.12.mm. Deviatorie stress at failure lod undrained condition (da = 8) generally A Qdoo Agz TF [37. 17x102) ? .297x109 min 1 - 12:12 - 74003 TJ = 127 x 16 KM 97.918 kW/m. 1-297X163 mm Shearstrength from mohorcivele Gs. Ditog = 450 +97.918 5470918 Tg = e = 450 km Sscanned with CamScanner

Og = first point of Mohr's circle = hast point of mnohr's circle 1 .450 shear strength a 547.918-45o = 1 48-959 • KNM ③ Eritical state failure plane Angle: 45+ 4 7=45+24 - 570 plastic equilibrium for effective-stress Analysis [0,- 0) = (oz- u) taraf + actanas, CSScanned with CamScanner

0,- Istanas +2ctandf (for Total stress undrained analysis] 2f = 45+% = 45° (@= bº]. 5471918 - 450 +20. (C =.480959 kN/mm (6-0) = (3-0) tanacs i ectandf 1549-918-1] -[450-] tañ [589]+ 2x486959 10 = 488.552 kN/m]. Total stress. Effective stress Dis 547.918 0.947-918-488-42. - 59.398 kN/m DE 450-488.52 2-38152 kN/m Total stress mohn's circle x tan 579 og = 450 450 549.918 CSScanned with CamScanner

Effective stress mohu's circle F -24 59.398 30.52 CScanned with CamScanner