Licture_5 - Vane Shear test (VST) Vane • Used to...

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Unformatted text preview: Vane Shear test (VST) Vane • Used to determine the in situ undrained shear strength of Clay, cu. Filed Tests (Cont’d) • It is a four blades connected to a rod with H = 2D. (1) (2) (3) VST Analysis • The torque, T is applied at a standard rate of 0.1º/sec. 0.1º Vane Test Procedures T = f (cu , H, and D) H, cu = T/ K D, H = Vane diameter and Height, respectively, (cm) T = Torque (N.m) (N.m) cu = Shear strength (kN/m2) The constant K depends on the Vane dimensions ⎛π K =⎜ 6 ⎝ 10 2 D⎞ ⎞⎛ D H ⎞⎛ ⎜ ⎟⎜ 1 + ⎟⎜ ⎟ ⎟ 3H ⎠ ⎠⎝ 2 ⎠⎝ 1 VST Analysis • In case of English unites VST Correction • The strength from VST recommended to be corrected according to: D⎞ ⎛ π ⎞⎛ D H ⎞⎛ ⎟⎜ 1 + K =⎜ ⎟⎜ ⎟ ⎜ ⎟ 3H ⎠ ⎝ 1728 ⎠⎝ 2 ⎠⎝ 2 cu (corrected) = λ cu (VST- field) (VSTCorrection Factor < 1.0 D, H = Vane diameter and Height, respectively, (in) T = Torque (lb.ft) (lb.ft) For Vanes with H = 2D cu = Shear strength (lb/ft2) K = 366 x 10-8D3 Standard unites (cm) λ = 1.7 – 0.54 log [PI (%)] Plasticity index PI = LL - PL K = 0.0021 D3 (in) English system unites VST Correlations • Pre-consolidation pressure, σ’c : Pre- Example_1 Vane shear test was conducted on a layer of clay. The vane dimensions were 63.5 mm (D) x 127 mm (H). At a certain depth, the torque required to cause failure was 0.051 N.m. N.m. The liquid limit of the clay was 46 and the plastic limit was 21. Estimate the undrained cohesion of the clay for use in the design. σ = 7.04 [cu( field ) ] ' c 0.83 • Over-consolidation Ratio, OCR: Over- OCR = β cu ( field ) σ ' o Cu(field) = filed vane shear strength (kN/m2) u(field) and, and, β = 22 (PI)-0.48 Plasticity index PI = LL - PL 2 Example_2 Vane shear test was conducted on a saturated layer of clay. The height and diameter of the vane were 4 in. and 2 in., respectively. During the test, the maximum torque applied was 23 Ib.ft. Ib.ft. The liquid limit of the clay was 58 and the plastic limit was 23. Estimate the undrained cohesion of the clay for Design purpose. Cone Penetration Test (CPT) Friction Sleeve fc = sleeve resistance Friction ratio = fc /qc Conical tip qc = cone tip stress cone Cone Penetration Test (CPT) • A cone with certain base area is pushed into the ground at a steady rate of 20 mm/sec. • Boreholes are not necessary to perform it. • The tip of the cone is usually advanced about 40 mm before the sleeve engaged • The cone resistance, qc: Is the vertical force applied to the cone divided by its horizontally projected area. • Friction resistance, fc: is the resistance measured by a sleeve located above the cone with the local soil surrounding it. 3 CPT Correlations 1. Relative density of Sand, Dr : Sand, Start of the next lecture Typical CPT reading ⎡q ⎤ c ⎢ ⎥ ⎛ ⎞ ⎢ pa ⎥ 1 Dr = ⎜ ⎜ 305 Q ( OCR ) 1.8 ⎟ ⎢ ' 0.5 ⎥ ⎟ c ⎝ ⎠⎢ ⎛ σ o ⎞ ⎥ ⎜⎟ ⎢ ⎜ pa ⎟ ⎥ ⎣⎝ ⎠ ⎦ qc and σ’o are the cone tip resistance and vertical effective stress. Pa = atmospheric pressure (100 kN/m2) OCR = over consolidation ratio Qc = 0.9 to 1.1 is the compressibility factor 2. Drained friction angle of Sand, φ’ : Sand, 4. Pre-consolidation pressure for Clay σ’c : PreClay ⎡ ⎛ q ⎞⎤ φ = tan ⎢0.1 + 0.38 log10 ⎜ c' ⎟⎥ ⎜σ ⎟ ⎝ o ⎠⎦ ⎣ ' −1 σ 'c = 0.243(qc ) (MN/m2) 0.96 qc and σ’o are the cone resistance and effective vertical stress. 3. Undrained shear strength of Clay, cu: Clay, (MN/m2) 6. Over-consolidation Ratio for Clay, OCR : OverClay, q −σo cu = c NK Consistent units is required ⎛ q −σ ⎞ OCR = 0.37 ⎜ c ' o ⎟ ⎜σ ⎟ o ⎝ ⎠ 1.01 σo is the total vertical stress. NK = 15 for electronic cone, and 20 for mechanical cone. σo and σ’o are the total and effective vertical stress. 4 ...
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This note was uploaded on 02/22/2010 for the course CVE 333 taught by Professor Magdy during the Spring '10 term at American Dubai.

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