Piezocone Penetration M R Selamat

Piezocone Machine

Piezocone Penetration (CPTU) Advantage - Able to carry out tests at depths and unlike in boring, on relatively undisturbed soils, cost effective Disadvantage - No samples retrieved, cannot penetrate hard layer or rock like boring can Why? - To gain a relatively quick information of soil to 30 m depth

Piezocone - CPTU Observe how soil changes with depth Observe how soil resistances (skin and end) change with depth Observe how water pressure changes with depth Cannot get samples Cannot do field tests (other than those done by the cone)

35.6 mm Sleeve Cone

Sleeve Resistance = Sleeve Friction = fs Cone Resistance = qc

Sleeve Resistance (Friction) corrected for pore pressure effects = ft Cone Resistance corrected for pore pressure effects = qt Friction Ratio = Rf = fs/qc = ft/qt = fs/qt

Sleeve (150 cm2) Porous Stone (Filter Element) Cone (600, 10 cm2)

Pore (water) Pressure = u

Pore Pressure Ratio = Du/(qt – svo) Where: Du = Excess pore (water) pressure = (u – u0) u0 = In situ equilibrium (static) pore (water) pressure svo = In situ total vertical stress = S(gihi)

Dissipation U = Percentage Dissipation (Degree of Dissipation) ut – u0 U = .100% ui – u0 It is frequently recommended that Dissipation be continued to at least U = 50% ut = u at time t u0 = In situ equilibrium (static) pore (water) pressure ui = Immediate (initial) in situ pore (water) pressure at t=0 ui = Up to 26 u0 in clays

Rate of Penetration 20 mm/s + 5 mm/s (International Society of Soil Mechanics and Foundation Engineering) Depth interval between reading >200 mm (ISSMFE) 10-50 mm (most system) >20 mm (Norwegian Oil Company) Saturation of piezocones Filter, filter-transducer tubing, and transducer cavity must be saturated using silicon oil or glycerin Calibration Effects of temperature

Soil Description vs. Soil Classification The description of soils as drilling progresses is not soil classification (such as USCS) but mere layman’s description such as: Sand, Yellow Sand, Clay, Clay and Sand, Yellow Clay, Dense Sand, Loose Sand, Stiff Clay, etc. True soil classification is done when a sample has been subjected to lab tests and the results produced. Some examples of classification names are MH, SM etc. A piezocone test also describes soils and appoint names but with another and different procedure, unlike in a drilling or in a soil test.

Soil Classification CPT chart cannot be expected to provide accurate predictions based on USCS, ASSHTO etc Charts of page 53, 54, 55, and 56 Coefficient of Consolidation Ch versus t50 is given in page 79

SPT in Boreholes Names of Test: Standard Penetration Test, SPT Data: Standard Penetration Test Number, SPT(N), Standard Penetration Resistance Hammer Weight: 63.5 kg (140 lb) Hammer Lift: 760 mm (30 in) Dimensions of Sampling Tube: 35 mm inside diameter, 50 mm outside diameter, 650 mm length Procedure: Number of blows per 75 mm advance is recorded for 6 consecutive advances starting at the bottom of a borehole. Numbers of blows of the last 4 advances (300 mm, 12 in, foot) are summed and is called SPT(N). Numbers of blows of the first two advances are discarded due to disturbance of the soil layer by washing and boring. Maximum number of blows per foot to record is 50.

How to get SPT(N) number 5.4 of MS1056 N30 is blows/30 cm N60 is blows/30 cm assuming 60% hammer efficiency Some demand blows/7.5 cm Discard the blows of the first 15 cm (2x7.5) Read blows for every 7.5 cm So readings of 2, 3, 6, 7, 10, 19, 20 will give N30 = 42 Normally read taken for every 1.5 m interval

SPT(N) versus f correlation for cohesion-less soils (Old JKR Document) SPT(N) versus f correlation for cohesion-less soils (Coyle and Castello, 1981) SPT(N) versus f correlation for cohesion-less soils (Old JKR Document)

SPT(N) versus f correlation for cohesion-less soils (McCarty, David F, 1998) 10 20 30 40 50 60 32 35 38 42

SPT(N) c, kPa 2 4 8 15 30 14 25 50 100 200

SPT in Sand Angle of Friction Description 0 – 4 0 – 28 degree Very Loose 4 – 10 20 – 30 Loose 10 – 30 30 – 36 Medium 30 – 50 36 – 41 Dense > 50 > 41 Very Dense

SPT in Clay Cohesion (c), kPa Description < 2 0 – 14 Very Soft 2 – 4 14 – 25 Soft 4 – 8 25 – 50 Medium 8 – 15 50 – 100 Stiff 15 – 30 100 – 200 Very Stiff > 30 > 200 Hard

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