Flat Plate Dilatometer Test & TEXAM Pressuremeter Tests in Soil Investigations with Special Emphasis on Evaluating Settlements

Transcription

1 Flat Plate Dilatometer Test & TEXAM Pressuremeter Tests in Soil Investigations with Special Emphasis on Evaluating Settlements David L. Tucker, P.E., Regional Technical Expert Deep Foundations U.S. Army Corps of Engineers - Baltimore District CENAB-EN-GF David.L.Tucker@usace.army.mil

2 Conventional Standard Penetration Test (SPT)

3 Computation of Settlement - e log p curve One Dimensional Consolidation Test or Oedometer Test Procedure consists of applying series of normal loads and measuring the corresponding vertical deformations. Specimen is constrained laterally simulating 1-D consolidation ONLY in the vertical direction relating vertical stress to the vertical strain by the CONSTRAINED MODULUS (M)

4 Disturbed Sample What am I going to do with this????

5 Confined Compression Laboratory Oedometer Tests Stress-strain data from consolidation test typically plotted as Traditional stress versus strain (linear scales for both axis) to obtain 1-D constrained tangent modulus M e log p (p on semi- log) mid 1930 s (utilizing void ratio in lieu of strain) (After Lamb and Whitman, 1969)

6 Constrained Modulus (D and M) For Poissons ratio = , D = 0.85E (After Lamb and Whitman, 1969)

7 Constrained Modulus (M) from DMT The modulus (M or D) determined from the DMT is the vertical drained constrained (one-dimensional) tangent modulus at σ vo and is the same modulus as obtained by the oedometer test.

8 Flat Dilatometer Test (DMT) Developed in the 1970 s in Italy by Dr. Silvano Marchetti Introduced in the US in 1981 by Dr. John Schmertmann and Dr. David Crapps (University of Florida) ASTM standard D6635

9 Dilatometer Test Applications Settlement Prediction Sands and Clays Soil strength Parameters Undrained Shear Strength Friction Angle OCR and Preconsolidation Pressure Total unit weight of soil Coefficient of lateral earth pressure (Ko) P-y Curves for Laterally Loaded Piles Monitoring Densification (i.e. monitoring of deep compaction etc.)

10 Calibrated static deformation test Dilatometer Test Performed at 0.1 to 0.2m (4 8 inch) intervals (near-continuous) Low volumetric and shear strain induced during penetration measures significance of lateral stress and stress history Accurately measures deformation modulus, drained friction angle in sands and undrained shear strength in clays Test Repeatability Error: <10% Easy to use from drill rig, barge, but most efficient w/ direct push rig

11 Flat Dilatometer Test (DMT) (Marchetti Dilatometer)

12 Distortions due to Insertion (OK for rupture, but for deformation??) The shape of the blade causes less soil disturbance (insertion strains) than the conical shape of the SPT or CPT (1/3 less shear strain than those developed by the CPT). Because of less soil disturbance, the dilatometer results in a better measurement of the stress history of the soil and better correlations with the true in-situ conditions. (After Marchetti,2001 )

13 Video

14 Working Principle Displacement Controlled Test 1) Push Dilatometer blade to test depth (typ. 20 cm (8-inch) intervals at 2 cm/sec and measure thrust with load cell Thrust used to compute φ, ko, OCR, p c for sands ) Soil presses against membrane creating electrical contact--inflate membrane with nitrogen gas until it separates from blade 0.05mm and record pressure ( A reading) Take Off Pressure (After Marchetti, 1996) 3) Continue to inflate membrane until it moves 1.1 mm. Electrical switch will turn on and record pressure ( B reading) 4) OPTIONAL - Depressurize and record pressure when membrane in original A- position ( C reading pore water pressure)

15 Dilatometer Test Can be pushed with Cone Rig into soil with N 60 up to 40 blows for foot (w/18 tons thrust) Heavy drill rig (CME 55) with N 60 up to blows per foot (w/7.5 tons thrust) ISSUES IN DENSE and GRAVELLY SOILS Spacing of DMT from adjacent open boring shall be 25 boring diameters and 5 feet from adjacent DMT or CPT sounding.

16 Flat Plate Dilatometer Test (DMT) Load cell for thrust measurements 16

17 Flat Plate Dilatometer Test (DMT) 180-mm 60-mm dia. 96-mm 15-mm 17

18 Flat Plate Dilatometer Test (DMT)

19 Constrained Modulus (M) from DMT The constrained modulus determined from the DMT is highly reproducible. Comparisons both in terms of (1) M DMT versus M from high quality odometer test and (2) Predicted versus measured Settlements, result in accurate and dependable results Norway Clays (After Marchetti,2001 ) DEFORMATION MODULUS -- DMT DATA, M (bars) Residual Alluvial Virginia Soils (After Failezger & Bullock,2004 ) Tokyo Bay Clay DEFORMATION MODULUS -- OEDOMETER DATA, M (bars)

20 Flat Dilatometer Test (DMT) Data Reduction The A and B pressure readings are corrected for membrane stiffness and then converted using relationships established by Marchetti (1980), Schmertmann (1982) and others to obtain soil parameters AT EACH 10 TO 20-CM INTERVAL Interpretation to common soil parameters derived from INTERMEDIATE DMT PARAMETERS. Material Index I D Horizontal Stress Index K D Dilatometer Modulus E D

21 Flat Dilatometer Test (DMT) Data Reduction Intermediate DMT parameters Note the necessity in applying the correction R M to E D ; R M = f(i D, K D ). E D lacks information on stress history and lateral stress, drained versus undrained behavior.

22 Importance of Thrust Measurements for I D > 1.2 (Sands) φ Ko for sands OCR and P c for sands

23 Membrane Calibration A & B account for Membrane Stiffness at 0.05mm and 1.1mm respectively and gage zero offset Calibration outside the tabulated values indicates either membrane to be replaced or exercised. MEMBRANE SHALL BE REPLACED IF IT CAN T BE EXCERCISED TO FALL WITHIN PUBLISHED RANGE. Calibration extremely important in very soft soils.

24 Settlement Predictions DMT accurately measures the constrained deformation modulus (M or D) of the soil. DMT performed at 10 to 20 cm (8-inch) intervals (near-continuous) to create a Modulus versus depth profile for use in the settlement analysis. Because of the close test depth intervals, the increase in the applied vertical stress is accurately computed utilizing Boussinesq, Westergaard or other stress distribution method. (After Monaco,2006 )

25 Settlement Predictions Schmertmann (1986) developed the Ordinary and Special methods for computing settlements beneath structures and embankments. Special method adjusts M to the average vertical effective stress during the loading that produces the settlement. (i.e. does stress occur below or above p c or start below the p c and extends beyond the p c etc) Generally Ordinary versus Special Methods are within 10% of one another.

26 Accuracy of Settlement Prediction Accuracy of settlement computations have been demonstrated by numerous researchers from a data base of a variety of soils. Average ratio of predicted to actual settlement utilizing the DMT is 1.07 with a coefficient of variation (standard deviation/average value) of (Failmezger,2006 )

27 Example Poplar Island Soft Clays DH 626 Worst Case

28 Example Poplar Island Soft Clays DH 626 Worst Case Similar to the settlements observed in Dike near borings M- 72/73 Area

29 Example Poplar Island Soft Clays DH 629/DMT 718 S S

30 Example Poplar Island Soft Clays DMT 718 Laboratory Consolidation Test 17 (5.2 m) Slope = M = σ/ ε = 27 σ vo of 0.32 bar

31 Example Poplar Island Soft Clays DMT 718 Laboratory Consolidation Test 31 (9.5meters) Slope = M = σ/ ε = 29 σ vo of 0.57 bar

32 Example Poplar Island Soft Clays DMT 718 DMT versus Lab 17 (5.2 meters) & 31 (9.5m) Slope = M = σ/ ε = 27 σ vo of 0.32 bar 27 bar 29 bar Slope = M = σ/ ε = 29 σ vo of 0.57 bar

33 Example Poplar Island Soft Clays DMT 718

34 Example Poplar Island Soft Clays DMT 718 DMT versus UC Lab 31 (9.5meters) Su: 0.23 vs 0.36 bar Note: Sand in UC test will lower Su results & UC tests will generally yields lower values than UU test.

35 DMT CPT Comparisons feet meters

36 DMT CPT Comparisons feet meters

37 DMT CPT Comparisons feet meters

38 TEXAM Pressuremeter (PMT)

39 Texam Pressuremeter Test (PMT) 39

40 Check for leaks. Pressuremeter Calibrations Establish zero volume of the probe. Calibration for System Compressibility. Calibration for Membrane Resistance.

41 Pressuremeter Test (PMT) Developed in the 1950 s by Louis Menard at the University of Illinois. ASTM standard ASTM D 4719 utilizing. Method A Equal Pressure Increment Test (pressure increment = est. limit pressure/10) Method B Equal Volume Increment Test (volume increment =.05 to 0.1 V o ) Mono Cell probe (length/diameter ratio larger than 6). Main design applications include: Shallow Foundation Design under vertical loadings Deep foundation design under vertical and horizontal loadings (p-y analysis)

42 Video

43 Quality of Borehole The making of a Quality Borehole is the most important step. Drilling a quality pressuremeter borehole is much different from SPT sample drilling. SPT Sample Drilling keep disturbance to a minimum below the drill bit Pressuremeter Drilling Keep disturbance to a minimum along the borehole walls above the drill bit Nominal hole diameter shall not be more than 1.2 times the nominal probe diameter. Per ASTM D4719, D H = diameter of borehole D = Diameter of deflated probe

44 Quality of Borehole

45 Quality of Borehole Hole too small Greatly disturbed Hole too large

46 Typical Construction of Borehole for with an N-size probe (2.9 diameter) The hole is prepared utilizing mud rotary drilling. The top of the hole is cased with 4-inch diameter casing and the hole advanced with a 3-7/8 inch (roller bit for sands and 3 wing bit for clays) with AW rods to the depth of the top of the PMT test. The 3-7/8 inch roller bit is then removed and a 3-1/16 inch roller bit is inserted and advanced two feet to construct the hole for the PMT test. Following completion of the test, the 3-7/8 inch roller bit was reinserted and the procedure repeated at 5-foot intervals until the bottom of the hole. Note D H = 3.06 and D = 2.9, therefore 2.987<3.06<3.48 satisfying ASTM D4719

47 Test Procedure Calibration of Equipment (saturation and leaks, establish zero volume of probe (V o ), calibration for system compressibility, membrane resistance, hydrostatic pressure) PMT test (Method B) consists of placing a cylindrical probe into the ground and expanding the probe by injecting equal volume increases (say 40 cm 3 increments) of water into the probe to pressurize the soil horizontally and record corresponding pressures until limit of equipment is reached. Take readings after 30s and 1 min after volume (or pressure) increments are applied.

48 Schematic of TEXAM Mono-Cell Pressuremeter Actuator is turned forcing a known volume of water through the nylon tubing and recording the pressures via the pressure gage.

49 Corrected Pressuremeter Test Curve The Corrected pressuremeter curve is obtained by plotting the corrected pressure (p c ) versus the corrected relative increase in probe radius ( R/R o ) ( R/R o ) is a function of initial volume of probe and volume increments = ((V o +v c )/V o )) 1/2-1 The Coefficient of Earth Pressure Coefficient at rest (K o ), pressuremeter modulus (E o ), reload modulus (E R ), limit pressure (P L ) and net limit pressure (P L *) are determined from the curve.

50 PMT Parameters Coefficient of Earth Pressure at rest (K o ) σ ov = total vertical stress u o = hydrostatic pressure

51 PMT Parameters Pressuremeter Modulus (E o ) and Reload Modulus (E R ) E o = modulus calculated from the slope of the pseudoelastic portion of the corrected pressure-volume curve experiencing little to no creep. E R = modulus calculated from the unload-reload loop.

52 PMT Parameters Limit Pressure (P L ) and Net Limit Pressure(P L* ) P L = The pressure at which the probe volume reaches twice the original soil cavity volume. P L corresponds to a relative increase in probe radius ( R/R o ) L = ( R/R o ) c where ( R/R o ) c is the relative increase in probe radius corresponding to the initial size of the cavity. ( R/R o ) L may not be reached requiring extrapolation techniques to obtain P L Net Limit Pressure (P L* ) = P L - σ OH Net limit pressure is a measure of the strength of the soil

53 PMT Strength Parameters Undrained shear Strength Su: Su = 0.21P L * 0.75 (Su and P L* in tsf) Friction Angle φ : Not recommended

54 Preliminary Values/Soil Identification Preliminary Values for P L* and E o are illustrated below: General guidelines for soil identification:

55 Example Curve

56 Shallow Foundations Bearing Capacity Use k/1.2 for strip footings.

57 Shallow Foundations Settlement

58 Shallow Foundations Settlement Comparison Settlement summary for net bearing pressure = 4.0 tsf PMT Menard & Rousseau 1962 Tucker & Briaud 1986 DMT Schmertmann 1986 SPT Burland & Burbidge 1985 SPT Schmertmann Converting N 60 to E 1970, 78 SCPT Schmertmann E = 2.5qc 1970, 78 Q = 1210 T B= Q=225 T B =

59 References Roger Failmezger, P.E at Marchetti Briaud, J.-L, The Pressuremeter, Balkema/Rorredam/Brookfield, 1992 FHWA IP , The Pressuremeter Test for Highway Applications, Briaud, 1989

60 Questions??