USING UNSATURATED SOIL SHEAR STRENGTH

Transcription

1 DESIGNING TEMPORARY SOIL NAIL WALLS USING UNSATURATED SOIL SHEAR STRENGTH T.J. Ju, P.E., Design Engineer, Wan Soo Kim, Ph.D., P.E., Soils Engineer, Virginia Dept. of Transportation Roy H. Borden, Ph.D., P.E., Professor, North Carolina State University

2 Saturated vs. Unstaurated Saturated : Effective stress (σ-u w ) controls behavior Unsaturated : Net stress (σ-u a ) and Matric suction (u a -u w ) control behavior Have seen cut slopes that are steeper than we would allow but they are stable. Why? Their strength is not being governed by effective stress parameters (c, φ ) but unsaturated shear strength parameters.

3 ( from Anderson and Ogunro, 2008 )

4 Unsaturated Shear Strength Unsaturated soil (Fredlund et al., 1978) τ = c ' + ( σ u ) tan φ ' + ( u u ) tan φ b f n a f a w f Extended Mohr-Coulomb failure envelope b φ Shear stress, τ Matric suction, (u a -u w ) φ ' (u a -u w ) f tanφ b b φ c b φ φ ' c Net normal stress, (σ-u a )

5 Triaxial Test Modified triaxial cell for testing unsaturated soils (Rahardjo et al., 2004)

6 Direct Shear Test Modified direct shear apparatus (Gan et al., 1988)

7 Soil Water Characteristic Curve (SWCC) Degree e of satura tion (%) Water Soil particle Residaul degree of saturation Boundary effect zone Air entry value Transition zone Air Residaul zone of unsaturation Matric Suction (kpa)

8 Pressure Plate Test (ASTM 2325) Schematic Diagram of pressure plate extractor Wang and Benson (2004)

9 Pressure Plate Test (ASTM 2325) (NC State University Soils Lab)

10 Pressure Plate Test (ASTM 2325) (NC State University Soils Lab)

11 Pressure Plate Test (ASTM 2325) (NC State University Soils Lab)

12 Typical SWCCs for Various Soils De egree of satu uration (%) Matric Suction (kpa) Regina clay Indain Head Till Silt Sand (Vanapalli et al., 1999)

13 Estimation of SWCC Experimental determination of SWCC is generally difficult, time-consuming and relatively expensive. SWCC could be reasonably estimated from : o Fredlund et al model (2002) : grain size distribution o Zapata et al model (1999) : grain size distribution (D60), Plastic Index, % 200 passing o SoilVision : a database system for Saturated/Unsaturated soil properties for 6,200 soil samples (98% of them have a SWCC measured in the lab) o NCHRP 9-23a : a national catalog of subgrade SWCC default inputs for use with MEPDG

14 Prediction Methods Fredlund et al. s approach (1996) κ ' ( ) τ = c' + ( σ u )tan φ' + ( u u )[ θ (tan φ )] f n a a w where = I + I + 2, κ P P 1 Vanapalli et al. s approach (1996) S S τ σ φ φ ' r f = c' + ( n ua)tan ' + ( ua uw)[(tan ) ] 100 Sr Khallili and Khabbaz model (1998) τ σ φ χ φ ' f = c' + ( n ua)tan ' + ( ua uw) f[ (tan )] 0.55 ( ua uw) f where, χ = for ( u u ) > ( u u ) ( ua uw) b χ = 1 for ( u u ) < ( u u ) a w a w b a w a w b

15 Kim and Borden Study (2011) Each of the procedures was developed based on limited experimental data obtained from a few soils Comparisons between measured and predicted values of unsaturated shear strength are presented for different soil types (sandy soil, low plasticity soil, silts, etc.) o Shear strength data of fifteen soils published in the literature (soils A thru O) o Net normal stress (0-200kPa) o Matric suction (0-1500kPa) Shear strength (kpa) Measured Fredlund 1st approach (Vanapalli et al.) K&K Khallili and Khabbaz 2nd approach (Vanapalli et al.) Vanapalli Matric suction (kpa)

16 Low Plasticity Clays Pre edicted shear strength (kpa) (a) 1st approach Fredlund (Vanapalli et al.) 1:1 y = 0.99x R 2 = 0.79 S L =0.99 R 2 =0.79 Pre edicted shear strength (kpa) (b) 2nd approach (Vanapalli et al.) 1:1 y = 0.97x R 2 = 0.86 S L =0.97 R 2 =0.86 (kpa) Pre edicted shear strength (c) Khallili & Khabbaz model K&K 1:1 y = 0.90x R 2 = 0.82 S L =0.90 R 2 = Measured shear strength (kpa) Measured shear strength (kpa) Measured shear strength (kpa) (Kim and Borden, 2011)

17 Sandy Soils Pre edicted shear strength (kpa) (a) 1st approach (Vanapalli et al.) y=120x 1.20x R 2 = 1.00 S L =1.20 R 2 =1.00 1:1 (kpa) Pre edicted shear strength (b) 2nd approach (Vanapalli et al.) y=126x 1.26x R 2 = 0.99 S L =1.26 R 2 =0.99 1:1 (kpa) dicted shear strength Pre (c) Khallili & Khabbaz model S L =1.17 R 2 =0.99 y = 1.17x R 2 = : Measured shear strength (kpa) Measured shear strength (kpa) Measured shear strength (kpa) (Kim and Borden, 2011)

18 APPLICATION OF UNSATURATED SOIL SHEAR STRENGTH

19 PROJECT SUMMARY Project : Art Commons at University of North Carolina Location : Chapel Hill, North Carolina Shoring Method : Temporary Soil Nail Wall Soil Description : Silty SAND or Clayey Sandy SILT

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21 PREPARATION OF SUBSURFACE INFORMATION 1. Five additional soil borings were done. 2. Soil samples were collected from various depths at boring locations. 3. Additional tests were performed on sampled soils. 4. Locations of all nearby utilities were carefully reviewed. ed

22 GRAIN SIZE DISTRIBUTION CURVE OBTAINED 100% 90% 80% 70% t % Passin ng by Weigh 60% 50% 40% 30% 20% 10% 0% Particle Diameter, mm

23 SOILVISION is a knowledge-based database software including unsaturated t soil data on over 6,200 soil samples. 98% of these soil samples have a soil-water characteristic curve measured in a laboratory. These data are used to estimate unsaturated soil properties.

24 SWCC OBTAINED FROM SOILVISION Gravimet tric Water Content E E E E E E E E E+06 Matric Suction (kpa)

25 SHEAR STRENGTH PREDICTION METHODS Fredlund et al. s approach (1996) κ ' ( ) τ = c' + ( σ u )tan φ' + ( u u )[ θ (tan φ )] f n a a w where = I + I + 2, κ P P 1 Vanapalli et al. s approach (1996) S S τ σ φ φ ' r f = c' + ( n ua)tan ' + ( ua uw)[(tan ) ] 100 Sr Khallili and Khabbaz model (1998) τ σ φ χ φ ' f = c' + ( n ua)tan ' + ( ua uw) f[ (tan )] 0.55 ( ua uw) f where, χ = for ( u u ) > ( u u ) ( ua uw) b χ = 1 for ( u u ) < ( u u ) a w a w b a w a w b

26 COMPARISON OF TOTAL COHESIONS OBTAINED per Fredlund et al. s approach per Vanapalli et al. s approach Khallili & Khabbaz s approach 14.5 kpa 14.5 kpa 14.6 kpa Some soils showed much great differences in total cohesions calculated from three approaches.

27 CONSTRUCTING TEMPORARY SOIL NAIL WALL

28 COMPLETED TEMPORARY SOIL NAIL WALL

29 AERIAL PHOTO OF PROJECT SITE AFTER COMPLETION

30 PROJECT SUMMARY Project : Wake County Parking Deck Location : Raleigh, North Carolina Shoring Method : Temporary Soil Nail Wall Soil Description : Silty SAND or Sandy SILT

31 PLASTIC COVER AND SUPPLEMENTAL DRILLED SOIL NAIL

32 ENCOUNTERING ROCK DURING DRIVEN SOIL NAIL INSTALLATION

33 WATER SERVICE CAP VACUUM GAUGE PLASTIC TUBE FILLED WITH DEAIRED WATER CERAMIC POROUS TIP TENSIOMETER SO 1. Negative pore-water pressure in soil can be directly measured. 2. The measured negative porewater pressure is numerically equal to the matric suction when the pore-air pressure is atmospheric (i.e., u a = 0). 3. The measuring capacity is limited to 100 kpa.

34 USE OF TENSIOMETER TO MEASURE ACTUAL MATRIC SUCTION

35 USE OF TENSIOMETER TO MEASURE ACTUAL MATRIC SUCTION

36 USE OF TENSIOMETER TO MEASURE ACTUAL MATRIC SUCTION

37 SHORTCOMINGS FOUND DURING THIS PRACTICE 1. How can the Soil Water Characteristic Curve obtained from SOILVISION software be confirmed for site specific soils? 2. How can moisture content (or matric suction) be confirmed during the project life? 3. Which h shear strength th prediction method is most appropriate to use?

38 CONCLUSIONS 1. Unsaturated soil shear strength properties estimated with information from additional soil tests and SOILVISION software have been used to design temporary soil nail wall. 2. Special care needs to be taken for control of natural moisture content (or matric suction) in soil (i.e. e surface water run-off, ground water and etc.). 3. Studies on shortcomings found during this projects should be explored for more confident and wider use.

39 Thank you! CONTACT TJ T.J. Ju, PE P.E. Tel : tjju@subsurfaceconstruction.com t ti