ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering - PowerPoint PPT Presentation

IIT Bombay ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering DR. D. N. SINGH dns@civil.iitb.ac.in www.civil.iitb.ac.in/~dns

IIT Bombay Slide 1 27.10.2009 Lecture No. 22 Lecture Name: Swelling, Shrinkage and Cracking Characteristics of Soils Sub-topics • Swelling Characteristics • Intrinsic expansive behaviour • Potential expansive behaviour • Swelling: A concern • Swelling: A boon • Swelling and Soil Suction • Mechanism of swelling • Methodologies for Identification and Determination of Swelling Soils • Classical methodologies • Neo-methodologies • Determination of Swelling Potential of Soils • A Novel Technique to determine Swelling Characteristics of Soils • Shrinkage Characteristics • Cracking Characteristics Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 2 Swelling characteristics of Soils The volume change due to “in-take” or “taking up” of water Exhibited by clays and fine silts Such soils are termed as “expansive soils” They shrink, as moisture content decreases and swell with increase in it Swelling caused due to Presence of swelling clay minerals (montmorillonite) in soils Exposure to water Results in Damage to structures founded on/in/over/with it Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 3 Intrinsic Expansive Behavior The property by virtue of which soil holds considerable amount of adsorbed and double layer water. Determined by: The mineralogy of the soil Pore-fluid chemistry The degree of aggregation of the particle The particle size and shape however are the fundamental variables. Indicators of intrinsic expansiveness: Liquid limit Clay Fraction C s = ∆ e / log σ′ Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 4 Potential expansive Behavior Defined as the pressure exerted against an unyielding support due to swelling. Determined by both intrinsic expansiveness and void ratio. For a soil of low void ratio, if external agent is removed, soil asserts potential expansiveness Na- Montmorillonite exhibits high intrinsic expansiveness and very high potential expansiveness Kaolinite exhibits low intrinsic expansiveness and potential expansiveness. Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 5 Swelling: A Concern Swelling of expansive soils poses a major problem with respect to serviceability performance of light weight structures on shallow footings This occurs due to Moisture near ground fluctuates as a result of rainfall, watering of gardens Seasonal change in rainfall Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 6 Swelling: A Boon Expansive soils are attracting greater attention as buffer and backfill materials for repository sites of high-level nuclear wastes. The function of the buffer materials is to create an impermeable zone around containers (self sealing) Buffer materials like bentonite and sand bentonite mixtures are used together A good buffer must exhibit swelling properties (and hence development of impermeability in the backfill) Choice of Bentonite: Swelling characteristics of compacted bentonite and sand-bentonite mixture need to be established Estimation of the swell pressure resulting from swelling on the structures of the disposal sites. Need to establish Hydro-thermo-mechanical properties Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 7 Swelling and Soil Suction As soil dries, the suction increases and soil shrinks As soil wets, the soil suction decreases and soil swells Soil suction = f (moisture conditions) depending on environmental factors Changes in soil suction are quite significant on the surface, where maximum soil moisture fluctuations occur. Development of “A Novel Technique” for establishing Swelling Characteristics of Expansive Soils Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 8 Mechanisms of swelling Swelling pressure osmotic pressure created by the cations Relates to the double layer theory of clay expansion Swelling pressure Reduction in the potential energy of the interlayer water Swelling Buildup of pressure in the trapped air of an advancing wetting front Clay swelling Balancing of forces of interaction among clay surface, ions, and water Environmental Geomechanics Lecture No. 22 D N Singh

Methodologies (Classical) for Identification of IIT Bombay Slide 9 Swelling Soils • Differential swell test • Free swell test Neo-Methodologies (Methods resorting to Advanced Instrumentation • Differential thermal analysis • X-ray diffraction method • Electron microscopy Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 10 Determination of Swelling Potential of Soils Direct Methods  Free swell test  Expansion Index  CBR Test  Potential Volume Change Test  Coefficient of Linear Extensibility (COLE) test Indirect Methods Estimated from  Plasticity Index  Activity Ratio  Cation Exchange Capacity Most of these tests are Too simple Time consuming and cumbersome Empirical & soil specific Need for rapid and easy determination of Swelling properties/Swelling Pressure Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 11 A New Method - Using Suction Measurement (Soil Water Air) A Three Phase system- Creates negative pressure or Suction Unsaturated soil Suction State of unsaturation Higher tendency to suck water and swell Suction Swelling Pressure Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 12 Physical properties of the Soils used Property Bentonite (B) Montmorillonite (M) Color Yellowish brown Dark brown Specific Gravity 2.76 2.78 Liquid limit (%) 227 411 Plastic limit (%) 65 71 Plasticity Index (%) 162 340 Free swell Index (%) 264 410 Specific surface area 48 310 (m 2 /g) (@150 ° C for 60 min.) An oedometer is used for determining swelling pressure Ps as per the guidelines presented by ASTM D 4546 Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 13 The Methodology Mineral Sample w (%) LVDT B1 13.6 Loading Guide rods arrangement B2 30.1 Bentonite B3 41.3 Load frame B4 60.5 B5 69.9 Acrylic cylinder M1 11.4 M2 28.8 Water bath Montmorillonite M3 55.3 Drainage M4 72.3 Base plate M5 99.8 Oedometer top and bottom Soil sample Dry unit weight =12 kN/m 3 plates Porous plate Oedometer ring Filter paper Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 14 30 M1 B1 M2 B2 M3 B3 25 M4 B4 M5 B5 20 (%) Swelling 15 10 5 0 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 Time (min.) Free swelling response of various samples Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 15 30 B1 M1 B2 M2 B3 M3 25 B4 M4 B5 M5 20 (%) Compression 15 10 5 0 -5 -10 0 50 100 150 200 250 300 0 100 200 300 400 500 600 P a (kPa) Compression characteristics of various samples Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 16 Variation of suction with molding water content 6 10 Bentonite Montmorillonite 5 10 4 10 ψ (kPa) 3 10 2 10 1 10 0 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 w Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 17 Results from the study Ψ Mineral Sample w P S (%) (kPa) (kPa) Bentonite B1 13.6 194.26 46000 B2 30.1 114.80 9100 B3 41.3 99.22 3400 B4 60.5 44.69 410 B5 69.9 27.69 150 Montmorillonite M1 11.4 454.17 83000 M2 28.8 369.26 26000 M3 55.3 201.77 4070 M4 72.3 169.83 1500 M5 99.8 113.73 330 Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 18 Relationship between swelling pressure and suction 1000 Bentonite (Present study) Montmorillonite (Present study) 900 Garbulewski and Zakowicz (1995) 800 700 600 P s (kPa) A =30 500 25 22.5 400 P s = A ( ψ ) 0.25 300 15 200 12.5 10 100 0 1 2 3 4 5 10 10 10 10 10 ψ (kPa) Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 19 Inference from the Study Higher suction Lesser water content Higher swelling pressure Knowing Suction- Swelling potential can be determined easily and rapidly Challenges Ahead Generalize the influence of each mineral on the overall swelling characteristics of the soil Physico-chemico-mineralogical characterization of various clay minerals should be taken up Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 20 Effect of valence of cations on the swelling of Na-Bentonite Environmental Geomechanics Lecture No. 22 D N Singh

IIT Bombay Slide 21 Effect of concentration on the swelling of Na- Bentonite Environmental Geomechanics Lecture No. 22 D N Singh