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 3.11.2009 Lecture No. 23 Lecture Name: Swelling, Shrinkage and Cracking Characteristics of Soils Sub-topics • Shrinkage Characteristics • Importance • Mechanism of Swelling • Swelling and Shrinkage • Factors Influencing Shrinkage • Mechanism of Shrinkage • Some Definitions • Determination of Shrinkage • Classification of Soils (Based on swelling, shrinkage and Suction) • Some Important Relationships • Cracking Characteristics Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 2 Importance Soils Swelling soils Non-Swelling soils Wetting drying Swelling Shrinkage Montmorillonite Quartz Bentonite Kaolinite IIlite Dolomite Vermiculite Chlorite Minerals Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 3 Mechanisms of Swelling Swelling Inner crystalline swelling Osmotic swelling Due to the large difference in ion Due to the hydration of exchangeable concentrations close to the clay cations of the dry clay Surfaces and in pore water Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 4 Swelling and Shrinkage Swelling and Shrinkage are due to change in distance between the clay platelets. Increase in distance during wetting is due to development of Wanderwall forces and exchangeable cations (Montmorillonite), Hydrogen bonds (Kaolinite), Potassium bonds(Illite) and reduction in distance during drying process. Wanderwall forces are weaker than both hydrogen, potassium bonds and Double layer formation due to exchangeable cations, which causes more swelling in Montmorillonite. Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 5 IMPORTANCE of SHRINKAGE Cracking of soils is a function of shrinkage, thermal processes, tensile strength and fracture. Most concrete cracks are due to shrinkage. Unfavorable environment to plant growth due to root zone shrinkage. Shrinkage cracks are responsible for land slides. Overall health of pavements. Shrinkage cracks appearing on turfs/pitches Paint coatings Pottery Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 6 FACTORS INFLUENCING SHRINKAGE Initial bulk density Clay content Organic (carbon) content REMIDIES Cation exchange capacity Mica-smectite content Soil-cement Liquid limit Reinforcement Presence of salts Vegetation Initial water content pH Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 7 Mechanism of Shrinkage As moisture content decreases, capillary stresses in voids increase due to the increased surface tension. This increased surface tension tends to pull adjacent soil particles closer together resulting in overall volume decrease. The reduction in moisture content is due to (a) evaporation of water from surface of the soil in dry climates (b) lowering of the ground water table (c) desiccation of soil by trees in humid climates Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 8 Response of Bentonite 0 hrs 24 hrs 48 hrs 72 hrs 84 hrs Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 9 SOME DEFINITIONS Shrinkage : the reduction in volume of the soil, due to change in moisture content. Shrinkage limit : the boundary between the solid and semi-solid states of consistency. Volumetric shrinkage : the reduction in volume of soil mass expressed as a percentage of its dry volume when the soil mass is dried from a water content above the shrinkage limit to shrinkage limit. Shrinkage ratio : the ratio of reduction in volume of soil mass expressed as percentage of its dry volume to the corresponding reduction in water content.

IIT Bombay Slide 10 Shrinkage Limit Determination Remolded soil sample is prepared with moisture content > LL Then allow air drying (so that cracks don’t appear) Volumes to be measured by mercury displacement water Air water W w V 1 , W 1 V 2 SL = [(w 1 -w s )- γ w (V 1 -V 2 )]/W s V 2 Solids Solids Solids Ws W s = γ w {V 2 -(W s /G. γ w )}/W s W s Dry state Shrinkage limit state Plastic state (soil is fully saturated) Shrinkage Ratio (R) = (W s /V 2 ). ( γ w ) -1 = Apparent specific gravity G=(R -1 -SL/100) -1 Environmental Geomechanics Lecture No. 23 D N Singh

The change in volume of the soil with respect IIT Bombay Slide 11 to the moisture content 30 Montmorillonite Bentonite 25 20 V (cc) 15 10 5 0 1 10 100 1000 10000 w (%) % fraction % Sample A c Sand Silt Clay LL PL PI SL G BT 10 8 82 305 140 165 30 2.50 2.01 MT 7 21 72 434 166 268 30 2.72 3.72 Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 12 SHRINKAGE CURVE FOR SOILS In a shrinkage curve the inverse of bulk density (soil specific volume) is plotted to volumetric water content θ of the soil. Inverse of specific volume e Volumetric water content Specific volume= (1+e o ) Volumetric water content Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 13 Classification of Soils Degree of Swelling L 1 : Low M 1 : Moderate N 1 : Non swelling H 1 : High VH 1 : Very high EH 1 : Extremely high 80 140 FSI=4 (b) (a) L 1 M H 1 VH 1 EH 1 2 70 120 1 A=0.73(wl-20) 60 VH 1 100 1.5 H 1 50 M 80 1 PI (%) V w(cc) L 1 1 40 60 30 40 N 1 20 20 10 0 0 20 40 60 80 100 120 140 160 0 0 5 10 15 20 25 30 35 40 LL (%) VK(cc) V w volume of the soil in water FSI = ( V w - V k )/ V k ×100 V k volume of the soil in kerosene Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 14 30 70 (c) (d) 60 25 VH 1 50 20 40 SL (%) FSI (%) H 1 15 30 M 1 10 20 L 1 L 1 M 1 H 1 5 VH 1 10 0 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 PI (%) PI(%) Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 15 30 (e) (f) 25 20 N 1 VH 1 H 1 M 1 L 1 SL (%) 15 10 L 1 M 1 H 1 5 0 0 10 20 30 40 50 60 0 -5 -10 -15 -20 -25 -30 PI (%) ∆ log PF/ ∆ w PF: soil suction Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 16 5 5 10 10 (b) (a) AEV=a ⋅ CL b A 4 4 10 10 C B AEV=c+a ⋅ F b 3 3 10 10 AEV (kPa) D 2 =0.71 R 2 2 10 10 2 =0.49 2 =0.25 R R 2 =0.64 1 1 R 10 10 0 0 10 10 -1 -1 10 10 =30 % -2 -2 10 10 0 20 40 60 80 100 0 20 40 60 80 100 CL (%) F (%) Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 17 4 10 log(AEV)=1.06+0.76 ⋅ logS p log(AEV)=0.8+1.3 ⋅ logS 2 =0.62 R 2 10 AEV (kPa) 2 =0.71 R 0 10 =0.25 =0.02 -2 10 -2 -1 0 1 2 3 -2 -1 0 1 2 3 10 10 10 10 10 10 10 10 10 10 10 10 S (%) S p (kPa) S: Percentage swelling S p : Swelling pressure Environmental Geomechanics Lecture No. 23 D N Singh

IIT Bombay Slide 18 4 10 log(AEV)=1.13+0.6 ⋅ log FSI log(AEV)=2.82-0.075 ⋅ SL AEV (kPa) 2 =0.79 R 2 10 2 =0.67 R 0 =0.9 10 -2 10 -2 -1 0 1 2 3 10 10 10 10 10 10 0 10 20 30 40 50 SL (%) FSI (%) Environmental Geomechanics Lecture No. 23 D N Singh