Lecture_6_-_water_in_soils - WATER IN SOILS Effect of water...

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Unformatted text preview: WATER IN SOILS Effect of water on soils can be dramatic. Numerous soil failures are caused by water actions Examples of water-related soil failures Static soil-water interaction Dynamic flow of water through soils; seepage forces SOIL FAILURE CAUSED BY WATER Slope stability problems Piping at the toe of dams Failure of retaining walls Failure of road embankments STATIC SOIL-WATER INTERACTION Water pressure Capillarity Shrink-swell problems in soils Frost action Effective stress concept WATER PRESSURE At any depth, the pressure in a static water can be calculated as the product of the depth and the unit weight of the water (62.4 pcf or 1 g/cc). Z A U = (Z)( ) CAPILLARITY FORCES All materials have intermolecular forces which are called: ADHESION: for attraction between molecules of different materials COHESION: for internal forces CAPILLARITY FORCES If ADHESION forces between a liquid and a solid are LARGER than the intermolecular attraction (COHESION) of the liquid, the surface of the solid will be "wetted" by the liquid. WATER: has very little internal cohesion; it will wet almost any material MERCURY: has very strong cohesion, it will wet only a very limited number of materials CAPILLARITY FORCES Surface tension is a phenomenon occurs at the interface between two materials. In soils, it occurs at the interface between water, soil grains and air. In a glass tube, the surface tension between the liquid in the tube and the glass causes the fluid to either rise or fall below its pool hydrostatic surface as illustrated below. TS hc D CAPILLARITY The capillary height (hc) can be calculated by summing the vertical forces as follows: Down force = water weight = r hc CAPILLARITY FORCES The water pressure in the capillary zone is negative. In soil, it may create apparent cohesion (sand castle) Pressure - TS hc D + SHRINKAGE LIMIT Shrinkage Limit The shrinkage limit is defined as the water content at which the volume of the drying soil becomes constant as shown below. Volume Color change Vi Vd VS Shrinkage limit (SL) Weight Wi SHRINKAGE LIMIT V Color change Vi Vd VS Shrinkage limit (SL) Wi w Water Content (%) SHRINKAGE The shrinkage limit (SL) can be calculated as follows: SL = 100*[(V V )* ]/W OR SOIL SWELLING The swell potential of a given soil is expressed by the Skempton defined activity (A) of the soil, where A can be calculated as follows: A = (PI)/[(percent passing the 2m sieve) 5] Some soils swell when its water content increases, therefore Build structure on dry soil Structure prevents evaporation Capillary water moves in Attraction for dipolar water allows expansion If building pressure < swelling pressure => HEAVE! SWELLING DAMAGE REQUIRES Montmorillonite w (%) > PL Source of water PREVENTING SWELLING DAMAGE Compact the soil at wet of optimum, low d Pre-wet the soil Install moisture barriers Use chemical stabilization Construct special foundations FROST ACTION When the pore water freezes, it causes 10% volume expansion (uneven) When ice lenses are formed, additional water drawn in by vapor and by capillary action When the ice lenses melt they cause high water content, low strength, and pavement failure (potholes, depressions, etc.) FROST ACTION When water is present in soil and the temperature drops below the freezing point, ice lenses will form. The ice lenses will grow due to water vapor and other water sources causing the ground surface to heave. GS GS H Ice lenses GWT Bedrock FROST ACTION During spring, the ice lenses will melt at the top and bottom of the ice lenses. The top water will be trapped because the ice below it is impermeable. This is the main mechanism of pothole formation in the spring. GS GS H Ice lenses GWT Bedrock ELEMENTS NEEDED FOR FROST ACTION Temp. below freezing Source of capillary water Frost susceptible soil ...
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