IIT Bombay 04/08/2009 Lecture: 6 Soil-water-air relationships Sub-topics Simple laboratory investigations Determination of : Determination of � water content, specific gravity of solids & water content in-situ unit weight CE 303 6 Instructor: AJ
IIT Bombay Determination of water content 2 Oven dying method Pre-weighted moist soil sample is put in an oven for 24 hours after which dry weight of the sample is taken Note: Oven temp of 105 to 110 o C • Lower temp (~ 60 o C) for organic soils to avoid oxidation of organic matter • In sandy soils � complete drying achieved in ~ 4 to 6 hours; clay • samples may require ~ 16 to 20 hours CE 303 6 Instructor: AJ
IIT Bombay Determination of water content 2 Pycnometer method • Pycnometer is approx 900ml capacity glass bottle • Conical cap provided with 6mm diameter hole at top • Rubber washer provided between the cap and rim of the bottle to avoid leakage CE 303 6 Instructor: AJ
IIT Bombay Determination of water content By definition water content, 2 W = w w x 100 W s Weight of water, ( ) = − − W W W W w 2 1 s If from W 3 the weight of solids W s is removed and replaced by the weight of an equivalent volume of water, then weight W 4 is obtained: ( ) W G − + γ = ⇒ = − s s W W W W W W γ − 3 s w 4 s 3 4 G G 1 s w s CE 303 6 Instructor: AJ
IIT Bombay Determination of water content 3 Sand bath method (quick field test when the facility of electric oven is not available) Wet soil sample in a container is dried by placing the container on sand-bath heated over kerosene stove CE 303 6 Instructor: AJ
IIT Bombay Determination of water content 4 Rapid moisture meter method (Portable equipment conveniently used in the field) Calcium carbide is introduced in (5g) sample CaC 2 reacts with free moisture and releases acetylene C 2 H 2 gas Amount of released gas depends on the amount of free moisture The confined gas pressure is measured in the sealed chamber Calibrated to interpret pressure as % of moisture on total weight basis CE 303 6 Instructor: AJ
IIT Bombay Determination of water content Torsion balance moisture meter method 5 (quick determination of water content in the laboratory) The balance works on infrared radiation A torsion wire is pre-stressed to an extent equal to 100 % of the scale Sample is evenly distributed on the balance pan to counteract the pre- stressed torsion and the scale is brought back to zero As the sample dries, loss in weight is continuously balanced by rotation of the drum The drum is calibrated to read water content on wet-weight basis CE 303 6 Instructor: AJ
IIT Bombay Determination of specific gravity of solids Pycnometer Weight of empty & dry pycnometer bottle = W 1 Weight of pycnometer + oven-dried soil (~300g) = W 2 Fill pycnometer + soil + water = W 3 weight after filling it with water up to the top = W 4 CE 303 6 Instructor: AJ
IIT Bombay Specific gravity of solids = − Weight of dry soil W W W s 2 1 Weight of water in = W − W 3 2 observation (3) Weight of water in = W − W 4 1 observation (4) ∴ Weight of water that has the same volume as of solids ( ) ( ) = − − − W W W W 4 1 3 2 ( ) ( ) = − − − On rearranging the terms W W W W 2 1 3 4 − − W W W W = = Specific gravity of solids, 2 1 2 1 G ( ) ( ) − − − − + s W W W W W W W 2 1 3 4 s 3 4 CE 303 6 Instructor: AJ
IIT Bombay Determination of in-situ unit weight Unit weight is necessary to compute overburden pressure at given depth within the soil Methods commonly used : Core-cutter Sand replacement Water displacement CE 303 6 Instructor: AJ
IIT Bombay Core-cutter method 3 ) driven into the soil Core of known volume (1000cm Usually mounted on top by 25mm high dolly The core is driven by hitting the dolly using a suitable hammer The core with soil is removed and weighed after trimming the excess soil In-situ unit weight is determined CE 303 6 Instructor: AJ
IIT Bombay Sand replacement method Suited for hard soils A hole is made into the ground and the excavated soil weighed Volume of the hole is determined by replacement with sand of known “loose” unit weight Knowing the weight of excavated soil and volume of the hole, γ b is determined CE 303 6 Instructor: AJ
IIT Bombay In-situ density in boulder deposits? A large hole is excavated and the excavated soil collected Volume of the hole is determined by pushing a thin polythene sheet bag inside the hole and filling it with measured volume of water Volume of water is the measure of the hole-volume and is used to calculate γ b CE 303 6 Instructor: AJ
IIT Bombay Water displacement method (suitable for soils which can be extracted in lumps) A small specimen is trimmed and weighed (W 1 ) The specimen is coated with thin layer of paraffin wax and weighed again (W 2 ) Take a metal container filled with water and with an overflow mechanism The coated specimen is lowered into the container and the overflow measured V w (V w is the volume of the coated specimen) ⎛ ⎞ − W W ⎜ ⎟ = − 2 1 Volume of the uncoated specimen is calculated as V V ⎜ ⎟ γ w ⎝ ⎠ p where γ p = unit weight of paraffin wax; the bulk and the dry unit weights of the specimen are then calculated in the usual manner CE 303 6 Instructor: AJ
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