04/08/2009 Lecture: 6 Soil-water-air relationships Sub-topics - - PowerPoint PPT Presentation

IIT Bombay

CE 303 6 Instructor: AJ

04/08/2009 Lecture: 6 Soil-water-air relationships Sub-topics Simple laboratory investigations Determination of :

• water content, specific gravity of solids &

in-situ unit weight Determination of water content

IIT Bombay

CE 303 6 Instructor: AJ

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 110oC
• Lower temp (~ 60oC) 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 Determination of water content

2

IIT Bombay

CE 303 6 Instructor: AJ

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

2

Determination of water content

IIT Bombay

CE 303 6 Instructor: AJ

2

Determination of water content By definition water content, 100 x W W w

s w

= water,

• f

Weight

( )

s 1 2 w

W W W W − − = If from W3 the weight of solids Ws is removed and replaced by the weight

• f an equivalent volume of water, then weight W4 is obtained:

4 w w s s s 3

W G W W W = γ γ + −

( )

1 G G W W W

s s 4 3 s

− − = ⇒

IIT Bombay

CE 303 6 Instructor: AJ

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

3

Determination of water content

IIT Bombay

CE 303 6 Instructor: AJ

Rapid moisture meter method (Portable equipment conveniently used in the field) Calcium carbide is introduced in (5g) sample CaC2 reacts with free moisture and releases acetylene C2H2 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

4

Determination of water content

IIT Bombay

CE 303 6 Instructor: AJ

Torsion balance moisture meter method

(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

• f the drum

The drum is calibrated to read water content on wet-weight basis

5

Determination of water content

IIT Bombay

CE 303 6 Instructor: AJ

Determination of specific gravity of solids Weight of empty & dry pycnometer bottle = W1 Weight of pycnometer + oven-dried soil (~300g) = W2 Fill pycnometer + soil + water = W3 weight after filling it with water up to the top = W4

Pycnometer

IIT Bombay

CE 303 6 Instructor: AJ

Specific gravity of solids Weight of dry soil 1 2 s W W W − = Weight of water in

• bservation (3)

2 3 W W − = Weight of water in

• bservation (4)

1 4 W W − = ∴ Weight of water that has the same volume as of solids ( ) ( ) 2 3 1 4 W W W W − − − = On rearranging the terms ( ) ( ) 4 3 1 2 W W W W − − − = Specific gravity of solids, ( ) ( ) 4 3 s 1 2 4 3 1 2 1 2 s W W W W W W W W W W W G + − − = − − − − =

IIT Bombay

CE 303 6 Instructor: AJ

Determination of in-situ unit weight

Methods commonly used : Core-cutter Sand replacement Water displacement Unit weight is necessary to compute overburden pressure at given depth within the soil

IIT Bombay

CE 303 6 Instructor: AJ

Core of known volume (1000cm

3) driven into the soil

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 Core-cutter method

IIT Bombay

CE 303 6 Instructor: AJ

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, γb is determined Sand replacement method

IIT Bombay

CE 303 6 Instructor: AJ

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 γb

IIT Bombay

CE 303 6 Instructor: AJ

A small specimen is trimmed and weighed (W1) The specimen is coated with thin layer of paraffin wax and weighed again (W2) Take a metal container filled with water and with an overflow mechanism The coated specimen is lowered into the container and the overflow measured Vw (Vw is the volume of the coated specimen) Volume of the uncoated specimen is calculated as

where γp = unit weight of paraffin wax; the bulk and the dry unit weights of

the specimen are then calculated in the usual manner Water displacement method

(suitable for soils which can be extracted in lumps)

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ γ − − =

p 1 2 w

W W V V