Page 31 - ebook_soil engineering practical workbook_15112021

3 WATER CONTENT DETERMINATION

3.1 Experiment outcomes
 Determine the mass of soil solids.
 Determine the mass of pore water.
 Determine the water (moisture) content of soils

3.2 Theory

The soil moisture content may be expressed by weight as the ratio of the

mass of water present to the dry to the dry weight of the soil sample, or by
volume as ratio of volume of water to the total volume of the soil sample. To
determine any of these ratios for a particular soil sample, the water mass

must be determined by drying the soil to constant weight and measuring the
soil sample mass after and before drying. The water mass (or weight) is the

difference between the weights of the wet and oven dry samples. The
criterion for a dry soil sample is the soil sample that has been dried to

o
o
constant weight in oven at temperature between 100 – 110 C (105 C is
typical). It seems that this temperature range has been based on water
boiling temperature and does not consider the soil physical and chemical
characteristics.

Water content determination by the gravimetric method involves collecting

and weighing a moist sample, removing the water by oven drying, and
weighing the dry sample. Water content is determined by calculation.
Two factors are important in this simple process. First, the sample must not

be allowed to lose water between collection and weighing. This implies use
of an air tight sample collection can (taped closed if storage time will be more

than a few hours) and weighing as soon as possible after collection. Plastic
bags, which are somewhat permeable to air, should be used only if storage

time will be short and high accuracy is not critical.

Second, a "standard" dryness must be reached. Some water (called
o
hygroscopic or molecular) is held by soil even after drying at above 100 C for
several days. However, high temperatures will oxidize the organic matter.

0
Consequently, drying at 105 C until a constant mass is attained (usually less

23 | Soil Engineering Practical Workbook