Semi Direct Methods (Boring) - Soil Exploration, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering PDF Download

Semi Direct Methods-Boring

Making or drilling bore holes into the ground for obtaining soil or rock samples from specific depths is called boring. The different types of borings are:

(i)  Auger boring

(ii) Wash boring

(iii) Rotary drilling

(iv) Percussion drilling

The auger is useful for advancing a bore hole into the ground. It may be hand-operated or power-driven. The hand-driven augers are used for relatively small depths (less than 3 to 5 m), while the power-driven augers are used for greater depths (upto 60 to 70 m). The soil auger is advanced by rotating it while pressing it into the soil. As soon as the auger gets filled with soil, it is taken out and the soil sample collected. The soil samples obtained from this type of borings are highly disturbed. The Augur boring is suitable for partially saturated sands, silts and medium to stiff cohesive soils.Wash boring is commonly used for exploration below ground water table for which the auger method is not suitable. This method may be used in all kinds of soils except those mixed with gravel and boulders. A casing pipe is pushed in and driven with a drop weight. A hollow drill bit is screwed to a hollow drill rod connected to a rope passing over a pulley and supported by a tripod. Water jet under pressure is forced through the rod and the bit into the hole. This loosens the soil at the lower end and forces the soil-water suspension upwards along the annular surface between the rod and the side of the hole. This suspension is led to a settling tank where the soil particles settle while the water overflows into a sump. The water collected in the sump is used for circulation again. The soil particles collected are very disturbed sample and is not very useful for the evaluation of the engineering properties. Wash borings are primarily used for advancing bore holes; whenever a soil sample is required, the chopping bit is to be replaced by a sampler. The change of the rate of progress and change of color of wash water indicate changes in soil strata.

Rotary drilling can be used in sand, clay and rocks. A drill bit, fixed to the lower end of a drill rod, is rotated by power while being kept in firm contact with the hole. Drilling fluid or bentonite slurry is forced under pressure through the drill rod and it comes up bringing the cuttings to the surface. Even rock cores may be obtained by using suitable diamond drill bits. When soil samples are required, the drilling rod is raised and drilling bit is replaced by a sampler.

In case of Percussion drilling, a heavy drill bit is suspended from a drill rod or a cable and is driven by repeated blows. The water is added to facilitate the breaking of stiff soil or rock. The slurry of the pulverized material is bailed out at intervals. The method cannot be used in loose sand and is slow in plastic clay. The formation gets badly disturbed by impact.

The soil samples taken out of natural deposits for testing may be classified as:

(i) Disturbed sample

(ii) Undisturbed sample

A disturbed sample is that in which the natural structure of the soil gets modified partly or fully during sampling. An undisturbed sample is that in which the natural structure and other physical properties remain preserved. Disturbed but representative samples can generally be used for determination of the following purposes:

(i) Grain-size analysis

(ii) Determination of liquid and plastic limits

(iii) Determination of specific gravity of soil solids

(iv) Organic content determination

(v) Soil classification

The Undisturbed samples must be used for

(i) Consolidation test

(ii) Hydraulic conductivity test

(iii) Shear strength test (to determine the cohesion and angle of friction)

For collecting good quality undisturbed soil samples the area ratio [Ar = (O.D² - I.D²)/I.D² x 100 (%), where O.D and I.D are the outside and inside diameter of the sample tube] of the sample tube should be less than 10%. Thicker the wall, greater is the disturbance. Proper care has to be taken for transport and handling of the soil samples. The soil samplers are classified as:

(i) Thick wall samplers (Split spoon sampler)

(ii) Thin wall samplers (Shelby tubes)

The standard size of the spoon sampler is of 35 mm internal and 50.8 mm external diameter. Thus, the area ratio is 112% and the obtained samples are highly disturbed sample. The sampler is lowered to the bottom of the bore hole by attaching it to the drill rod. The sampler is then driven by forcing it into the soil by blows from a hammer. The assembly of the sampler is then extracted from the hole and the cutting edge and coupling at the top are unscrewed. The two halves of the barrel are separated and the sample is thus exposed. Samples are generally taken at intervals of about 1.53 m (5 ft). When the material encountered in the filed is sand (particularly fine sand below the water table), a spring core catcher is placed inside the split spoon. Shelby tubes are commonly used to obtain undisturbed clayey samples. The Shelby tube has outside diameter: 50.8 mm (2 in) and 76.3 mm (3 in).