BORING METHODS - SITE EXPLORATION.

The following are the various boring methods commonly used:

(i) Auger boring.
(ii) Auger and shell boring.
(iii) Wash boring.
(iv) Percussion boring.
(v) Rotary boring.

(i) Auger boring
Augers are used in cohesive and other soft soils aboye water table. They may either be operated manually or mechanically. Hand augers are used upto a depth upto 6 m. Mechanically operated augers are used for greater depths and they can also be used in gravelly soils. Augers are of two types: (a) spiral auger and (b) post-hole auger.

FIG. 2.12  AUGER.

Samples recovered from the soil brought up by the augers are badly disturbed and are useful for identification purposes only. Auger boring is fairly satisfactory br explorations at shallow depths and for exploratory borrow pits.

(ii) Auger and shell boring
Cylindrical augers and shells with cutting edge or teeth at Iower end can be used for making deep borings. Hand operated rigs are used for depths upto 25 m and mechanised rigs up to 50 m. Augers are suitable for soft to stiff clays, shells for very stiff and hard clays, and shells or sand pumps for sandy soils. Small boulders, thin soft strata or rock or cemented gravel can be broken by chisel bits attached to drill rods. The hole usually requires a casing. Fig. 2.13 shows a typical sand pump.

FIG. 2.13  SAND PUMP

(iii) Wash boring
Wash boring is a fast and simple method for advancing holes in all types of soils. Boulders and rock cannot be penetrated by this method. The method consists of first driving a casing through which a hollow drilled rod with a sharp chisel or chopping bit at the lower end is inserted. Water is forced under pressure through the dril rod which is alternativety raised and dropped, and also rotated. The resulting chopping and jetting action of the bit and water disintegrates the soil. The cuttings are forced upto the ground surface in the form of soil-water slurry through the annular space between the drill rod and the casing. The change in soil stratification could be guessed from the rate of progress and colour of wash water. The samples recovered from the wash water are almost valueless for interpreting the correct geo-technical properties of soil.

(iv) Percussion boring
In this method, soil and rock formations are broken by repeated blows of heavy chiesel or bit suspended by a cable or drill rod. Water is added to the hole during boring,  if not already present and the slurry of pulverised material is bailed out at intervals. The method is suitable for advancing a hole in all types of solis, boulders and rock. The formations, however, get disturbed by the impact.

(v) Rotary boring
Rotary boring or rotay drilling is a very fast method of advancing hole in both rocks and soils. A . drill bit, fixed to the lower end of the drill rods, is rotated by a suitable chuck, and is always kept in firm contact with the bottom of the hole. A drilling mud, usually a water solution of bentonite, with or without other admixtures, is continuously forced down to the hollow dril rods. The mud returning upwards brings the cuttings to the surface. The method is also known as mud rotary drilling and the hole usually requires no casing.

Rotary core barrels, provided with commercial diamond-studded bits or a steel bit with shots, are also used for rotary drilhng and simultaneously obtaining the rock cores or samples. The method is them also known as core boring or core drilling. Water 15 circulated down drill rods during boring.

FIG. 2.14  WASH BORING.

Record of borings

In all exploration work it is very important to maintain an accurate and explicit record of borings. Soil/rock samples are collected at various depths, during boring. These samples are tested in the laboratory for identification and classification. The samples are suitabty preserved and arranged serially according to the depth at which they are found. A boring chart, similar to the one shown in Fig. 2.15 Is prepared for each bore hole. A site plan should be prepared, showing the disposition of various bore holes on it.



Number and disposition of trial pits and borings

The number and disposition of the test pits and borings should be such as to reveal any major changes in the thickness, depth or properties of the strata affected by the works, and the immediate surroundings.

(a) for a compact buliding site covering an area of about 0.4 hectares, one bore hole or trial pit in each comer and one in the centre should be adequate.
(b) For small and less important buildings, even one bore hole or trial pit in the centre will suffice.
(c) For very large arcas covering industrial and residential colonies, the geological nature of the terrain will help in deciding the number of bore holes or trial pits. Dynamic or static cone penetration tests may be performed at every 100 metres by dividing the area into grid patterns and number of bore holes or trial pits decided by examining the variation in the penetration curves.

                                                    FIG. 2.15  DETAILS OF BORING.