Topic:Different methods of surfaCE investigation SUBJECT:-Building Construction Topic:Different methods of surfaCE investigation

Henil Shah(10) Vishwam Sheth(49) Karan Joshi(12) Bhumil Shah(46) Group no:-1 Henil Shah(10) Vishwam Sheth(49) Karan Joshi(12) Bhumil Shah(46) Abhay Nainani(01) Dharamraj Vaghela(57)

DIRECT METHOD Trial pits and trenches are excavated at the site to inspect the strata. As per IS:4453-1980 a clear working space of 1.2m x 1.2m is required at the bottom of the pit Shallow pits upto 3m depth can be made without any lateral support. For deep pits,the lateral support in the form of sheeting and bracing system is required. For depths greater than 6 m, bore holes are more economical than open pits. Trenches are long shallow pits. As a trench is continuous over a considerable length, it provides exposure over a line. The trenches are more suitable than pits for exploration on slopes.

Test Pit

2)Drifts and shafts Drifts are horizontal tunnelsmade in the hill side to determine the nature and structre of the geological formation Drift should have minimum clear dimension of 1.5m width and 2.0m height in hard rock. In soft rock,an arch roof is more advantageous,than a flat roof. Drifts are useful for establishing the minimum excavation limits to reach sound rock and for locating faults and shear zones and buried channels in the river section Drifts are also known as audits Shafts are large vertical holes made in the geological formation. These may be rectangular or circular in section dimensions for shafts are 2.4 m and for circular shaft diameter is 2.4 m Shafts are used to reach a particular strata at a depth of 4 m or more Shafts are also used to extend the exploration below the river bed already done by the means of tunnels

Semi direct methods Borings Anger boring Auger snd shell boring Wash boring Rotary drilling Percussion drilling

1)Auger boring Hand operated augers Mechanical augers An auger is a tool used for drilling a bore hole into the ground Auger are of two types :- Hand operated augers Mechanical augers auger boring is generally used in soils which can stay open without casing Clays,silts and partially satuarted sands can stand unsupported

For soils which cannot stand unsupported, especially for sandy soils below water table, a casing is normally required. Auger boring is fairly satisfactory where the depth of exploration is small, such as for highways, railways ,air fields,borrow pits etc.

2)Auger and shell boring If the sides of the bore hole cannot remain unsupported, the soil is prevented from falling by the means of cylindrical “shell” or “casing” used along with the auger The equipment used for drilling bore holes is generally known as ‘boring rig’ The hand operated boring rigs may be used for boring holes upto 25 m and power driven or mechanical boring rigs for boring holes upto depth of 50 m

3)Wash boring Wash boring is a fast and simple method for boring holes into the ground. This method may be used in all types of soil except those gravel, boulders,rocky and strata Process:- A hole is drilled for a short depth by using an auger. A casing pipe is pushed into the hole and driven with a drop weight or with the aid of the power A hollow drill bit screwed to the lower end of a hollow drill rod connected to a rope passing over a pulley and supported by a tripod is inserted into the casing pipe. Water jet under pressure is forced into the hole through the rod and the bit, which is alternatively raised and dropped, and rotated as well

The resulting chopping and jetting action of the bit and water loosens the soil at the lower end and forces the water-slurry upwards through the annular space between the drill rod and the casing. The soil water slurry is led into a settling tank where the soil particles settle where the water overflows into a sump

The soil particles collected represent a very disturb sample and is not very useful for the evaluation of the engineering properties of the soil. The changes in soil strata may be indicated by the change in the rate of progress and the change in the colour of the wash water.

4)Rotary drilling This method is a fast method of drilling holes in rock formations In this method by using suitable diamond studded drill bits or steel bits with shots, the rock cores may be obtained. The method is then known as core drilling or core boring

5)Percussion drilling The percussion drilling method is used for making holes in rocks, boulders and other hard strata. In this method a heavy drilling bit called ‘churn bit’ suspended from a drill rod or a cable is alternatively raised and dropped in the vertical hole By the repeated blows of the drill bit the material in the hole gets pulverised. if the point where the drill bit strikes is above the ground water table, water is added to the hole to faciliate the breaking of stiff soil or rock One of the major disadvantages is that the material at the bottom of the hole is disturbed by the heavy blows of the chisel

Indirect methods Geophysical methods:- Seismic refraction method Electrical resistivity method

Geophysical techniques: Advantages- Relatively low cost; Obtain results quickly; Can be undertaken in rough , inhospitable terrains by small teams; and Can assist planning of expensive drilling programs. Limitations- Techniques all identify boundaries between two layers with appreciably different properties. Little contrast - poor definition of layers. Requires confirmation by independent means. Techniques- Seismic reflection & refraction; Electrical resistivity (ER);

Seismic Methods involve propagation of waves through earth materials Electrical methods involve measurement of electrical properties of earth materials either- measurement of natural earth currents, or the resistance to induced electrical flow. Natural earth current flow generated under geological conditions in which anode & cathode develop naturally. Measurement of strength & extent of current helps establish geologic conditions. Electrical resistivity is resistance to electrical flow through earth materials. Current induced & resistivity measured- identifies basic property of earth material.

Seismic refraction: Theoretical treatments of theory of Elasticity,& wavelength of seismic waves confirms that velocity of P waves > than S waves; Also velocities of seismic waves dependent on rock density & Young’s modulus. Both increase with depth, so wave velocity also increases; P waves behave like visible spectrum waves- obey Snell’s law; Man-made seismic wave created& times of arrival of P waves sensed by regularly spaced geophones; Both refracted & reflected events measured on same seismic signal; Engineering relies on P wave (  rock strength) Cannot rip apart material whose seismic velocity exceeds 2,500 m/sec.(compacted sand with 40% porosity has P wave velocity = 1800 m/sec).

Used for many years to predict ease of excavation of earth materials; Changes in jointing, dipping beds & cementation changes will affect seismic velocity; Limitation- geologic units must increase in velocity with depth to ensure that refracted ray can return to the Earth’s surface. Seismic reflection: provides a detailed picture of sub surface structure & interfaces; depths determined by observing travel times of P waves generated near surface & reflected back from deep formations; comparable to echo sounding of water depths. Advantages- permits mapping of many horizons for each shot; can determine depths to dipping interfaces, as well as angle of dip; Not used as much as refraction, but refraction will not work where a high velocity layer overlies a low one; Reflection profiling in permafrost areas is not affected by the high velocity permafrost, whereas refraction techniques can be nullified completely.

Electrical Resistivity: The range of resistivities( ) of rocks is enormous: Amount of ground water& dissolved ions in rocks & water of great importance; Dry rock has virtually no electrical conductance:

Water-bearing rock resistivity is function of amount of groundwater present & salinity; Resistivity of saturated fine-grained sedimentary rocks tends to be lower than coarse-grained sedimentary rocks because of greater porosity; Gravels have more ground water recharge & less total dissolved solids (TDS) than fine-grained material such as colluvium or till; Resistivity used to map overburden thickness, faults, fractures, specific rock units, etc; Difficult to relate resistivity value directly to rock type. Fortunately show profound anomalies; Therefore should be compared to drill log data; In practice use Werner array- constant spacing, and moving whole array- This is resistivity profiling;