SITE INVESTIGATION AND GROUND TECHNIQUESIt is desirable to visit the site of work and inspect the same carefully from the view point of foundation details. The nature and thickness of strata of strata of soil may be estimated by studying the excavation details of near by constructions or by examining the open side of a nearby well, etc. The general inspection of site of work serves as a good guide for determining the type of foundation to be adopted for the proposed work and in addition, it helps in getting the data with respect to the following items :
1. Behaviour of ground due to variations in depth of water table;
2. Disposal of storm water at site ;
3. Nature of soil by visual examination;
4. Movement of ground due to any reason; etc.
The load of the structure is ultimately transferred to the soil. It becomes therefore essential to know the quality and thickness of sol underground and such a study would assist in selecting an economical but safe design for the foundation of the structure.
The characteristics of the foundation soils i. e., behaviours under loading and also under different environmental conditions, are very important and it governs the selection and type of footing / foundation to be selected e.g., black cotton soil which is typically a cohesive soil requires a particular type of foundation and in most of the cases, under-reamed pile foundation is finding out the acute value of soil parameters, that needs to be used in the design of the foundation.
The sub-soil exploration gives precise information with respect to the following conditions at site of the proposed world
1. Location of ground water and its variation ;
2. Nature and engineering characteristics of the soil and rock formation; and
3. Order of occurrence and extent of different soil strata.
DEPTH OF EXPLORATION
The depth of exploration will have to be decided very carefully. The term significant depth is used to indicate the depth up to which the increases in pressure due to structural loading is like which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundations. The general rules to be adopted to decide the depth of exploration for various types of structures and site conditions.
METHODS OF SITE EXPLORATION
Following are the various methods of site exploration :
1) Test pits
3) Auger boring
4) Wash boring
5) Sub-surface soundings
6) Test piles
7) Deep boring
8) Geophysical method.
Each of the above method of site exploration will now be discussed in detail.
1 ) Test pits : A square pit, known as a trial pill or a test pit, with side as about 1.50 m, is excavated up to a depth at which sufficiently hard soil is available The various strata of the soil can be inspected, studied and Classified accordingly This method is useful when hard soil is available within a maximum depth of m
Following two pints should be noted :
1 ) A sufficient number of test pits should be dug on the site to know the variation of the ground.
2) The test pits should be examined at all the exposed level sofa the ground and these observations should be carried out as soon as the excavation of the test pits is completed.
2 ) Probing : The probing consists of driving either a hollow tube or a steel rod or an iron rod into the ground.
A hollow tube of diameter 35 mm to 50 mm is taken. It had a slit of 3 mm thickness in the bottom portion of 600 mm The tube is driven into the ground 300 mm or so at a time. It is then withdrawn and the material caught in the slit is inspected.
In other case, a solid rood of steel or iron having a diameter of about 30 mm to 35 mm is taken and driven into the ground. The rod had a pointed end as shown in fig. 4-3 The rod is frequently withdrawn and the material stuck up at the pointed end is examined .
With the help of this method, it is possible to examine tee ground for a maximum depth of 3 m
3) Auger boring : An auger may be of post – hole type or screw type or shell type. They all work in the same way. A post-hole auger, a screw auger and a shell auger.
It consists of a hollow tube of diameter 75 mm to 100 m The tube is provided with a cutting edge at the bottom. The slit extending over a length of 600 mm to 900 mm is provided at the bottom of the tube of catch the material. The length of the tube is about 1.50 m and it is provided with a threaded end of that it can be suitably extended with the help of lengthening pieces. The auger is worked by applying leverage at the top. The auger is driven into the ground and turned like a screw. The auger is withdrawn and the material caught in the slit is inspected.
When the auger is to be driven in loose sand, it becomes essential to prevent the collapse of the loose material, when the auger is being withdrawn. A casing is a thin the auger. The lengthening of the casing can be done by connecting one pipe to the other. With the help of this method, it is possible to inspect the ground for a depth of 6 m to 8m and in case of loose sand, the auger may be useful even up to a depth of 15 m or so.
4 ) Wash boring : The wash boring is the term used to denote a method in which a casing I s driven into the ground and the material inside the casing is washed out and brought to the surface for inspection.
The process of wash boring consists an driving an inner tube of diameter 25 mm to 50 mm , inside an outer tube of diameter 100 mm to 150 mm. The water is simultaneously forced with driving of tube under pressure though the inner tube This facilitates easy driving of the tube and makes the soil loose enough to flow the quantity of water required is usually about 100 to 300 litres per minute under a pressure of 36. 28 KN /m2 . The process is continued till hard surface is met with. The washed material is collected in a tank and studied carefully.
Following two pints should be noted :
1 ) In case a boulder is met, a small charge of dynamite should be sued to dislocate the boulder and then the work should be started again. The fact whether a hard surface is reached should be properly ascertained.
2 ) In the process of washing, the finer particles such as loam, clay, etc disappear and they are separated from the coarse particles. Thus it becomes difficult to ensure whether the samples obtained represent the real nature of the subsoil. However the results obtained by this process are reliable when depths are about 30 m to 45 m.
5 ) Sub-surface soundings : In this method, the resistance of the soil with depth help of blows of the sol with depth is measured by means of a tool known as the Penetrometer under static or dynamic loading. The penetrometer may consist of a sampling spoon, a cone or tool of other shape. The procedure is as follows :
1) The Penetrometer is driven in the ground with the help of blows from a 650 N weight falling from a height of 750 mm.
2) The number of blows required to drive the penetrometer into the ground through a distance of 300 mm is measured. It is known as the standard penetration resistance or SPT of the soil.
3) The values of SPT of soil at different depth and determined.
4) The bearing capacity and other engineering properties of certain types of soil are then known by referring to the curves correlating SPT and the corresponding characteristics of soil. These curves are available for ready reference.
The sub-surface sounding tests are useful for :
1 finding the depth of bed rock or stratum;
2 Knowing the general exploration of erratic soil profiles; and
3 Testing Cohesion less soils from which it is difficult to obtain the undisturbed samples.
6 ) T est. piles : Sometimes the test piles are driven into the ground to obtain the information of the solid strata. With the help of this process, it is not possible to know definitely the kinds of strata through which the test piles pass, as the material is not available for inspection. But the factors such as resistance of soil to driving of piles. Load bearing data and any other available local information serve as useful guides.
7 ) Deep boring : It becomes essential to carry out deep boring for big important engineering structures such as dame. In such structures, in addition to the stability of the superstructure, the importance is to be given to various other factors such as non-leakage of the stored water, seepage through porous strata, etc
The machines used for deep boring are as following
1) Percussion boring machine
2) Core or rotary drilling machine.
1 Percussion boring machine : In this process, the heavy cutting tool is dropped into the ground by means of a series of blows, The broken material is brought ot the ground by adding water into the core and then the paste is lifted to the ground. The material thus obtained is made dry and it is then examined. The percussion boring machine is very much useful for hard material like rock.
2 ) Core or rotary drilling machine : In this process, a hollow tube is driven by rotary motion which cuts a solid core. The water is used is used to facilitate the cutting process. The machine can be sued either for soft or hard material. If the tube passes through the soft material, no core is obtained and the slurry formed had to be pumped out after the tube is withdrawn. When the tube passes through the hard material, the core is retained and this had to be cut at the bottom and lifted up. This is done by purring sand at the inner side I.e. Between the core and the inner surface of the tube and then the tube is slightly rotated, The core is then broken and caught in the tube along with sand and it is lifted up. A core chart is then prepared. In order to prevent the falling of loose material, when the tube is removed, the casing may be used.
3 Geophysical method : In favourable circumstances, the geophysical method is adopted to know the nature of soil strata. These methods are used when the exploration depth is substantial and the speed of investigation is of primary importance. These methods are mainly adopted to ascertain the depths at which useful minerals and oils are available. The two most commonly adopted methods for civil engineering purposes are as follows :
1 Electrical resistivity method; and
2 Seismic refraction method
3 Each of the above geophysical method will now be briefly described.
4 Electrical resistivity method : In this method, four electrodes are driven in the ground at equal distance apart and in a straight line as shown in fig. The distance between the two electrodes indicates the depth of exploration or depth up to which the ground resistance is to be measured An electrical current is then passed between the two outer electrodes and the potential drop between the two inner electrodes is measured with the help of the potentiometer. Te means resistivity is then calculated by the following equation ;