Site Investigation

Site Investigation

Site Investigation is the gathering of information about the proposed location of the construction project. The reason for this can be twofold, firstly to assist in the location of the project and secondly to ascertain ground conditions. A site investigation should be taken for every site. Without a properly procured, supervised and interpreted site investigation, hazards which lie beneath the site cannot be known.

A site investigation is made up of five parts –

1) Desk Study

The desk study is work taken up prior to commencing the work on site and the Ground Investigation. It should always be the first stage of the Site Investigation and is used to plan the Ground Investigation. The work involves researching the site to gain as much information as possible, both geological and historical.

Geological Maps and memoirs are probably the most important source of information as these give an excellent indication of the sort of ground conditions likely to be encountered. It gives a good indication of the types of material and the structures occurring in the region.

Arial Photography is another extremely useful source of information. These records

can be obtained from one of several sources such as the Department of the Environment, local authorities and air-survey firms. Such records can be extremely useful in getting a good knowledge about the topography of the land.

Records of Previous Site Investigation reports are also helpful in a desk study.

2) Site Reconnaissance

The Site Reconnaissance phase of a site investigation is normally in the form of a walk over survey of the site. Important evidence to look for is:

Hydrogeology: Wet marshy ground, springs or seepage, ponds or streams and Wells.

Slope Instability: Signs of slope instability include bent trees, hummocks on the ground and displaced fences or drains.

Mining: The presence of mining is often signs of subsidence and possibly disused mine shafts. Open cast mining is indicated by diverted streams replaced or removed fence/hedge lines.

Access: It is essential that access to the site can be easily obtained. Possible problems include low overhead cables and watercourses.

Following information can be obtained by the step of investigation.

i. The general topography of the site, the existence of drainage ditches. ii. Existence of settlement cracks in the structure already built near the site.

iii. The evidence of landslides, creep of slopes and the shrinkage cracks.iv. The stratification of soils as observed from deep cuts near the site.v. The location of high flood marks on the nearby building.

vi. The depth of ground water table as observed in the wells.vii. Existence of springs, swamps, etc. at the site.

viii. Type of the vegetation existing at site. (The type of vegetation gives a clue to the nature of the soil).

ix. Existence of underground water mains, power conduit, etc. at the site.

Under this step, there are four geophysical methods used to locate the boundaries between different strata of soil.

Magnetic method Gravity method Seismic method Electrical method

3) Exploratory investigations

The depth, thickness, extent, and composition of each stratum at the site to determine preliminary investigation is carried out as a third step. The depth of the bed rock and the ground water table is also determined. The preliminary investigations are generally in the form of a few borings or trial pits. Test are conducted with core peretro meters and sounding rods to obtain information about the strength and compressibility of soils.

4) Laboratory Testing

Disturbed and undisturbed soil samples collected from boreholes were used for following laboratory testing.

1. Natural moisture Content2. Atterburg Limits3. Sieve Analysis4. Specific Gravity5. Consolidation Test6. Unconfined Compression Test

Results of the laboratory testing are given in Annex.

5) Report

Classification of Soil

General classification

According to the particle size, soil can classify as below,

Table 1 Soil classification

Clay Particle size is less than 0.002 mmSilt Particle size is between 0.002 to

0.06 mm Sand particle size is between 0.06 to 2

mmGravel Particle size is between 2 to 60 mm

Cobbles Particle size is between 60 to 200 mm

Cohesive soil and Cohesion less soil,

Cohesive soil – Silt, Clay, Mud

This type of soil consists of particles with smaller and separated voids. And water retains in soil.

Cohesion less soil – Sand, Gravel

This type of soil consists of interconnected larger pores. And water may not retain in the pores.

Field Work

The field work carried out at the site in the subsoil investigation as bellow.

Field work of this investigation comprised advancing three boreholes (BH 01, BH 02 and BH 03) at the site. These boreholes were advanced through overburden cutting tools using rotary drilling technique and adopting the wash boring technique to remove the cuttings from the bottom of the borehole.

The boreholes were initially advanced up to hard rock. Thereafter, in the case of boreholes BH 01 and BH 03 only, these boreholes were further advanced by, coring the rock using a double tube core barrel. The depths of drilling are as follows.

Borehole No BH 01 BH 02 BH 03

Depth to rock (m)

Depth of borehole (m)

24.1

25.5

22.9

22.9

21.6

22.7

Standard Penetration Tests (SPT) was carried out regularly within the boreholes in order to obtain a continuous strength profile of the subsoil.

Disturbed soil and rock core samples were collected both from SPT tube and the cuttings collected from the washings for visual classification.

Ground water level was determined as the depth of the water level inside the borehole prior to extracting the casing.

The results of the field investigation together with the results of the SPT’s are shown as logs of boreholes in Annex.

Boring methods used for the selected site

1) Rotary Drilling Method

Two main types of rotary drilling are carried out in rock. Rock coring using diamond or tungsten carbide tipped core bits provides samples and information on rock types, fissuring and weathering. This method is useful as a quick method for detecting major strata changes and for the location of coal seams and old workings. Water, air, foam or drilling mud may be used as the flushing medium in either case.

2) Wash boring method

In this method, water is forced under pressure through an inner tube which may be rotated or moved up and down inside a casing pipe. The chopping and jetting action of the bit and water disintegrates the soil. The soil water slurry comes up to the ground through the annular space between the drill rod and the casing. The slurry flowing out gives an indication of the soil type. In this method heavier particle of different soil layers remain under suspension in the casing pipe and get mixed up, hence this method is unsuitable for obtaining reliable samples for classification. Whenever a sample is to be taken, washing should be stopped and a tube sampler should be attached to the end of the drill rod or the inner tube. By driving the sampler into the soil, by hammering or jacking, samples can be obtained. Jacking should be used when undisturbed samples are required. Below is an Auto CAD drawing of a setup for a wash boring.

Borehole Test Results

Three boreholes were used to find the stratification of the soil below the surface. The borehole plane is as bellow,

The borehole test deciphered the following results –

Borehole investigation reveals that surface horizon at the site consists of clayey sand of average thickness of 0.6 m, which is mixed with building debris.

Underlying this fill, a fine clayey sand layer followed by a layer of very dense sand was observed and its depth varying from 5 m to 6 m.

The residual formation is encountered at a depth of around 8-9m. This formation consists of the in-situ weathered products of the parent rock lying beneath. The residual formation is very strong, although there are a few intrusions of bands of peat between depths of 17m and 22m. These soils are followed by highly weathered rock, and basement rock.

The depth to hard basement rock varied between 21.6m and 24.1m.

Ground water level was lying just below the fill at a depth of 5.6 m from the surface at BH-01.

Foundations

Introduction to foundations

A foundation is the base on which a building rests, and its purpose is to safely transfer the load of a building to a suitable subsoil.

Requirement of foundation in a building is to,

Safely sustain and transmit the combined dead and imposed loads to the ground, as not to cause any settlement or other movement in any part of the building or of any adjoining building or works.

Be of such a depth, or be so constructed, as to avoid damage by swelling, shrinkage or freezing of the subsoil.

Be capable of resisting attack by deleterious material, such as sulphates, in the subsoil.

Foundations are usually made of either mass or reinforced concrete and can be considered into two main categories.

Shallow Foundations – Those that transfer the loads to subsoil at a point near to the ground floor of the building such as strips, pad and rafts.

Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils.

Deep Foundations – Those that transfer the loads to subsoil some distance below the ground floor of the building, to a deeper, more competent strata at depth, if unsuitable soils are present near the surface; such as piles, piers and caissons.

Shallow Foundation

a. Strip Foundation

Strip foundations are use to support and transmit the loads from heavy walls. The effect of the wall on the relatively thin foundation is to act as a point load, and the resultant ground pressure will induce tension on the underside across the width of the strip. Tensile reinforcement is therefore required in the lower face of the strip, with distribution bars in the second layer running longitudinally.

Figure RC Strip Foundation

b. Isolated or Pad Foundation

This type of foundation is used to support and transmit the loads from piers and columns. The most economic plan shape is a square, but if the columns are close to the site boundary it may be necessary to use rectangular plan shape of equivalent area. The reaction of the foundation to the load and ground pressure is to cup, similar to a saucer, and therefore main steel is required in both directions.

c. Raft Foundation

Raft foundations are made of a slab of reinforced concrete under the entire building designed

to transmit the load of the building to the subsoil below the raft. Raft foundations are used

where the soil is very weak and has low bearing capacity. It also has a uniform distribution of

the load and thus there is no surprise settlement of the soil beneath. The two types of Raft

foundations commonly used are the flat slab rafts and the wide toe rafts.

Figure Raft Foundation for two way beam and slab

Figure Raft foundation for Flat plate with pedestals

Deep Foundation

d. Pile FoundationWhen the soil beneath the ground level at which the footing would normally be established is too weak to provide adequate support, the loads are transferred to more suitable strata at a greater depth by means of piles. Piles are structural members with a small cross-sectional area compared to their length. Piles can be roughly classified into four groups which are timber, concrete, steel and a combination of these.

Depending upon their function or use piles may be classified into the following types, I. End bearing piles II. Friction piles

According to the method of installation piles can be classified as follows, I. Displacement piles II. Replacement piles

End Bearing Piles End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel. They derive most of their carrying capacity from the resistance of the stratum at the toe of the pile.

Figure End Bearing Pile

Friction Piles

When piles are required to be driven at a site where the soil is weak or soft to a considerable depth, the load carried by a pile is borne by the friction developed between the sides of the pile and the surrounding ground (skin friction). In such cases the pile is named as friction or floating pile. Thus friction piles are driven in the type of soil whose strength does not increase with depth or, where rate of increase in strength with depth is very slow.

Following figure describes the difference between End bearing pile and Friction pile.

e. Caisson Foundation

A well or caisson foundation is a box-like structure which is sunk on either land or water to a

required depth. It is mainly used in bridges and structures close to water beds like oceans and

rivers.