Thursday, October 27, 2011

Site Investigation

Site investigation or soil explorations are done for obtaining the information about subsurface conditions at the site of proposed construction. Soil exploration consists of determining the profile of the natural soil deposits at the site, taking the soil samples and determining the engineering properties of soils. It also includes in-situ testing of soils.

GENERAL: Soil is used as:

a)Construction material as, for example, in the construction of dams, pavements, buildings etc.,
b)Supporting material (Foundation) for carrying the loads of the super-structure through their foundations.
The function of a properly designed foundation is to support loads resting on it without causing excessive stresses within the soil mass at any depth beneath foundation. Stresses are considered excessive if a complete rupture within the soil mass occurs (Shear failure), or if detrimental settlements result (failure due to excessive settlement). Therefore it is apparent that one of the most important steps in the solution of a foundation problem is determining underground conditions that will affect the design. Field and laboratory investigations required to obtain necessary information about geology, hydrology, and soil conditions; geotechnical properties of soil at the prospective building site, and the performance of various soil types encountered when acted upon by structural loads, water and temperature are called sub-surface investigations or soil exploration programme.


Soil exploration programme are influenced by a number of factors some of these are:
a) Size and type of the project;
b) General characteristics of the soils in the work area;
c) Time available for exploration; and
d) Degree of risk or safety involved.

Tall buildings or heavy industrial structures founded over a deposit of fairly homogeneous clay require an extensive soil exploration programme.
An erratic soil mass in one in which the soil or soil strata are not uniforms or consistent in properties, elevation, thickness, or extent. In the case of erratic strata it is very difficult to have an accurate picture of the subsurface as it require an infinite number of boring and tests. In such cases only the location and extent of the weaker strata and lenses and possibly the properties of these weak soils are determined.
The degree of risk or safety involved will be another governing factor in the extent and thoroughness of soil survey. Failure of a dam, for example, can take many lives and cause virtually irreparable property damage. To a lesser degree the same is true for tall buildings, bridges and many industrial structures. Soil conditions for these structures must be carefully and thoroughly investigated. On the other hand the damage caused by road pavement failure or large overall settlements of Storage tanks is relatively minor, and a greater degree of risk is justified. However, it is emphasized that even light structures, such as private dwellings and filling stations cannot be expected to remain stable if founded on loose or compressible soils, and a soil investigation is warranted prior to the design and construction of a light structure, as well as for projects involving greater loads and larger investments.
Adequate and accurate sub-surface data will enable architects and engineers to design foundations for both safety and economy. Savings in time and money will offer more than offset the cost of exploration.
CLASSES: Sub-surface investigations may be subdivided into three classes:
a) Foundation investigations to investigate sites for new structures.
b) Stability or failure investigations to investigate causes of distress or failure of existing structures.

c) Earthwork investigations to evaluate suitability of natural materials for construction purposes.
The procedure of exploration can be divided into the following steps:
i) Collection of data about the project.
ii) Geologic study of the site.

iii) Site inspection.
i) Depth, extent, and composition of critical soil strata,
ii) Ground-water level and its fluctuations,

iii) Depth of bed rock, when necessary,
iv) Estimate of engineering properties of soil,
v) Initial selection of foundation possibilities.
i) Additional test borings.

ii) Undisturbed sampling if compressible soils are encountered at critical depth.
iii) Laboratory/Field tests if data on soil strength and deformation characteristics are needed.
i) Evaluation of settlement characteristics of various soil layers.
ii) Evaluation of bearing capacity of various soil layers.

<!–[if !supportLists]–>iii) Foundation design.
Tentative cost estimates of various possible foundation.
The purpose of the reconnaissance is to determine the nature of the site and to estimate the type of soils and rock likely to be encountered. This is accomplished by means of a geologic study and as inspection of the site and adjacent areas. The results of this phase of investigation are extremely valuable in planning the sub-surface exploration programme.
The geologic study will indicate the origin of the site, such as a buried prelacies valley, a former flooded plain, a former data deposit, or glacial deposit, or deposit formed by wind action etc.

The possibilities of such structural defects in the bed rock, as cracks fissures, be revealed. These in formations can be collected from:
i) Geological maps,
ii) Agronomy maps,
iii) Area photograph etc.

The site inspection includes a study of pertinent topographic features, open cuts, and existing structure. The surface drainage pattern and the location of springs may provide much information on groundwater conditions. Cuts made for construction purposes such as highways may shed light on expected soil profiles, particularly for shallow soil formation, at the site to be investigated.
Another phase of this programme should be to investigate the types of foundations used in surrounding buildings, including their depths, and settlement records if available. This should also include inspections of the surrounding buildings as to the structural conditions, such as the presence of settlement cracks. The depth of adjacent basements and foundation may have some influence on the type of foundation selected for the new structure if regulations exist to protect adjacent structure.
Before any sub-surface investigation programme is outlined, it is also desirable to establish the location of any underground sewer lines, water mains, and other utilities that many cross the site of the proposed structure.
Preliminary exploration can be of two types:
a) Shallow exploration usually used for light structures, highways, railways, airfield etc.
b) Deep exploration used for dams, bridges, tall buildings, heavy industrial structures etc.

i) Probing/Sounding Methods:
These methods have been developed for determining the consistency of cohesive soils or relative density of cohesion less deposits. In this method a rod encased in a sleeve is forced into the soil and the resistance to penetration or with drawl is observed. Variations in this resistance shows dissimilar soil layers and the numerical values of the resistance permits an estimate of some of the physical properties of the strata.
The advantages and limitations of soil sounding methods are as follows.
1- Soundings are generally considerably faster and cheaper than boring.
2- In boring a thin weak soil strata may pass unnoticed but sounding indicates its presence.
3- In erratic soil conditions soundings can be used between two borings.

4- Sounding gives an idea about the consistency of cohesive soils and degree of compactness of cohesion less soils. Hence where undisturbed sample is difficult to collect or is expensive sounding may be used as a substitute.
5- Bearing capacity of soil can be estimated by sounding.
6- Soundings alone cannot provide sufficient data for the final design of important or unusual foundations and earth structures.
7- Sounding gives no idea about the settlement characteristics of the soil in question.
8- Sounding gives misleading results when soil contains stones/boulders etc.

As a general rule, dynamic penetration tests are performed in cohesion less soils and static tests in cohesive materials.
Following are the various probing/sounding methods.

The oldest and simplest form of soils sounding consists in driving a rod into the ground by repeated blows of a hammer. The apparatus is shown in fig. 1. The penetration of the road for a given number of slows with a hammer of constant weight and drop is recorded or the number of blows required per foot penetration of the rod is noted. These information’s are used as an index of the penetration foundation experience.
Skin friction is also acting on the rod and is cumulative with depth, hence the penetration resistance does not directly represent the strength or density of the strata encountered.
{SKIN FRICTION = Perimeter x Length x Average

shear resistance per unit area developed between the soil and the rod
A sounding rod is forced into the soil partly by static load and partly by rotation of the rod. The assembly is shown in fig. 2. The rod is provided with a screw point with a diameter about 50 percent greater than that of the rod. The penetration is first recorded for successive static loads of 5,15,25,75 and 100 kg. The rod with a final static load of 100 kg is then rotated and the penetration is observed for each 25 half turns. A diagram of the variation of this penetration with depth is then plotted and compared with similar diagrams obtained for the same soil for which the bearing capacity has been determined by other means.
This method is relatively fast and inexpensive, even when compared with other sounding methods, but it is not suitable for exploration of coarse and gravelly soils or very compact or hard soils. Neither does the method furnish adequate details on the soil profile when soils are so soft that they are penetrated by the sounding rod without rotating it but simply by placing the above mentioned static loads on the self-locking clamp.
A cone penetrometer is used for shallow explorations in relatively soft deposits and determination of the capacity of such deposits to sustain various types of loads and traffic. At depths at which it is desired to determine the penetration resistance, the pressure on the handle is slowly increased until there is a perceptible but very slow and uniform downward movement of the cone. The corresponding pressure is measured by means of the proving ring.
ii) Auger Borings: Auger borings are classified soil exploration methods. These are made in cohesive soil or in non cohesive soils above groundwater table. The soil samples so obtained by auger borings are badly disturbed. However, the auger may also be used for advancing the hole down to the point where undisturbed soil samples are to be taken. Different types of augers are used for auger boring as shown in fig. (helical auger, post-hole auger etc.). Hand-operated augers can be used to reach depth up to 20 ft. to 30 ft. For greater depths motor-driven auger can be used. The size of boring varies from 2 in. to 12 in.

iii) Test Pits (Open pit exploration): Test pits permit a direct inspection of the soil strata in place, and taking of adequate disturbed and undisturbed soil samples.
Test pits are the most satisfactory method of disclosing the soil strata conditions. Also it is possible to take even undisturbed samples of sands by this method. The cost of test pit increases rapidly with depth; they are uneconomical beyond a depth of 12 ft. They are practically impractical when groundwater is to be handled.
iv) Deep Borings: Deep borings are required for heavy structures, dams, industrial buildings, bridges etc. For deep borings usually power drilling rigs are used.
There are two principal types of equipment for making borings;
a) Cable tool drilling rigs (wash borings); and b) Rotary drilling rigs.

Wash Boring: A diagrammatic sketch of wash boring is shown in figure 2. Wash boring is one of the most common methods of advancing a hole into the ground. In this method a hole is started by driving a casing to a depth of 5 ft. to 10 ft. casing is simply a pipe which supports the hole, preventing it from caving in. The casing is cleaned out by means of a chopping bit fastened to the lower end of the drilling rod, if water is pumped through the drilling rod and exists at high velocity through holes in the bit. The water rises between the casing and drill rod, carrying suspended soil particles, and overflows at the top of the casing through. T connection into a container, from which the effluent is reticulated back through the drill rod. The hole is advanced by raising, rotating, and dropping the bit on to the soil at the bottom of the hole. Drill rods, and if necessary caring are added as the depth of the boring increases. This method is quite rapid for advancing holes in all but the very hard soil strata.
Rotary drill is another method of advancing test holes. This method uses rotation of the drill bit with the simultaneous application of pressure to advance the hole. Rotary drilling is the most rapid method of advancing holes in the rock unless it is badly fissured; however, it can also be used for any other type of soil. If this is applied in soils when the sides of the hole tend to cave in, a drilling mud may be used. The drilling mud is usually a water solution of a thixotropic clay (Bentonite), with or without other admixtures, which is forced into the sides of the hole by the rotary drill. This provides sufficient strength to the soil so that it maintains the hole. The mud also tends to seal off the water flow into the hole from the permeable water bearing strata.

These methods have been used considerably in recent years for preliminary exploration of dams sites and for highways.
The seismic method is based on the principle that sound travels more rapidly through dense materials than through loose materials. Velocities as high as 18,000 to 20,000 fps have been recorded in dense, solid rock and velocities as low as 600 fps have been observed in loose sand.

The electric method consists of measuring charges in electric resistance of the soil. Dense rock has a very high electrical resistivity and soft, saturated clay has a low resistance.

a) Spacing of Borings:
No definite rules can be established for boring spacing. They depends upon many factors, for example, the nature and conditions of soil. The shape, extent and type of structure, loads and sensitivity of the structure.
However, borings should be so spaced as to detect the various soil layers in sequence number as well as type, to determine their extent, course and dip as precisely as possible.
The borings can be classified into:
i) Primary borings: They are deep borings, and serve to disclose the nature and stratification of the soil and to detect the position of a satisfactory from soil layer below the active zone. Some times these are know as informatory borings.

ii)Main Boarings: These borings are made for observing the position of groundwater table and its fluctuations, usually this act of borings is not as deep as primary borings. Additional samples are obtained from these boring. These are usually made between the primary borings.
iii) Supplementary Borings: To plot accurately the position and extent of the several soil strata and soil pockets in clay beds or clay pockets in sand, and to secure additional soil samples, a third intermediate set of borings is made. These are known as supplementary borings. Figure 3 gives a general layout borings for strs. And bridges.
iv) Depth of Borings: The borings should pass the critical zone of the strata. The depth of borings is governed by the topographical and geological conditions and type purpose of the structure.

Usually the depth of the borings is approximately estimated dependent upon the width and load of foundation. For strip footing and single footing a thumb role is that the boring depth should be at least 2 to 3 times the width of the foundation below the contact area or base of the footing. For mat foundation 1.5 times to 2 times, the width. Table No. 1 gives the commonly used depth and spacing of borings:

Reporting: The results of a soil boring are usually presented in the form of a boring log. A boring log should contain the following information’s:
i) Depth below ground surface, ii) Elevation of soil layers & groundwater table

iii) Thickness of layers, iv) Graphical symbol of the soil type.
v) Description of soil,
vi) Position where soil sample is taken; whether disturbed or undisturbed.
vii) Sample No.

viii) Natural m.c.
ix) S.P.T. Resistance.
x) Notes indicating position of groundwater table, encountered tree routs or other features.
A typical log is shown in figure 4.

BORING REPORT: A boring report should contain:
i) Location plan of the project.
ii) Location plan borings.
iii) Description of borings.

iv) Surface drainage conditions.
v) Probable source of free water,
vi) Groundwater conditions,
vii) Boring log drawn to scale,
viii) Information on difficulties met with during exploration.

ix) Soil identification and classification tests results.

a) Disturbed Samples:
A disturbed soil sample is one whose natural conditions such as structure, texture, density, natural m.c. and stress conditions, are disturbed. They can be obtained easily by shovel, auger boring and deep borings. These are used for classification tests, compaction test etc.
UNDISTURBED SAMPLES: Soil samples obtained by minimum disturbance of natural conditions such as structure, texture, density, natural moisture content and stress conditions are known as undisturbed samples.

Different Types of Samples:
a) This walled Shelby tubes,
b) Split Spoon Samplers,
c) Piston sampler etc.

Experience has shown that the degree of sample disturbance bears a direct relationship to the ratio of sample area to sampler area.

Ar =Degree of disturbance in per cent it should vary from 10-15% Another thumb rule is that the wall thickness should not exceed 3 to 4 per cent of the tube dia.
Do and Di =The external and internal dia. of sampler.

Types of Tests

i) Visual inspection:
a) Grain size, grain shape, and gradation of coarse-grained, cohesion less soils.
b) Texture and colour of fine-grained soils. Organic soils are distinguished by their coarse, fibrous texture and dark colour.

c) Moisture content, such as dry, moist, or wet.
ii) Dilatancy Test,
iii) Feel Test,
iv) Dry Strength Test,
v) Shine Test.

i) Atterberg limits:
a) Plastic limit Test,
b) Liquid limit Test.

c) Shrinkage limit test.
ii) Grain size analysis Test:
a) Mechanical analysis
(Sieve analysis Test)
b) Wet analysis Test.

(Hydrometer analysis Test)

i) Field Vane Shear Test.
ii) Standard Penetration Test.
iii) Penetrometer Tests.

i) Direct Shear Test.

i) Triaxial Test
ii) Unconfined Compression Test

iii) Laboratory Vane Shear Test
i) Constant head permenmeter Test.
ii) Falling head permeameter Test.

iii) In-situ permeability Tests.

i) Procter compaction test.
ii) Modified AASHO Test.
iii) C.B.R. Compation.




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