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FOUNDATION ENGINEERING UNIT-1 SITE INVESTIGATION AND SELECTION OF FOUNDATION
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CONTENTS
1.1 DEFINITIONS OF KEY TERMS 1.2 SCOPE AND OBJECTIVES 1.3 METHODS OF EXPLORATION 1.3.1 General 1.3.2 Classification of investigation method 1.3.3 Direct methods of explorations 1.4 BORING: 1.4.1 Displacement boring. 1.4.2 Wash boring. 1.4.3 Auger boring. 1.4.4 Rotary drilling. 1.4.6 Percussion drilling. 1.4.7 Continuous sampling. 1.5 DEPTH AND SPACING OF BORE HOLE 1.6 SAMPLING 1.7 REPRESENTATIVE AND UNDISTURBED SAMPLING 1.8 SAMPLING TECHNIQUES 1.8.1 Split spoon sampler and thin tube sampler 1.8.2 Stationary piston sampler 1.9 PENETRATION TESTS 1.9.1 Standard penetration tests 1.9.2 Standard core penetration test 1.10 SELECTION OF FOUNDATION
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TECHNICAL TERMS:
Site Investigation:An investigation is essential for judging the suitability of engineering works, and for preparing adequate, economic designs.
Site Reconnaissance:An inspection of the site and study of topographical features helpful in deciding the future programis termed as site reconnaissance.
Site Exploration:The study of details about sub-soil and ground water conditions is termed as Site Exploration.
Test pits are the trenches in which soil can be inspected in their natural conditions and samples can be conveniently taken.
Boring is done to take representative or undisturbed samples for conducting laboratory tests.
Auger and Shell Boringis the cylindrical augers with cutting edge used for making deep borings up to 50m.
Wash Boring is fast and reliable method for advancing holes in all types of soils more than 3m deep
Percussion. Boringmethod consists of breaking up of the soil sub strata by repeated blows from a bit or chisel, used for pulverizing rock boulders.
Rotary Drilling is done to get Undisturbed Samples from rocks and soil by rotating a hollow steel tube having a cutting bit at its base
SPT is nothing but Standard Penetration Test used to determine the relative density, angle of shearing resistance of cohesionless soil.
Penetration Resistance: Number of blows required to driven the sample through 30cm
beyond the seating drive. Is termed as Penetration Resistance 'N'. UNIT-I
SCPT is nothing but Static Cone Penetration Test used in places of SPT particularly for 1. 3
soft clays and silts and fine to medium sand deposits.
Overburden Pressure:N value in granular soil is influenced by overburden pressure.N values recorded from field tests at an effective Overburden Pressure are corrected by
standard effective overburden pressure.
DCPT is nothing but Dynamic Cone Penetration Test is a cone with apex angle 60° is attached to drill rods into the soil by blows of a hammer of 65 kg,falling free from a height of 750mm.
Sample is actually a small portion which is removal from natural medium for testing its properties in laboratory under the conditions similar to its original position.
Samplingthe natural soil structure gets disturbed, can be used to determine the index properties of soil such as grain size, specific gravity.
Representative Samples: The samples where the natural moisture content, proportion of mineral constituents can be preserved with suitable precautions are called Representative
Samples.
Undisturbed Samples:During sampling the natural soil structure and water content of sample are undisturbed, used to determine the engineering properties of soil.
Non-Representative Samples:In addition to alteration in the original soil structure, soils from other layers get mixed up, virtually of no use.
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UNIT - I - SITE INVESTIGATION AND SELECTION OF FOUNDATION 1.1 DEFINITIONS OF KEY TERMS
Site Investigation:An investigation is essential for judging the suitability of engineering works, and for preparing adequate, economic designs.
Site Reconnaissance:An inspection of the site and study of topographical features helpful in deciding the future programis termed as site reconnaissance.
Site Exploration:The study of details about sub-soil and ground water conditions is termed as Site Exploration.
Test pits are the trenches in which soil can be inspected in their natural conditions and samples can be conveniently taken.
Boring is done to take representative or undisturbed samples for conducting laboratory tests.
Auger and Shell Boringis the cylindrical augers with cutting edge used for making deep borings up to 50m.
Wash Boring is fast and reliable method for advancing holes in all types of soils more than 3m deep
Percussion. Boringmethod consists of breaking up of the soil sub strata by repeated blows from a bit or chisel, used for pulverizing rock boulders.
Rotary Drilling is done to get Undisturbed Samples from rocks and soil by rotating a hollow steel tube having a cutting bit at its base
SPT is nothing but Standard Penetration Test used to determine the relative density, angle of shearing resistance of cohesionless soil.
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Penetration Resistance: Number of blows required to driven the sample through 30cm beyond the seating drive. Is termed as Penetration Resistance'N'.
SCPT is nothing but Static Cone Penetration Test used in places of SPT particularly for soft clays and silts and fine to medium sand deposits.
Overburden Pressure:N value in granular soil is influenced by overburden pressure.N values recorded from field tests at an effective Overburden Pressure are corrected by
standard effective overburden pressure.
DCPT is nothing but Dynamic Cone Penetration Test is a cone with apex angle 60° is attached to drill rods into the soil by blows of a hammer of 65 kg,falling free from a height of 750mm.
Sample is actually a small portion which is removal from natural medium for testing its properties in laboratory under the conditions similar to its original position.
Samplingthe natural soil structure gets disturbed, can be used to determine the index properties of soil such as grain size, specific gravity.
Representative Samples:The samples where the natural moisture content, proportion of mineral constituents can be preserved with suitable precautions are called
Representative Samples.
Undisturbed Samples:During sampling the natural soil structure and water content of sample are undisturbed, used to determine the engineering properties of soil.
Non-Representative Samples:In addition to alteration in the original soil structure, soils from other layers get mixed up, virtually of no use.
Introduction The knowledge about the site forms a vital role in the safe and economic development of a site. A thorough investigation of the site is an essential preliminary to the construction of any civil engineering works. Public building officials may require soil data together with the recommendations of the geotechnical consultant prior to issuance of a building permit. Elimination of the site exploration, which usually ranges from about 0.5 to1 percent of total construction costs, only to find after construction has started that the foundation must be UNIT-I
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redesigned is certainly false economy. This is generally recognized, and it is doubtful if any major structures are currently designed without exploration being undertaken. 1.2 SCOPE & OBJECTIVE
An investigation of site is essential for judging its suitability for the proposed engineering works and for preparing adequate and economic designs.
In general, any investigation should start with the collection and examination of the already existing data about the soil and geological conditions of the site.
Site investigation is equally necessary for analyzing the safety or causes of failure of existing works, for selecting construction materials and for deciding upon the
construction methods to be applied.
In general, the purpose of a site investigation is to obtain necessary information about the soil and hydrological conditions at the site and to know the engineering properties of soil
which will be affected.
A timely and intelligently planned site investigation should be considered a pre-requisite for efficient, safe, economical design and construction.
The methods of site investigation are largely dependent upon the nature of the engineering project and the site.
In many areas, the existing local knowledge, records of trial pits, bore holes, etc., in the vicinity and the behavior of existing structures, particularly if they are similar to the
proposed ones, are very helpful.
If the existing information is not sufficient or is inconclusive, the site should be explored in detail. The exploration should be preceded by site reconnaissance.
To access the general suitability of the site.
To achieve safe and economical design of foundations and temporary works.
To know the nature of each stratum and engineering properties of the soil and rock, this may affect the design and mode of construction of proposed structure and foundation.
To forecast and provide against difficulties that may arise during construction due to ground and other local conditions.
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To find out the sources of construction material and selection of sites for disposal of water or surplus material.
To investigate the occurrence or causes of all natural and manmade changes in conditions and the results arising from such changes.
To ensure the safety of surrounding existing structures.
To design for the failed structures or remedial measures for the structures deemed to be unsafe.
To locate the ground water level and possible corrosive effect of soil and water on foundation material.
1.3 METHODS OF EXPLORATION 1.3.1 General
The various types of site investigation are:
Open excavation
Boring
Subsurface Sounding
Geophysical Methods
These available methods of exploration can be broadly classified into two categories:
Direct methods
Indirect methods
The direct method of soil exploration usually consists of sinking a borehole at a predetermined location to the required depth by a method suitable for the site and to obtain fairly intact samples of soils from every stratum encountered or at suitably selected depths. The samples obtained are utilized to get necessary information about the soil characteristics by means of laboratory tests. During recent years, indirect methods of soil exploration have also been used for civil engineering structures. These methods include various sounding and geophysical methods. In sounding methods, the variation in penetration resistance of sample or cone is utilized to UNIT-I
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interpret some of the physical properties of the strata. In geophysical methods, the change in subsoil strata are identified by measuring certain physical characteristics, e.g. electrical conductance, wave velocity of subsurface deposits. In addition to these methods, projectiles, probes, and aerial photographs are also useful in interpreting the soil characteristics. 1.3.2 Classification of investigation method 1. Logging method
Penetration test (SPT, CPT)
DCPT
Field vane shear test
2. Specials Method
Dilatometer test or Pressure meter test
Plate load test
Classify the Site Investigation in another way
Investigation of site for new works
Investigation of defects or failure of existing works
Investigations as the safety or stability of existing works.
Investigations relating to the suitability of material for various constructional purposes.
Information Extraction from site investigation topography
General topography of sites which affects foundation design & construction. Ex:-surface configuration & waterways.
Location of buried services Ex:-Telephone lines, Water lines Power lines
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General Geology of the India with particular reference to the principal geological formation underlines the site. 1. 9
Any special features such as possibility of earthquake, folding, seasonal swelling etc.
Availability and quality of local construction materials.
Detailed record of soil and rock strata & ground water condition within the zone affected by foundation loadings & of any deeper strata affecting site conditions in any way.
Design data which comprises strength and compressibility characteristics of different strata.
Results of chemical analysis on soil or groundwater to determine possible deleterious effect on foundation structures
1.3.3 Direct Methods of Exploration General
Test pits, trenches, shafts and tunnels are the direct excavation methods of exploration which not only affords sampling and testing in situ but also permit visual inspection of the soil and rock formations in their natural state. They are considered the only means of obtaining reliable information in a soil deposit of mixed sand, gravel and boulders where boring may prove difficult. However, they are slow and become relatively expensive with increasing depths of exploration. Test/Trial pit Trial pits are the cheapest way of site exploration & do not require any specialized equipment. A pit is manually excavated to get an indication of the soil classification & obtain undisturbed & disturbed samples. Trial pits allow visual inspection of any change of strata & facilitate in-situ testing. They are suitable for exploration of shallow depth only. 1.4 Boring methods of exploration The boring methods are used for exploration at greater depths where direct methods fail. These provide both disturbed as well as undisturbed samples depending upon the method of
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boring. In selecting the boring method for a particular job, consideration should be made for the following:
The materials to be encountered and the relative efficiency of the various boring methods in such materials.
The available facility and accuracy with which changes in the soil and ground water conditions can be determined.
Possible disturbance of the material to be sampled.
The different types of boring methods are: 1
Displacement boring.
2
Wash boring.
3
Auger boring.
4
Rotary drilling.
5
Percussion drilling.
6
Continuous sampling.
1.4.1 Displacement borings It is combined method of sampling & boring operation. Closed bottom sampler, slit cup, or piston type is forced in to the ground up to the desired depth. Then the sampler is detached from soil below it, by rotating the piston, & finally the piston is released or withdrawn. The sampler is then again forced further down & sample is taken. After withdrawal of sampler & removal of sample from sampler, the sampler is kept in closed condition & again used for another depth. Features:
Simple and economic method if excessive caving does not occur. Therefore not suitable for loose sand.
Major changes of soil character can be detected by means UNIT-I
Figure 1.1 Wash Boring
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. of penetration resistance.
These are 25mm to 75mm holes.
It requires fairly continuous sampling in stiff and dense soil, either to protect the sampler from damage or to avoid objectionably heavy construction pit.
1.4.2. Wash boring: It is a popular method due to the use of limited equipment. The advantage of this is the use of inexpensive and easily portable handling Figure.1.2 Augers
and drilling equipment. Here first an open hole is formed on the ground so that the soil sampling or rock drilling operation can be done below the hole. The hole is advanced by chopping and twisting action of the light bit. Cutting is done by forced water and water jet under pressure through the rods operated inside the hole. In India the ―Dheki‖ operation is used, i.e., a pipe of 5cm diameter is held vertically and filled with water using horizontal lever arrangement and by the process of suction and application of pressure, soil slurry comes out of the tube and pipe goes down. This can be done up to a depth of 8m –10m (excluding the depth of hole already formed beforehand) Just by noting the change of color of soil coming out with the change of soil character can be identified by any experienced person. It
Figure.1.3 Rotary Drilling System
gives completely disturbed sample and is not suitable for UNIT-I
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very soft soil, fine to medium grained cohesion less soil and in cemented soil.
1.4.3. Auger boring This method is fast and economical, using simple, light, flexible and inexpensive instruments for large to small holes. It is very suitable for soft to stiff cohesive soils and also can be used to determine ground water table. Soil removed by this is disturbed but it is better than wash boring, percussion or rotary drilling. It is not suitable for very hard or cemented soils, very soft soils, as then the flow into the hole can occur and also for fully saturated cohesionless soil. 1.4.4. Rotary Drilling Rotary drilling method of boring is useful in case of highly resistant strata. It is related to finding out the rock strata and also to access the quality of rocks from cracks, fissures and joints. It can conveniently be used in sands and silts also. Here, the bore holes are advanced in depth by rotary percussion method which is similar to wash boring technique. A heavy string of the drill rod is used for choking action. The broken rock or soil fragments are removed by circulating water or drilling mud pumped through the drill rods and bit up through the bore hole from which it is collected in a settling tank for recirculation. If the depth is small and the soil stable, water alone can be used. However, drilling fluids are useful as they serve to stabilize the bore hole. Drilling mud is slurry of bentonite in water. The drilling fluid causes stabilizing effect to the bore hole partly due to higher specific gravity as compared with water and partly due to formation of mud cake on the sides of the hole. As the stabilizing effect is imparted by these drilling fluids no casing is required if drilling fluid is used. This method is suitable for boring holes of diameter 10cm, or more preferably 15 to20cm in most of the rocks. It is uneconomical for holes less than 10cm diameter. The depth of various strata can be detected by inspection of cuttings. 1.4.5. Percussion drilling In case of hard soils or soft rock, auger boring or wash boring cannot be employed. For such strata, percussion drilling is usually adopted. Here advancement of hole is done by alternatively lifting and dropping a heavy drilling bit which is attached to the lower end of the drilling bit which is attached to the cable. Addition of sand increases the cutting action of the drilling bit in clays. Whereas, when coarse cohesionless soil is encountered, clay might have to UNIT-I
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Figure.1.4 Percussion Drilling
be added to increase the carrying capacity of slurry. After the carrying capacity of the soil is reached, churn bit is removed and the slurry is removed using bailers and sand pumps. Change in soil character is identified by the composition of the outgoing slurry. The stroke of bit varies according to the ground condition. Generally, it is 45-100cm in depth with rate of 35-60 drops/min. It is not economical for hole of diameter less than 10cm. It can be used in most of the soils and rocks and can drill any material. One main disadvantage of this process is that the material at the bottom of the hole is disturbed by heavy blows of the chisel and hence it is not possible to get good quality undisturbed samples. It cannot detect thin strata as well. 1.4.6. Continuous Sampling
Fig 1.4
The sampling operation advances the borehole and the boring is accomplished entirely by taking samples continuously. The casing is used to prevent the caving in soils. It provides more reliable and detail information on soil condition than the other methods. Therefore it is used extensively in detailed and special foundation exploration for important structures. It is slower method and more expensive than intermittent sampling. When modern rotary drilling rigs or power driven augers are not available, continuous sampling may be used to advantage for advancing larger diameter borings in stiff and tough strata of clay and mixed soil. In the Boston district, corps of Engineers has made faster progress and reduced cost by use of continuous sampling in advancing 3-inch diameter borings through compact gravelly glacial till, which is difficult to penetrate by any boring method. 1.5 NUMBER AND DEPTHS OF BOREHOLES: It is practically impossible and economically infeasible to completely explore the whole project site. You have to make judgments on the number, location, and depths of borings to provide sufi cient information for design and construction. The number and depths of borings should cover UNIT-I
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the zone of soil that would be affected by the structural loads. There is no i xed rule to follow. In most cases, the number and depths of borings are governed by experience based on the geological character of the ground, the importance of the structure, the structural loads, and the availability of equipment. Building codes and regulatory bodies provide guidelines on the minimum number and depths of borings. The number of boreholes should be adequate to detect variations of the soils at the site. If the locations of the loads on the footprint of the structure are known (this is often not the case), you should consider drilling at least one borehole at the location of the heaviest load. As a guide, a minimum of three boreholes should be drilled for a building area of about 250 m2 (2500 ft2) and about i ve for a building area of about 1000 m2 (10,000 ft2). Some guidelines on the minimum number of boreholes for buildings and for due diligence in subdivisions are given in Table below. Some general guidance on the depth of boreholes is provided in the following: TABLE 1.1
• In compressible soils such as clays, the borings should penetrate to at least between 1 and 3 times the width of the proposed foundation below the depth of embedment or until the stress increment due to the heaviest foundation load is less than 10%, whichever is greater. • In very stiff clays and dense, coarse-grained soils, borings should penetrate 5 m to 6 m to prove that the thickness of the stratum is adequate. • Borings should penetrate at least 3 m into rock. UNIT-I
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• Borings must penetrate below any i lls or very soft deposits below the proposed structure. • The minimum depth of boreholes should be 6 m unless bedrock or very dense material is encountered.General guidelines for the minimum number or frequency of boreholes and their minimum depths for common geotechnical structures are shown in Table. 1.6 SAMPLING: The soil samples can be of two types: disturbed and undisturbed. A
disturbed
sample is that in which the natural structure of soils get partly or fully modified and destroyed,
although
with
suitable
precautions the natural water content may be preserved. Such a sample should, however, be representative of the natural soil by maintaining the original proportion of the various soil particles intact. An undisturbed sample is that in which the natural structure and properties remain preserved.
Figure 1.5 sampler
1.7 REPRESENTATIVE SAMPLING:
Disturbed samples can be obtained by direct excavations, augers and thick wall samplers.
For sampling saturated cohesion less soils, a trap valve or a spring sample retainer is inserted in the drive shoe (cutting edge).
The disturbed samples may be used for mechanical analysis, water content determination, index properties tests, and compaction and stabilization tests.
The split spoon samplers can be used for approximate determination, of unconfined compressive strength.
The samplers should be so transported and stored that the original composition is preserved and the water content also does not change, if desired.
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UNDISTURBED SAMPLING
Undisturbed samples may be required for tests on shear, consolidation, and permeability.
They can also be used for other tests like the disturbed samples.Undisturbed samples are obtained by forcing a thin wall sampler into the soil at the bottom of the bore hole or in a
test pit.
The penetration of the sampler into the soil should be continuous and rapid. The sampler should never be over driven so as to compress the sample.
A piston sampler may be used with advantage in soft soils, especially below water table.
Undisturbed samples of cohesionless soils, especially from below the water table are difficult to be obtained.
A compressed air sampler may be used. It enables the sample to be removed from the ground into an air chamber and then lifted to the ground surface without contact with
water of the drill hole.
A piston sampler with bore holes kept filled with drilling mud can also be used.
Alternative methods may be to impart cohesion to sand by asphaltic emulsions, or to freeze the sand at the sampling depth or near the lower end of the sampler 1.8 SAMPLING TECHNIQUES: 1.8.1 THIN TUBE SAMPLER &SPLIT SPOON SAMPLER: The objective of soil sampling is to obtain soils of satisfactory size with minimum disturbance for observations and laboratory tests. Soil samples are usually obtained by attaching an open-ended, thin-walled tube called a Shelby tube or, simply, a sampling tube—to drill rods and forcing it down into the soil. The tube is carefully withdrawn, hopefully with the soil inside it. Soil disturbances occur from several sources during sampling, such as friction between the soil and the sampling tube, the wall thickness of the sampling tube, the sharpness of the cutting edge, and the care and handling of the sample tube during transportation. To minimize friction, the sampling tube should be pushed instead of driven into the ground. Sampling tubes that are in common use have been designed to minimize sampling disturbances. One measure of the effects of sampler wall thickness is the recovery ratio defined as L/z, where L is the length of the sample and z is the distance that the sampler was pushed. Higher wall thickness leads to a greater UNIT-I
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recovery ratio and greater sampling disturbance.
Figure 1.6 (a) Thin - Walled tube (b) Split barrel sampler One common type of soil sampler is the ―Shelby tube,‖ which is a thin-walled, seamless steel tube of diameter 50 or 75 mm and length of 600–900 mm (Figure 3.4a). Another popular sampler is the ―standard‖ sampler, also known as the split spoon sampler (split barrel sampler), which has an inside diameter of 35 mm and an outside diameter of 51. The sampler has a split barrel that is held together using a screw-on driving shoe at the bottom end and a cap at the upper end. In some countries a steel liner is used inside the sampler, but in the United States it is standard practice not to use this liner. Consequently, the soil sample has a greater diameter. The results of SPT are different for lined and unlined samplers. The thicker wall of the standard sampler permits higher driving stresses than the Shelby tube, but does so at the expense of higher levels of soil disturbances. Split spoon samples are disturbed. They are used for visual examination and for classification tests. 1.8.2 Stationary piston sampler
It contains a piston or plug attached to a long piston rod extending up to the ground surface through the drill rod.
During lowering of the sampler through the hole, the lower end of the sampler is kept closed with the piston.
When the desired sampling elevation is reached, the piston rod is clamped, thereby keeping the piston stationary, and the sampler tube is advanced down into the soil.
The sampler is then lifted up, with piston rod in the clamped position.
The piston prevents the entry of water and soil into the tube, when it is being lowered, and then greatly helps to retain the sample during lifting, operations.
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Thus the sampler is more suitable for sampling soft soils and saturated sand.
Rotary samplers
These are the core barrel types (Earth Manual, 1960) having an outer tube provided with cutting teeth and a removable thin wall liner inside.
It is used for firm to hard cohesive soils and cemented soils.
Figure 1.7 shows a few typical samplers 1.9 PENETRATION TEST 1.9.1 STANDARD PENETRATION TEST (SPT) It is difficult to obtain undisturbed samples of coarse-grained soils for testing in the laboratory. Consequently, the allowable bearing capacity and settlement of footings on coarsegrained soils are often based on empirical methods using test data from field tests. One popular method utilizes results from the standard penetration test (SPT). It is customary to correct the N values for overburden pressure. Various correction factors have been suggested by a number of investigators. Two suggestions for correcting N values for overburden pressure are included in this text. These are
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Where cN is a correction factor for overburden pressures, and
is the effective overburden
pressure in kPa. A further correction factor is imposed on N values if the groundwater level is within a depth B below the base of the footing. The groundwater correction factor is
Where z is the depth to the groundwater table, Df is the footing depth, and B is the footing width. If the depth of the groundwater level is beyond B from the bottom of the footing base, cw = 1. The corrected N value is
The ultimate bearing capacity for a shallow footing under vertical loads is
Where B is the width in m. In practice, each value of N in a soil layer up to a depth 1.5 B below the footing base is corrected, and an average value of N1 is used in above Equation. Meyerhof (1965) proposed that no correction should be applied to N values for the effects of groundwater, as these are already incorporated in the measurement. Furthermore, he suggested that qultcalculated from its Equation using N1= cNN be increased by 50%. In using Equation, the settlement is assumed to be less than 25 mm. Burland and Burbidge (1985) did a statistical analysis of settlement records from 200 footings located in quartzitic sand and gravel. They proposed the following equation for the settlement of a footing in normally consolidated sand at the end of construction:
Where
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is a correction factor if the thickness (Ho) of the sand stratum below the footing base is less than an influence depth z1 a is the vertical stress applied by the footing or allowable bearing capacity (kPa), B and L are the width and length of the footing (m), respectively,
and N is the uncorrected N value. However, for very fine sand and silty sand, Burland and Burbidge recommended using a corrected in Equation. Further, if the soil is gravel or sandy gravel, use N‘=1.25N in Equation. The influence depth is the depth below the footing that will influence the settlement and bearing capacity. If N increases with depth or N is approximately constant, the influence depth is taken as z1 = B0.763. If N tends to decrease with depth, the influence depth is z1 = 2B. If the sand is over consolidated,
Burland and Burbidge also recommended a time factor to account for time-dependent settlement. You can check the original reference for this factor. 1.9.2 STANDARD CONE PENETRATION TEST (SCPT): Schmertmann (1970) and Schmertmann et al. (1978) proposed a methodology to determine settlement from the quasi-static cone test data for sands. They assumed that the sand is a linearly elastic material, and only stress changes within depths of 2B for ax symmetric conditions and 4B for plane strain conditions influence the settlement. Settlement is calculated by integrating the vertical strains; that is,
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The equation proposed for settlement (mm) by Schmertmann et al. is
Where
ax symmetric condition (plane strain condition L/B > 10), qnet is the net footing pressure in kPa (applied stress minus soil zo is the original vertical effective stress in kPa at the
depth of the footing, t is time in year (t > thickness of the ‗ith‘ layer, and ‗i‘ is the influence factor of the ‗ith‘ layer given as: Ax symmetric: L = B
Plane strain: L > 10B
The procedure to determine the settlement from cone data is as follows:
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1. Divide the soil below the footing into a number of sub layers. For square footings, the total depth of the sub layers is 2B and a reasonable number of sub layers are four. For strip footing, the total depth is 4B and a reasonable number of sub layers are eight. 2. Determine the average value of (qc)i for each sub layer from the field data of qc versus depth. 3. Find Ico at the center of each sub layer.
The bearing capacity from the CPT test is estimated by taking a weighted average of the cone resistance over a depth of 2B for ax symmetric condition and 4B for plane strain condition below the bottom of the footing base. 1.10 SELECTION OF FOUNDATION BASED ON SOIL CONDITION
For most of the fine grained soils (containing silt and clays) it might be sufficient to use simple spread footings, it is largely depending on the magnitude of the load. The location of the foundations in relation to the soil (need to be aware of foundation walls and
hydrostatic pressure as moisture is present in the soil).
If the soil is poor and structure loads are relatively heavy, then alternate methods are required.
Pile foundations might be required in some cases where fine cohesive silt and clay soil is present. (CH, OH).
Sometimes it might be desirable and economically feasible to over excavate remove such soils that are not of bearing capacity; can remove compact and fill back or import other
engineered soil.
The geotechnical engineer based on borings will recommend suitable foundations systems or alternative solutions, also beating capacity, minimum depths, and special
design or construction procedures might be established.
Safe bearing capacity of soil equals to the ultimate bearing capacity divided by a safety factor (usually 2-4). Ultimate bearing capacity is defined as the maximum unit pressure a
soil can sustain without permitting large amounts of settlements.
Bedrock has the highest safe bearing capacity.
Well graded gravel and sand that are confined and drained have a safe bearing capacity of 3,000 - 12,000 PSF.
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Silts and clays have lower safe bearing capacity of 1,000 – 4,000 PSF. Role of Foundations:
1. Transfer the building load to the ground. 2. Anchor building against wind and seismic load. 3. Isolate building from frost heaving. 4. Isolate building from expansive soils. 5. Holds building up from moisture. 6. Provide living spaces (basement, storage). 7. Houses mechanical systems.
Foundation configurations are: Slab on Grade, Crawl Space, and Basement.
Foundation Types: Spread Footings:
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Used for most buildings where the loads are light and / or there are strong shallow soils. At columns there are single spot square pads where bearing walls have an elongation form. These are almost always reinforced. These footing deliver the load directly to the supporting soils.
Area of spread footing is obtained by dividing the applied force by the soils safe bearing capacity (f=P/A). Generally suitable for low rise buildings (1-4 Stories).
Requires firm soil conditions that are capable of supporting the building on the area of the spread footings. When needed footings at columns can be connected together with grade beams to provide more lateral stability in earthquakes. These are most widely used because they are most economical.
Depth of footings should be below the top soil, and frost line, on compacted fill or firm native soil. Spread footings should be above the water table.
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Concrete spread footings are at least as thick as the width of the stem.
As the weight of the building increases in relation to the bearing capacity or depth of good bearing soil, the footing needs to expand in size or different systems need to be used.
Drilled Piers or Caissons:
For expansive soils with low to medium loads, or high loads with rock not too far down, drilled caissons (piers) and grade beams can be used. The caissons might be straight or belled out at bottom to spread the load.
The grade beam is designed to span across the piers and transfer the loads over to a column foundation. Caissons deliver the load to soil of stronger capacity which is located not too far down.
Piles:
For expansive soils or soils that are compressive with heavy loads where deep soils cannot take the building load and where soil of better capacity if found deep below. There are two types of piles.
1. Friction piles – used where there is no reasonable bearing stratum and they rely on resistance from skin of pile against the soil. 2. End bearingpiles – it transfer directly to soil of good bearing capacity.
The bearing capacity of the piles depends on the structural strength of the pile itself or the strength of the soil, whichever is less. Piles can be wood, steel, reinforced concrete, or cast in place concrete piles.
Cast in place piles are composed of hole drilled in earth and then filled with concrete, it is used for light loads on soft ground and where drilling will not cause collapse. Friction type, obtained from shaft perimeter and surrounding earth.
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Mat Foundations:
Reinforced concrete raft or mats can be used for small light load buildings on very weak or expansive soils such as clays. They are often post tensioned concrete.
They allow the building to float on or in the soil like a raft.
Can be used for buildings that are 10-20 stories tall where it provides resistance against overturning.
Can be used where soil requires such a large bearing area and the footing might be spread to the extent that it becomes more economical to pour one large slab (thick),
more economical – less forms.
It is used in lieu of driving piles because can be less expensive and less obtrusive (i.e. less impact on surrounding areas). Usually used over expansive clays, silts to let foundation settle without great differences. QUESTION BANK PART A
1.
What are components of total foundation settlement?
2.
What are the types of shear failure?
3.
What are assumptions in Terzaghi‘s bearing capacity theory?
4.
List out the methods of computing elastic settlements?
5.
What are the limitations of Terzaghi‘s analysis?
6.
Define ultimate bearing capacity?
7.
Define net ultimate bearing capacity?
8.
Define allowable bearing capacity?
9.
Write the expression for correction due to dilatancy submergence?
10.
What are the requirements for a stable foundation?
11.
What are the factors which foundation depend depth?
12.
Define net pressure intensity?
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13.
What are the zones used in the Terzaghi‘s bearing capacity analysis for dividing thefailure envelope of the soil?
14.
Define Shallow foundation.
15.
Define differential settlement
16.
What type of shear failure of soil is more likely to happen in the case of very dense soil?
17.
Write down the reduction factors for water table level to be applied in the ultimate bearing capacity equation.
18.
Draw the pressure distribution diagrams under a footing on cohesion less and cohesive soils.
19.
When will the Consolidation settlement get completed?
20.
For which type of foundation, Terzaghi‘s bearing capacity equation is applicable. Why?
PART B 1. Explain any two methods of site exploration in detail? 2. Explain wash boring method of soil exploration? 3. Explain the arrangements and operations of stationary piston sampler? 4. Explain about standard penetration test? 5. Explain any two important types of samplers 6. Explain with neat sketch auger boring method of soil exploration. 7. Explain dynamic cone penetration test. 8. Describe the salient features of a good sub-soil investigation report? 9.Describe the selection of foundation based on soil condition? 10. Briefly explain the following two methods with neat sketch: (i) Rotatory drilling (ii) Split spoon sampler
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