CIVIL ENGINEERING MATERIALS &

CONSTRUCTIONCourse Code: 15C203

TITLE: “SITE INVESTIGATION &

GROUND IMPROVEMENT TECHNIQUES”

By: G.GOWTHAM (15C206)

I year B.E., Civil EngineeringPSG College of Technology

Essence of presentation:

Site Investigation

Soil basicsPurpose of soil investigation

Preliminary Soil Investigation

Method of Site Exploration

IS Codes on Site

investigation

Ground Improvement Techniques

Need for Ground

Techniques

Safe Bearing Capacity of

Soil

Tests for Safe Bearing Capacity

of Soil

Improvement of the bearing

capacity

IS Codes for Tests

and Improvem

ent

SITE INVESTIGATION:

Soil – Basics:

To a Civil Engineer, soil is “loose unconsolidated inorganic material on the earth crust produced by the disintegration (or) weathering of rocks, on the Earth’s crust overlying the hard rock with or without the organic matter”

Rocks and soil constitute the main structural materials and thorough knowledge of it is essential for safety and economy in Foundation design and construction.

Hence site investigation is significant for the safe construction of a building.

Site Investigations:

Objectives:

The following are the objectives of Soil Investigation:

To know the nature, thickness and variation in the soil strata in the region.

To assess the physical properties of the soil strata. This will help in determining the design and mode of construction of the foundation.

To check the variation and the effects of Ground water table on the soil strata.

To know the values of strength and compressibility of soil bed.

To know the depth of underlying rock, soil bed.

Typical Soil Profile – Cross section of the soil

Preliminary Investigations:

The following are the preliminary investigations to be taken for the Soil Investigation:

Reconnaissance:

This involves the inspection of the Topography of the site.

Gives information about the soil and ground water conditions.

Aerial Reconnaissance is done for a major area.

This activity gives the basic idea of the soil in the site.

Study of Maps:

It gives the information about the sub – surface features of the site.

Maps may be acquired from SURVEY OF INDIA and GEOLOGICAL SURVEY of INDIA. These maps are often called as “Topo Sheets” – Topographical Maps.

Geological Investigation of the site to know the Seismic Reactions is also indispensable.

Maps are also used to know the presence of Ground water table in that particular site.

Aerial Photography:

These are the photographs that are obtained by flying more or less straight lines over the site and mapping. For conventional mapping the scale of 1:20,000 is used

It is one of the most advanced preliminary methods of soil investigations.

It requires thorough knowledge of Geology, geomorphology, Hydrology and Agriculture, etc.,

Geological Map Of India

Aerial Photograph of a site

Methods of Soil Exploration:

Direct Method:

Open Test Pits (or) Trial Pits:

• They are the open type of accessible explanatory methods.

• These can be used for testing the soil strength and applicable to any type of soil.

• It is also useful for field test like Plate Load Test (for determining the Safe Bearing Capacity (SBC) of the soil)

• As the cost increases with depth, it is limited only for depth of 3 metres.

• For greater depths, for pervious soils, lateral supports or bracing should be done that makes the process slow and expensive.

• Generally for greater depths, bore holes are used.

• Also at greater depths, the ground water table may also intervene in the process and it may have to be lowered which is an additional expenditure.

Indirect Methods:

Sub – Surface soundings:

• This technique is used for investigation of the strata with varying soil nature.

• This is used to determine the consistency of cohesive soils and density index of cohesion less soils at different depths.

• The apparatus used for this is called as “Penetrometers”.

• The field test undertaken for this purpose is called as “Penetration Test”.

Penetrometer

Methods in Sub – surface Sounding

Standard Penetration Test:

Its is also known as Dynamic Method of soil sounding.

In this no: of blows required for a 300 mm of penetration gives a measure of the penetration resistance.

This test consists of a split spoon sampler in to the soil through a bore hole of 55 to 150 mm diameter to the desired depth.

A hammer of 450 N (65 kg) with a free fall of 750 mm is used to drive the sampler. The number of blows for a penetration of 300 mm is designated as “Standard Penetration Value” denoted by “N”.

Penetration Resistance: This is nothing but the resistance offered by the

soil strata for penetration of the cone present in the Penetrometer. The

resistance offered by the cone to penetrate gives a clear idea on the

Relative density of the soil.

The test procedure is standardised by the Indian Standard “ IS : 2131 – 1972”

In case of fine sand or silt, the value of ‘N’ is found to be greater. So Terzaghiand Peck recommended the following correction

𝑁 = 15 +1

2𝑁′ − 15

Here

• N’ = observed SPT value

• N = corrected SPT value

The following table shows the relationship of the density of the soil with N value‘N’ VALUE CONDITIONS OF

SOIL

0 – 4 Very Loose

4 – 10 Loose

10 – 30 Medium

30 – 50 Dense

Greater than 50 Very Dense

Probing (or) Rod driving (or) Rod Test• This is method of soil investigation is adopted where the soil is in soft nature such

as clay, sand, gravel, etc.,

• Probing is done by

Iron rod

Crow bar

Screw Rod

Hollow tube of steel

• IRON ROD: Diameter – 30 to 35 mm. It is driven in to the ground and withdrawn frequently to examine the material struck at the pointed end.

• CROW BAR & SCREW ROD: It is used similar to the iron rod.

• HOLLOW STEEL TUBE: Diameter – 35 to 50 mm having a split of 3 mm thickness at the bottom with the length of 600 mm is driven in to the ground at the time intervals of 300 mm at a time. It is also withdrawn frequently.

• The limitation of this is that it can be used for only up to 3 m depth.

Semi – Direct method:

Boring:

The following 4 types borings are adopted in practice:

Tube Boring

Auger Boring

Wash Borings

Mechanized drilling

These are used for deep explorations in the soil.

It is also used to determine the various ground water pressure due to flood conditions or draughts.

Tube Boring:

• It is used only for small jobs.

• In this method a tube of diameter 50 to 100 mm and having a length of 1.5 to 2 m.

• This method is used only to know the nature of the sub – surface .

• It does not give any clear indication of the structure of the soil layer.

Auger Borings:

Augers are used to drill bore holes in cohesive and other soft soils.

Augers may be hand – operated or power driven

Hand – operated augers are used to drill bore hole up to 6 m of depth.

Power driven augers (Mechanical Augers) used for greater depths and even in gravelly soil also.

Auger boring is the most simple and efficient way of soil exploration.

It is also applicable for sandy and silty soil.

It is a quick method that it gives fairly representative samples and correct idea of different soil layers.

Auger is a tool used for drilling a bore hole in to the ground.

Types of Augers used:

Post Hole Auger: - General purpose

Helical Auger: - Stiff clays

Shell Auger: - Very stiff and hard clays, for great depths and short duration

boring

POST HOLE AUGER

HELICAL AUGER

Wash Boring:

• Wash boring is simple method for sub – soil explorations.

• This method can be used for any types of soil except gravels and boulders.

• In this method a casing of steel of 1.5 m length is driven inside the soil.

• Then water is forced under pressure by using an electric motor.

• This pressurized water is used to drill the soil.

• The casing has to be washed repeatedly when it has advanced another 1.5 m approximately.

• This method can be used for depths up to 35 – 45 m.

• The change in the soil strata can be identified by the change in the progressing rate and the change in the colour of the wash water.

Mechanised Drilling:

This method is very useful for very deep boring especially in case of boulders and bed rock is encountered.

In this method also the bore hole is drilled in a similar way to the wash boring.

During the time of boring, if any hard rock is encountered, the boring is further done for small distance to eliminate any possibility of mistaking a boulder or a thin rock layer.

Types of Rock drilling:

1. Churn (or) Percussion Drilling

2. ‘Diamond’ drilling

3. Shot (or) Saw – tooth cutting drilling

Geophysical Methods: (Indirect Method)

The Geophysical methods help in determining the rapid changes in the soil strata.

Hence this method can be used where the speed of investigation is of prime importance.

The only limitation in this technique is that the interpretation of the results requires special knowledge and skill.

This method is based on the following principle:

“ The properties such as Elasticity / Seismicity, Electric, Radioactive,

Gravitational, Magnetic, etc., properties vary for different types of soil”

Seismic Refractive Method

• Principle: “ Sound waves travel faster in a solid rock than in fissured rock or in other soil strata

Electrical Resistivity Method

• Principle: “ The electrical resistance is different for different for different soils”

Seismic Refractive Method:

This based on the velocity changes in the sound when it encounters a different medium.

In this method the vibrations are created by striking a plate with a hammer (or) dynamite may be used.

This vibration passes in to the ground and reaches the “Geophones” –transducer, small elctromechanical device used to detect these vibrations and convert them to measurable electrical signals”

The sound gets refracted on encountering a different soil surface and this is acquired by the geophones.

From the graph the Critical distance 𝑑𝑐, and the velocities be 𝑉1𝑎𝑛𝑑 𝑉2 for the upper soft and the lower hard layer respectively. Then the thickness of the stratum 𝐻1 of the upper layer can be calculated as follows:

𝐻1 =𝑑𝑐2

(𝑉2 − 𝑉1𝑉2 + 𝑉1

Electrical Resistivity Method:

In this the resistance offered by the sub – surface layer is measured.

The electrical resistance is different for different soil layers. E.g., the clay has lower resistance than rock.

By measuring the electrical resistance values the type and the nature of the rock is determined.

This method is also used for ground water explorations. It is based on the fact that water conducts electricity and offers low resistance.

So the presence of low resistance in an area may indicate the presence of ground water table.

In this method, two current electrodes are placed at a distance say ‘d’ between them. These electrodes are connected to a battery of desired power output along with a ammeter to measure the current.

Two potential electrodes are also placed between the 2 current electrodes at d/3 distance between them. They are connected to a voltmeter or a potentiometer to measure the voltage.

Once the current (I) and voltage (V) are known, then

𝜌 = 2𝜋𝐷𝐸

𝐼Here

ρ = mean resistivity in ohm/m

D = distance between the individual electrodes

E = potential drop between the potential electrodes

I = current flowing through the outer electrodes

Vane Test:

• The principle of this test is that the “ the TORQUE values will give the idea of the different strata of the soil”

• This is carried out on cohesive soils with soft to medium consistency, such as soft clays, silt, etc.,

• In this method the vane is pushed in to the soil and the upper calibrated circular scale is rotated till it reaches the desired torque value.

• When the vane is inserted in the soil for the first time – strength of soil in undisturbed state.

• After giving necessary revolutions, the soil gets remoulded and the strength corresponds to the strength of the soil in the remoulded or disturbed state.

• The torque measured is the index for the shear strength of the soil.

INDIAN STANDARDS:

Standard Penetration Test – “ IS : 2131 – 1972”

Dynamic Cone Penetration Test – “ IS : 4968 (Part – I) – 1976” - Method for Subsurface Sounding for soils.

Static Cone Penetration Test – “ IS : 4968 (Part – III) – 1976” – Method for Subsurface sounding for soils.

Spacing for Borings: “ IS : 1892 – 1979” – Code of practice for subsurface investigation for foundation.

Wave velocities of soils – “ IS : 1982 – 1979 – Appendix B”

Electrical Resistivity of soils – “ IS : 1982 – 1979 – Appendix B”

Vane test – “ IS : 4434 – 1979”

GROUND IMPROVEMENT TECHNIQUES:

Introduction:

The testing of soil is important for any construction. But when the soil is not fit for construction, then the improvement of that site is much more important. So the factor that governs the strength of the soil is the SAFE BEARING CAPACITY.

Bearing Capacity:

It is defined as the load carrying capacity of the foundation bed that enables it to bear the loads transmitted to it from the structure. It is generally expressed in terms of loads per unit area.

Safe Bearing Capacity – (SBC):

It is defined as the maximum load per unit area that the foundation bed can withstand without the risk of shear failure.

Factors affecting bearing capacity:

Nature of soil, and its physical and engineering properties.

Nature of the foundation to be laid.

Total and differential settlements that the structure can withstand without functional failure.

Location of the ground water table relative to the level of the foundation.

Initial stress if any

Ultimate Bearing Capacity: It is defined as the maximum load per unit area

which the foundation bed can withstand without shear failure of the foundation

bed. It is also defined as the minimum load per unit area at the base of the

foundation at which the foundation bed fails in shear.

Methods of determining bearing capacity

Through local knowledge and experience

Soil sampling, examination and classification

Field test for Direct determination of Bearing Capacity

Plate loading test

Standard Penetration Test (SPT) and Vane Test

Soil resistance test

Laboratory tests

Plate Loading Test:

Working Principle:

In this a loading platform consisting of a bearing plate of steel or cast iron or composite material made of wooden sleepers and steel joists (beams), is subjected to gradual increment of load and the corresponding settlement values are noted. Finally SBC is calculated by

𝑆𝐵𝐶 =𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝐵𝑒𝑎𝑟𝑖𝑛𝑔 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦

𝐹𝑎𝑐𝑡𝑜𝑟 𝑜𝑓 𝑠𝑎𝑓𝑒𝑡𝑦

Procedure:

In this a trial pit with width at least 5 times the width of the test plate is made and the load is applied.

The test plate is made of Mild Steel (M.S) of 2.5 cm thickness is used.

The thickness of this M.S plate varies for different soils as follows:

This can be done in 2 ways:

Gravity Loading

Loading by trusses

Initially the platform is preloaded with a load of 70 gm/ square cm and increased by 200 kg or 1/5th of the approximate ultimate bearing capacity till about 1.5 times the expected ultimate bearing capacity is reached.

The settlements are recorded with 0.02 mm accuracy by using 2 dial gauges. The settlement of the soil should be observed at equal intervals of time. For clayey soils the ‘time – settlement’ graph is drawn.

SOIL TYPE AREA / SIZE OF M.S PLATE

Clayey, sandy and silty soil 60 cm square

Gravely and Dense sandy soil 30 cm square (min)

Presumptive bearing capacity values as per

IS 1904-1978.

Type of soil/rock Safe/allowable bearing

capacity (KN/ m2)

• Rock 3240

• Soft rock 440

• Coarse sand 440

• Medium sand 245

• Fine sand 440

• Soft shell / stiff clay 100

• Soft clay 100

• Very soft clay 50

Remarks:

• This method is very useful in obtaining the necessary information about the soil.

• The use of area under the plate should be large enough.

• The loading should be applied gradually at the centre.

• The test should be continued for the longest possible time.

• Zero Correction: This is the settlement in the soil strata even at zero loading. This is due to the adjustments of the soil particle on application of a load. This is known as Zero Correction. It is obtained from the load –settlement curve.

Ground Improving Methods:

By increasing the depth of the Foundation:

• Bearing capacity of soils – increases with increase in depth.

• So deep the foundation, more is the bearing capacity, more is the stability.

• The only limitation is that on increasing the depth, the weight and the cost of the structure increases.

By Draining the soils:

• The SBC of the soil increases with decrease in its moisture content.

• So, draining the soil will be an effective way of increasing the SBC of the soil

• Soils can be drained by using bore holes, porous pipes at gentle slopes, trenches, laying a bed of loose boulders etc.,

By compaction of the soil:

• Soil compaction increases the soil density and the strength. Thereby it increases the SBC of the soil.

• This can be done in 2 ways

Hand picking of rubble boulders (or) spreading broken gravel or sand & then ramming the bed.

By driving the piles, removing them and filling those holes by sand (or) concrete.

By increasing the width of the foundation:

As the width increases the bearing area also increases, hence the intensity of the load (or) the average load applied to a single point on the foot of the

foundation gets decreased.

The width of the foundation cannot be increased forever.

By replacing the poor soils:

In this method the loose soil particles are removed and then filled with superior materials like sand, gravel, rubble, concrete, etc.,

Finally the filled surface is rammed.

This method is very useful for areas of Black Cotton Soil

By this method, SBC is increased twice the original SBC.

By hardening the soils by Grouting:

In this method poor soil is hardened by injecting concrete under pressure and this process is called as Grouting.

By this process, the fissures, pores, cracks, unconformity (if any) present in the soil strata can be sealed and hardened.

By solidifying the soil using Chemicals:

Chemicals like silicates of soda and calcium chloride are injected to form a compacted solid mass with the soil particle. This process is called Chemical

stabilization. Though chemicals are added only in traces, the overall cost is high. So it is used in exceptional cases only/

INDIAN STANDARDS:

“ IS : 1888 – 1982” – Method of load test on soils.

“ IS : 6403 – 1981” – Code of practice for determination of the bearing capacity of shallow foundation.

“ IS : 1904 – 1978” – Code of practice for Structural safety of Building Foundation

“ IS : 8009 – (Part – I) – 1976” – Code of practice for calculations of settlement of shallow foundations subjected to Symmetrical Static Vertical

Loading.

References:

“ Building Materials and Construction” – By S.P.ARORA & S.P.BINDRA

“ Soil Mechanics and Foundation Engineering” – By Dr.P.N.MODI

www.google.co.in

www.wikipedia.com

Images:

www.constructioncivil.org

www.google.co.in

Maps:

www.mapsofindia.com