1.1 Introduction to soil exploration | unit 1 introduction to soil exploration

Unit-1

Introduction to soil exploration

1.1 Introduction to soil exploration

Soil exploration consists of determining the profile of the natural soil deposit at the site, taking the soil sample, and determining the engineering properties of soils using laboratory tests as well as in situ testing methods. Although info on the soil exposed at the bottom surface is incredibly valuable, geotechnical engineers additionally ought to judge the sub-surface conditions by taking samples by boring or by excavation beta pits. These activities are referred to as submerged exploration.

The extent of exploration depends on the importance of the structure, the complexness of the soil conditions, and also the budget obtainable for exploration. A detailed soil exploration program involves deep boring, field tests, and laboratory tests for determination of various properties of soils needed for the look of any structure

Soil investigation or subsoil exploration is done by obtaining the information about subsurface conditions at the site proposed for construction. soil exploration consists of determining the profile of natural soil deposit, soil sample taken, and determining the engineering properties.

Before deciding on any foundation, it is essential to obtain reliable and sufficient data about the strata, where the foundation will be located If a site is to be chosen from a large area, or alternative areas, such as the selection of a site for a dam. Bridge, tunnels, highways, etc. it is necessary that proper soil exploration is carried out.

Such exploration will enable the designer to provide viable and economical design. Although the cost of exploration is usually less than 1 to 2% of the project cost in most cases, exploration is not given due attention, which may prove to be uneconomical and disastrous in long run.

The safe and economical style of the foundation of any structure, supported on soil, needs the data of the physical and engineering properties of the soil up to a depth of 1. 5-2 times the dimension of the inspiration or structure below the bottom of the foundation.

For massive and necessary structures, appreciate multi-storied buildings, gas or oil storage tanks, and others, the choice of the foundation relies on the soil properties at the positioning up to a big depth below ground level.

For structures that need excavation below ground level, such as waste material treatment plants, underground reservoirs, or structures made in coastal areas, construction is probably going to be halted for many weeks or months if the inspiration trenches collapse, delaying the project and project costs.

For structures set in tough soil conditions, appreciate expansive soils or soft marine clays, data of soil properties up to a big depth is needed to style appropriate ground improvement techniques to make sure the long- term stability of such structures.

The design of earth-supporting structures, such as retentive walls, sheet piles, abutments, and piers of bridges, additionally needs knowledge of soil properties of the underlying soil for estimation of lateral earth pressure for finishing up design and stability analysis of such structures.

The choice of appropriate borrow areas for construction of earth structures appreciates earth dams and embankments for roads, railways, or canals, needs a determination of soil properties to settle on the foremost suitable and economical borrow area.

The field and laboratory investigations and testing are conducted to work out the physical and engineering properties of the soil likewise in the study the positioning conditions are referred to as soil exploration or geotechnical investigation. Though the name refers to soil exploration, its scope also includes the study of rock, groundwater conditions, and the other material encountered within the course of soil exploration.

The want for a correct soil exploration is highlighted by Karl Terazagisn in one of his notable quotes: Any decision to stop the settlement while not creating the planned preliminary investigation would be Associate in Nursing feckless gamble. Since I have witnessed several gamblers of this kind, I will state from personal expertise that the savings related to inadequate preliminary investigations are entirely out of proportion to the monetary risks. ”

SOIL EXPLORATION AND SITE INVESTIGATION:

The basic aim of sub-surface exploration or site investigation is to get the knowledge concerning the surface conditions at the positioning of planned construction.

For each massive or major engineering project site, investigations are essential, since it's useful for the design. of structures and for designing construction techniques

Purpose of the soil exploration program

Site investigation for one or a lot of the following purpose:

Determination of the bearing capability of the soil.

choice of sort and depth of foundation for a given structure

Investigation of the protection of the present structures and for necessary remedial measures

choice of appropriate construction technique

Calculation of the lateral earth pressure against retaining wall.

Prediction and solving of potential foundation problems

Establishing the groundwater table level and determination of the properties of water.

Determining the suitability of the soil as a construction material

Factor affecting the cost of soil investigation:

1. Non-uniformity of layers

2. Undisturbed sampling

3. Depth of exploration

4. Nature of the project

Non-uniformity of layers

Erratic deposits would like a lot of exploration price than homogenous deposits.

Undisturbed sampling

Need serious expenditure

Depth of explorations

The cost will increase with increases in comprehensive exploration.

Nature of the project

For stuff dam, the development cost is low however has a high total cost of exploration, and holding walls have a high cost of construction and low exploration cost vary is around zero. 1 to 2% of calculable cost betting on importance of project and rate of subsoil.

For lesser jobs, heavy expenditure on careful investigations isn't necessary.

For giant projects, extensive investigations are justified.

For advanced and erratic soil profile, heavy expenditure on investigation hardly improves the quality of information

For a regular uniform profile, even a small price of exploration might yield a top quality of information.

STEPS IN SOIL EXPLORATION

Soil exploration involves broadly the subsequent

1. Planning of a program for soil exploration

2. Collection of saturated and undisturbed soil or neck samples from the hales dilled within the field the amount and died of boles depend on the project

3 Conducting all the required in-situ for large the strength and compatibility characteristics of the soil or rock directly or indirectly

4. Study of groundwater conditions and action of water samples for analysis

The bottom investigation, regardless of the magnitude of the project, consists of 4 phases

Four phases of ground Investigation

1. Available information

2. Reconnaissance

3. Preliminary inventory

4. Detailed investigation

1. Available information

This is the first innovative collection of published Tropical and topographical information of the world hydrological data, details of existing or luster development, local regulations for construction activity S, are made

2. Reconnaissance

This within the phone during which the first examination of the world is made by the engineer together with other specialized, such the geologist, land surveyor, geotechnical engineer, etc.

At this stage, a radical study of the present structures for the sort of construction and defects such a hi and settlement, soil profiles in highway or railroad cuts and quarries, version in existing cuts, high watermarks on bridge abutments, outcrop, history cal food and scour levels from the local people are collected.

3. Preliminary investigation

This is a vital phase of the complete program during this stage the engineering plant the investigation program. The primary step towards a ground investigation could be a thorough understanding of the geology of the location, which cables an efficient understanding of the investigation program.

The second step is to get additional details of the under soil strata (e. g. thickness of individual strata) from one or 2 explorative drill holes. All alternative more steps rely upon the magnitude of the work and therefore the character of the soil profile.

During this stage, the attainable location of the bottom water is additionally found. For tiny jobs, the skinny preliminary investigation itself is also sufficient. It istypical to apply to limit the number of quality samples recovered except the one obtained from penetration tests.

The strength and settlement are calculable from standard correlations mistreatment index properties and supplemented by the results from samples obtained from penetration tests. Further, will be} the stage that much decide the flexibleness of the project.

4. Detailed investigation

Additional borings and deep borings are planned from the information obtained from the preliminary borings and deep borings. If the under soil is uniform in stratification, an orderly spacing is also planned.

Many times, extra borings are created to find weak soil or rock zones, outcrops and so on which can influence the planning and construction of the project. Drilling is needed to bore a hole into exhausting strata boulders. It can be done by percussion or rotary methodology of drilling.

Necessary unchanged tests ought to also be having sufficient samples ought to be procured to get relevant parameters for design and construction. Such extra samples should be recorded to redefine the planning or construction procedure.

Key Takeaways

Soil exploration -

Introduction

Purpose

Factor affecting

Steps of soil exploration

Bottom investigation- available investigation, reconnaissance, preliminary inventory, detailed investigation

1.2 Methods of boring and drilling

Following methods are used to obtained soil samples to identify the type of start and to send the same for testing.

Methods -

1 trial pits

2 Auger boring

3 wash boring

4. Rotary drilling, core drilling

5. Percussion drilling

1. Trial pits and trenches

This is one of the foremost dependable and informative methodologies of soil exploration however it's restricted to a depth of four to five m solely.

Trial pits are appropriate for all sorts of soils and permit a most careful visual examination of the soil formation for the whole depth. Another advantage of this methodology lies in obtaining comparatively undisturbed soil sample, from the walls or bottom of the pit, by pushing a skinny walled steel tube within the soil strata, Deeper pits ought to be supported by cloth and bracing or by cribbing to forestall collapse

Ventilation of deep check pits is important to forestall accumulation of interruption. This is often done by providing pipes beginning slightly on top of the ground and increasing about one meter on top of the highest of the pit

Fig no. 1 Trial pit and trenches

Special precautions got to be exercised if the presence of objectionable gases is anticipated (IS: 3764, 1966). A dewatering system must be used if pits square measure to increase below the geological formation.

TRENCHES

This square measure the same as check pits. They supply a protracted continuous exposure of the surface of the bottom on a desired line or section. They are best suited to exploration on slopes. Necessary safety precautions need to be taken as in deep check pits.

2. Auger boring

It is quite helpful in cohesive and alternative soft soils higher than groundwater level.

Hand-operated auger strategies are appropriate for distinguishing varied sorts of soils with depth and additionally for obtaining data concerning the depth to the bottom groundwater level for deeper boring. solid or hollow-stem, continuous-flight augers (rotary augers) are oftentimes used,

As the drill advances, further auger flights are other and the soil is dropped at the surface in an exceedingly disturbing type.

Fig no. 2 Auger drilling

In auger boring, it's attainable to spot even disturbed soils. Since the borehole is unbroken dry, it's significantly suitable for advancing borings on top of groundwater level to get undisturbed partly saturated samples. It additionally facilitates the determination of the free water level.

Auger will be operated automatically or manually Hand augers used for depths upto half a dozen m and automatically operated augers square measure used for larger depth and that they will be utilized in grave soil.

The hand augers utilized in boring square measure regarding fifteen to twenty cm In diameter. The lower finish of hand anger is attached to a pipe of eighteen mm diameter

For taking samples from the pronto driven hole, an auger referred to as 'Post-Hole Auger is employed.

Mechanical augers square measure driven by power. If the depth of the borehole will increase above twelve m, then mechanical augers become inconvenient, and alternative boring ways square measure used

Auger boring becomes trouble-some, once there square measure massive boulders or cobbles square measure met. The location investigation is completed quite a space and economically byauger boring

3. Wash boring

Wash boring is usually used for the boring in troublesome soil. to start with the outlet is advanced to a brief depth by auger then a casing pipe is pushed to stop the edges from caving in.

The hole is then continued by the utilization d a chopping bit fused at the top of a string of hollow drill steel.

A beam of water struggling forced that the rod and therefore the bit into the opening, that loosen the soil because the water flows up around the pipe.

The disentangled sail in suspension in water is discharged into a bath. The soil in pension settles down within the tub and therefore the clean water flows into s sump that is reused for circulation. The mobility for wash boring is either mechanical or man farmer

Builder's Engineer: BORING METHODS - SITE EXPLORATION.

Fig no. 3 Wash boring

The soil sample obtained by this methodology is understood as wash sample accustomed describe profile roughly by modification in the color of soil suspension. Approximate size gradation is determined by this sample

Mainly wash boring is employed for drilling holes within the ground. Once the hole is trained, a sampler is inserted to get soil samples for laboratory testing.

Advantages

1. Instrument used is relatively light-weight in weight.

2. Cheap technique

3. Quick and easy technique.

Disadvantages

1. it's slow in stiff and coarse-grained soils.

2. Can't be employed in rocky strata.

3. Sensible quality undisturbed samples can't be obtained

4. Not appropriate in areas wherever the H2O table is incrediblynear to the ground

4. Rotary drilling:

Rotary boring or rotary drilling could be an in no time technique of advancing hole within the rocks and soils. During this technique, the hole is trained by rotating hollow drill steel that features a cutting bit at its lower finish. At the highest of drill steel, a drill head is provided. It contains a rotary mechanism and a briefing for applying pressure

When the drilling rod is revealed, the cutting bit shears off chips of the fabric penetrated. A lubricator is introduced struggling through the drilling rod to an all-time low of the opening

The cuttings of the fabric penetrated square measure carried to the bottom by the fluid through the annular house between the drilling rod and also the walls of the opening. Another performance of fluid is to cool down the bit. The lubricator conjointly supports the walls of the opening, just in case of the un-cased hole.

CIRCULATION MUD DRILLING systems - Solids Control Shale Shaker

Fig no. 4 Rotary drilling

The drilling bit is replaced by a sampler when a soil sample is required.

Drilling may be done by a diamond-studded bit or a cutting edge having a chilled shot. diamond drilling is costlier but is superior to the other drilling type.

Advantages:

1 Rotary drilling can be used in clay, sand, rocks.

2 boreholes of diameter 50 mm to 200mm can be easily drilled by this method.

Disadvantages:

If soil is containing a large percentage of practical gravel size and larger this method is not well adapted because the particles of this size start rotating below the drill rod and it becomes difficult to drill the hole.

5. PERCUSSION DRILLING:

Percussion drilling is another methodology of drilling a hole within which an important bit is alternately raised and born in such a way that it powders the underlying materials and forms into suspension in water.

This suspension is removed out of the outlet by suggests that of bailers or sump pumps.

In all styles of drilling the perimeters of the holes could also be stable, if needed, by the employment of lubricator or casing pipes. A lubricator is nothing however clay mixed in water.

DEFINITION The machinery accustomed to advanced Holes and cake sample is named a drilling rig

Percussion drilling methodology is specially used for creating holes in rocks, boulders, and different laborious strata. Percussion drilling consists of lifting and dropping a very serious cut in a vertical hole. The fabric gets fine.

Percussion - an overview | ScienceDirect Topics

Fig no. 5 Percussion drilling

Water is added to the outlet if the chisel strikes on top of the groundwater level. The water forms a suspension of a disintegrated material that is removed by a sand pump or a boiler at intervals.

The casing could also be needed for this methodology. For drilling driven well additionally percussion drilling is employed.

Advantages

1. Percussion drilling is employed in every type of soil.

2. it's noticeably helpful for boring holes like that for well through rock or boulders.

Disadvantages

1. Due to significant blows of the chisel, the fabric at very cheap is disturbed.

2. Costlier as compared to alternative ways

Tough to sight minor changes within the properties of the strata penetrated.

Key takeaway

Methods of obtaining soil sample –

trial pits

augur boring

rotary drilling

wash boring

percussion drilling

1.3 Soil sampling and sampler

Type of samplers

1. Open Drive Sampler

2. Stationary Piston Sampler

3. Split Barrel or Split Spoon Sampler

4. Rotary Sampler

5. Disturbed and Undisturbed Sampling

6. Block or Chunk Samples

1. Open drive sampler

Open drive sampler consists of a steel tube with thread at every finish. The cutting shoe is hooked up to at least one finish of the tube.

The other finish of tube screws into a sampler head to that successively the boring rods are connected

The sampler head conjointly incorporates a non-return valve to permit air and water to flee because the soil fills the tube and to assist retain the sample because the tube is withdrawn.

The open drive sampler may be driven or jacked. when withdrawal the cutting shoe finish sampler head is detached and also the ends of the sample are sealed and it's then sent to a laboratory

The sampler is suited to clayey soil. Once accustomed acquire samples of sand a brief length of tube with spring-loaded flaps ought to be fitted between the tube and cutting shoe to stop loss of soil. Thanks to cutting shoe provided at heart Arc quantitative relation is higher (30%) than the skinny walled sampler

Igeotest

Fig no. 6 Open drive sampler

2. Stationary piston sampler:

Here we have a thin-walled tube fitted with a piston the piston is connected to a rod passing through the sampler head and run inside a hollow boring rod.

The sampler is first lowered into the borehole, the piston being located at the lower end of the tube. The tube and piston could be locked together by a clamping device which is provided at the top of rods. Piston thus prevents water or loose soil from entering the tube.

Now the piston is held against the soil by clamping the piston rod to the casing and the tube is pushed past the piston until the sampler head meets the top of the piston to obtain a sample.

The sampler is then withdrawn holding the piston at the top of the tube as this takes place. The vacuum between piston and sample helps to retain the soil in the tube. Piston samplers are always pushed by mechanical or hydraulic jacking and they are not driven. This sampler is generally used for soft clays.

25.5 Stationary piston sampler | bs5930-2015

Fig no. 7 Stationary piston sampler

3. Split barrel or Split Spoon Sampler

This sampler is used in a standard penetration test tube which splits longitudinally into two halves is used here there is a considerable disturbance in the sampler and useful in the sandy type of soils. The outside and inside diameter of the tube is 51 mm and 35 mm. Area ratio is 112%,Output

Diagram of a typical split-spoon sampler used for a standard... | Download Scientific Diagram

Fig no. 8 Split barrel

4 Rotary samplers:

The rotary sampler may be a double-walled tube sample with an associate inner removable liner, the outer tube is given a cutting bit. The bit cuts associate a rounded ring once the barrel is rotated.

The stationary tube slides over the cylindrical sample cut by the outer rotating barrel the standard of the sample obtained with a double-walled tube sampler is healthier than that obtained with a single tube sampler.

Rotary samplers square measure helpful for sampling in the firm to exhausting cohesive soils and notably in rocks. The rock quality may be calculable from the core recovery magnitude relation termed as rock quality designation, R Q D.

The magnitude relation of the whole length of core recovered to the length of sampler advanced on a given run, expressed as p. c, is that the price of R alphabetic character D. Whereas determinant the length of core recovered, solely those piece of the core which are at least 100 mm long and sound are considered breaks caused by drilling should be ignored.

The diameter of the core should preferably be not less than 54 mm. from RQD it is also possible to access the In-situ modulus of elasticity and comprehensive strength of the rock mass based on the corresponding valves obtained by laboratory tests on rock cores.

Quiz] Rotary Drilling - OILMAN Magazine

Fig no. 9 Rotary sample

The inside and outside diameter are 39 mm 51mm respectively and the area ratio 69%. The sample obtained is disturbed. the sampler is disturbed. is under heavy stress.

5. Disturbed and undisturbed sampling

Testing of soil samples within the laboratory should replicate, as shut as potential, the condition of the soil is that the field alternatively the prediction supported properties measured within the laboratory are completely different from the performance discovered within the field.

Samples obtained a lot of or less within the same conditions as within the field are acceptable as undisturbed samples for laboratory testing. For instance, chunk sample Engineering properties are determined on undisturbed samples,

However, a soil sample was obtained from the sector. Essentially involves some disturbance throughout sampling transport, extraction, and handling.

This disturbance could also be within the sort of amendment in water content, soil structure, stress condition, density, or grain size distribution such sample is known as disturbed samples which may be used for determination of properties like grain size water content, relative density, density index, consistency limits.

6. Block or Chunk Sample

Block samples are obtained from open excavations like check pits, shafts, etc. from the soil that contains a trace of cohesion

During excavation, a block of soil concerning forty cm x forty cm in arranging is left undisturbed. associate degree undisturbed of concerning thirty cm x thirty cm x thirty cm or any convenient size is sometimes cut with a knife is then slide over the cut block.

The house between the facet of the box and also the dry sand. the tip of the box is sealed with paraffin wax These samples if transported long-distance might get disturbed throughout transit

They can be used for any laboratory seed coat by careful trimming. However, these samples can't be obtained below water level

Undisturbed samples may additionally be obtained by means that of a sampling bumpkin of one-hundred-millimeter diameter with a pointy leading edge.

The soil close to the surface of the tube ought to be fastidiously removed whereas the tube is being pushed in

DEPTH OF EXPLORATION AND EXTEND

Depth of Exploration

Exploration, in general, ought to be applied to a depth up to that the rise in pressure thanks to structural loading is probably going to cause perceptible settlement or shear failure.

Such a depth, called the significant depth, Depends upon the sort of structure, its weight, size, form, and disposition of the loaded areas, and therefore the profile and its properties

It' is mostly safe to assume the many depths up to grade at that internet increase in vertical pressure becomes but 100 percent of the initial overburden pressure

Alternatively, a pressure bulb delimited by associate degree isobar of a common fraction or common fraction of the surface loading intensity is typically assumed to outline the minimum depth of exploration

This depth could also be assumed to be adequate to one and 0. 5 to 2 times the dimension (smaller lateral dimension of the loaded space.

The depth of exploration at the beginning of the work could also bedecided in step with the subsequent guideline given in (IS 1892-1979).

Extend of exploration

Some disposition of pits and boring holes ought to be like to reveal any major changes in thickness, depth, or properties of strata and immediate surroundings.

If ground conditions area unit uniform, min. three holes not in a very line area unit enough to allow a reliable profile.

For the building project, four holes at comers and one at the centre of the space area unit were counseled.

For large space, one hole per 250 sq. m. or holes spaced at 15-20 m could also be adequate or alternative sounding tests at each one hundred m.

For smaller, minor buildings one borehole or check pit in the centre could answer.

For dam sites preliminary borings at fifty m spacing on the highest line of the upstream face of the dam and across one or each abutment. Few wide scattered holes also are needed in the bottom of the reservoir on the upstream aspect of the dam.

In case of road sites: pits or boreholes on the planned centre line of road on ditch lines, alternating right and left inches one hundred m at t will increase to five hundred m for uniform soil or belittled to thirty m or less for erratic deposits (frequent changes in profile).

In erratic deposits, an increase in the variety of holes doesn't essentially improve the quality of data. In such cases,the less costly technique of exploration-like-sounding ought to be used.

FACTOR AFFECTING DISTURBANCE TO SOIL SAMPLE

1. Area ratio

2. Inside and outside clearance

3. Cutting edge angle

4. The effect of area ratio on sampling tube

5. Sampling speed

6. The material used for sampling tube

7. Roundness

8. Recovery ratio (Fr)

9. ROD, Rock Quality Designation

Key takeaways

TYPES OF SAMPLER

Open drive sampler

Stationary piston sampler

Split barrel

Rotary sampler

Disturbed and undisturbed sampling

Block sample

1.4 In-situ tests

In situ tests area unit tests conducted on or within the soil at the positioning. the foremost commonly utilized in situ checks area unit the quality penetration test (S P T) the sphere vane tests, the cone penetration check (CPT), the pressure meter check, and also the dilatometer check (DMT). The vane shear check (V S T) is employed in saturated fine-grained soils to get the un drain shear strength. The borehole shear check (BST) consists of drilling a borehole, removing the drilling tool, and inserting the borehole shear tester right down to the testing depth. The plate load check (PLT) is one in all the only and oldest in-place tests. The American state bearing magnitude relation check (CBR) may be a style of plate check performed within the field or the laboratory. The in-place checks most ordinarily used offshore area unit the cone penetrates meter test and also the V S T

1.5 Standard Penetration Test (SPT)

The tests essentially consist of diving a split spoon sampler of 50 mm outside diameter into the soil

Standard Penetration Test Arrangements | Download Scientific Diagram

Fig no. 10 Standard penetration test

The sample should be driven at the rock bottom of the borehole that is cased in a very pipe is forced into the borehole and take a look at conducted at rock bottom of the borehole.

In case the strata are stiff like gravels and sandy mallet, a solid cone-like 60° shoe is fitted to the sampler. The sampler is to be hooked up to the drill rods and driven by blows of a hammer. The hammer typically

The sampler is to be hooked up to the drill rods and driven by blows of a hammer. The hammer typically weighs sixty-five weight unitswith a free fall of 750 mm. typically thirty blows per minute are given.

The sampler is at first driven a distance of one hundred fifty mm below very cheap of the borehole and so an additional distance of three hundred mm.

The number of blows needed to drive this distance is thought of as customary penetration price N.

The number of blows needed to result in the primary one hundred fifty mm of penetration, known as the seating drive, is forgotten

The split-spoon sampler is then withdrawn and detached from the drill rods. The split-barrel is disconnected from the cutting shoe and also the coupling. The soil sample collected within the split barrel is rigorously collected top reserve the natural wet content and transported to the laboratory for tests. Sometimes, a skinny liner is inserted inside the split-barrel so that at the tip of the S P T, the liner containing the soil sample is scaled with liquefies wax at each of its ends before it's got rid of to the laboratory

The take a look at is halted If

(a) 50 blows area unit needed for any a hundred and fifty metric linear unit penetration

(b) one hundred blows area unit needed for three hundred penetration.

The following precautions ought to be taken whereas closing the take a look at

(a) The split spoon sampler should be in condition and the cutting shoe should be free from wear tear.

(b) The drop press should be of the correct weight and the fall ought to be free resistance.

(c)Very cheap of the borehole should be properly clean before the take a look at is administrated. Otherwise, the take a look at gets administrated within the loose, disturbed soil and not within the undisturbed soil.

(d) Once a casing is employed in the borehole, it ought to be ensured that the casing is driven simply by wanting level at that the S P T is to be administrated. Otherwise, the take a look at gets administrated in a very soil of very cheap of the casing

S P T is extensively used as a result of the take a look at is straightforward and economical. It is the sole take a look at that has representative soil samples each for visual scrutiny for natural wet content and classification tests within the laboratory.

Design charts for shallow foundations resting on cohesionless soils are developed.

Limitations of SPT

1. As SPT is subject to several errors, the S P T price isn't reliable.

2 In granular soils, the correlation between S P T and internal friction angle is incredibly poor.

3. Determination of allowable bearing pressure from N values isn't correct.

4. S P T price isn't acceptable for determinant the bearing pressure in fine-grained cohesive soils,

5. Corrections for overburden and impact square measure to be applied to record N values for correlation with soil properties

Applications of SPT

SPT is appropriate for each cohesive and non-cohesive soil variety of SPT is extensively used as a result of the take a look at is easy and comparatively economical.

It is the sole take a look at that gives representative soil samples each for visual scrutiny within the field and natural wet content and therefore the classification tests within the laboratory

This takes a look at what is employed to work out the density index of sandy soils and the consistency of cohesive soils. Variety of tried and true correlations square measure out there between N price and soil parameters each for cohesive and non-cohesive soils.

1.6 Cone Penetration Test (CPT)

The penetration used consists of a cone with an Associate in Nursing apex angle of sixty and a base space of ten cm and is employed as is as follows

1. At the start the cone and therefore the friction jacket assembly is during a stationary position

2. The cone is then pushed into the soil at the speed of a pair of20 mm /s by the inner sounding rod at a gradual rate until a collar engages the cone. The force Qc offered by soil for penetration measured on the gauge, and tip resistance purpose resistance. Qt is calculated by q c =Q c /Ac

Where Ac is the base area

3. The sounding rod is pushed more pushing the friction jacket and therefore the cone assembly along. The entire force alphabetic character, needed for this is often once more pressure gauge.

Qt = Qc + Q f, where Q f = force needed to push friction jacket

From this skin friction, fs is calculated

Fs =Q f/A f

Where A f is that the extent of friction jacket.

The cycle is recurrent until the desired depth is reached. Thus, with this check purpose resistance moreover, a skin operate f, is seen

Geotechnical devices. A: Dynamic cone penetration test. B: In situ vane... | Download Scientific Diagram

Fig no. 11 Cone penetration

The check is appropriate just in case of clayey strata, however unsuitable for sandy or gravely soil

Also, the check doesn't yield a sample as just in the case of a spilled spoon sampler. Check is speedy and easy. Enhancements are created in the latest equipment to record the forces mechanically.

Cone resistance Q c values rely on grain size distribution, soil density, the position of formation, and overburden pressure. Cone resistance is expounded to overburden pressure.

1.7 Dynamic Cone Penetration Test (DCPT)

In this take a look at a cone that has an m apex angle of sixty and attached to drill rods is a drive tool with a hammer of sixty-five kilo. Falling freely from a height of 750 millimeters.

The blow count for each one hundred millimeters of the cone is continuously recorded. The con la driven until refusal or up to the specified depth and drilling rods area unit withdrawn, going the cone behind within the ground.

The number of blows needed for three-hundred-millimeter penetration is noted because of the dynamic cone resistance. This takes a look provides a continued record of N. with depth. No sample, however, may be obtained during this take a look at

Compaction Quality Control of Earth Fills Using Dynamic Cone Penetrometer | Journal of Construction Engineering and Management | Vol 144, No 9

Fig no. 12 Dynamic cone penetration

Dynamic cone penetration tests area unit performed either by employing a fifty-millimeter diameter cone with clay suspension (IS: 4958- half 1-1976) or a sixty-five-millimeter diameter goes along with clay pay (1S: 4968).

When clay suspension is employed the set-up has an arrangement for the circulation of suspension so drill steel is eliminated.

The dynamic cone takes a look at maybe a fast take a look at and facilitate to hide an oversized space below investigation economically.

It helps in distinguishing the uniformity or the variability of the under soil profile and is found to be less costly and far faster than the SPT.

If the tests area unit allotted near to a couple of boreholes, the results obtained from DCPT may be compared with the SPT information and correlation between the 2 established for the actual web site conditions

The correlation can then be used to obtained N values from Ncd and N applicable for medium to fine sands are given below

1. 5 N for 3m depth

1. 75N for 3 to 6m depth

2. 0N for greater than 6 m depth

1.8 Geophysical methods

Geophysical strategies facilitate the US in checking and supplementing the soil take a look at results. They are usually helpful in the preliminary investigation stage, once they will provide US ideas concerning the position of the formation, strata boundaries of immensely differing soils, the depth of existing bedrock, etc.

The results inferred from such tests should, however, be checked and confirmed from the boreholes, by lifting soil samples and examining and testing them.

Most commonly used geophysical methods in soil exploration

1. Seismic refraction method

2. Electrical resistivity method (soil resistivity)

1.9 Soil resistivity methods

Principle –

This method is based upon the principle that electrical resistance offered by different types of soil to the flow of current is different

Soil impedance could be live of what proportion the soil resists or conducts current. It is a crucial considered style of systems that place confidence in passing a current through the surface.

Associate in the nursing understanding of the soil impedance and the way it varies with depth within the soil is important to style the grounding system in Associate in the nursing electrical station, or for lightning conductors.

It is required for the style of grounding electrodes for substations and High-voltage electricity transmission systems. It was necessary for earth-return telegraphy. It can even be a helpful live in agriculture as a proxy activity for wet content.

In most substations, the planet is employed to conduct fault current once there square measure ground faults on the system.

In single-wire earth come back power transmission systems, the planet itself is employed because of the path of the physical phenomenon from the top customers (the power consumers) back to the transmission facility.

normally there's some price higher than that the electric resistance of the planet affiliation should not rise, and a few most step voltage that should not be exceeded to avoid endangering individuals and placental.

Soil resistivity measurement

Fig no. 13 Soil resistivity method

The soil impedance price is subject to nice variation, because of wet, temperature, and chemical content. Typical values are:

Usual values: from ten up to one thousand (Ω-m)

Exceptional values: from one thousand up to ten thousand (Ω-m)

The SI unit of impedance is that the Ohm-meter (Ω-m); within the U.S. the Ohm-centimeter (Ω-cm) is commonly used instead. One Ω-m is one hundred-one hundred. Generally, the conduction, the reciprocal of the impedance, is quoted instead.

A wide variety of typical soil impedance values may be found in the literature. Military enchiridion 419 (MIL-HD BK-419A) contains reference tables and formulae for the resistance of varied patterns of rods and wires buried in the soil of glorious impedance. Being copyright-free, these numbers square measure wide derived, generally while not acknowledgment.

1.10 Seismic refraction methods

Principle

The rate of unstable waves passing through soil rock mass depends upon the kind of soil/rock and

The waves get refracted once they cross the boundary between differing kinds of soil.

Method

It consists of causing impact (by putting a plate on the soil with a hammer) or generating shock by exploding a tiny low charge at or close to the bottom surface.

The divergent shock waves area unit recorded by a tool referred to as geophone that records the time of travel of the wave.

The geophones area unit put in at appropriate legendary distances on the bottom in a very line from the supply of shock are identical is affected removed from the geophone to two. Is turn out shock waves at given intervals

These wave area units classified as either direct mirroredor refracted. Direct waves travel in close to straight lines from the supply of impulse to the surface.

Reflected or refracted waves endure a modification in direction once they encounter a boundary separating media of various unstable velocities.

Each lower strata must be denser than higher strata; as indicate in On plotting graph of your time versus distance, depth D, of higher strata and D, depth of lower strata may be observed by equations

D1 = 0. 5 * x1 *square root of [(v2-v1)/(v1+v2)]

Where

X1 = distance from the y-axis to point of intersection of first 2 straight lines.

X2 = distance from the y-axis to point of intersection of 2 straight lines.

$Seismic refraction - Wikipedia$

Fig no. 14 Seismic refraction method

Limitations of this technique

1. Materials like dense gravel, stiff clay, dirt, or rock have characteristic seismic speed, thus can't be known by the distance-time graph.

2. Just in case of blind zone wherever layer with speed not up to that of higher layer exists can't be detected by seismic refraction,

Key takeaways

There are mainly 3 tests

SPT

CPT

DCPT

Geophysical methods

Soil resistivity method

Seismic refraction methods

Reference:

1. “Geotechnical Engineering” by S L Gulhati and M Datta

2. “Applied Analysis in Geotechnics” by F Azizi

3. “Essentials of Soil Mechanics and Foundations: Basic Geotechnics” by DF McCarthy

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