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SITE INVESTIGATION

CHAPTER 4SITE INVESTIGATION PRACTICE

SITE INVESTIGATION PRACTICE

4.0Introduction

To design a foundation that will support a structure, an engineer must understand the types of soil deposits that will support the foundation. Moreover, foundation engineers must remember that soil at any site frequently is non-homogeneous; that is the soil profile may vary. Soil mechanics theories involve idealized conditions, so the application of the theories to foundation engineering problems involves a well judged evaluation of site conditions and soil parameters. To do this requires some knowledge of the geological process by which the soil deposit at the site was formed, supplemented by subsurface exploration. Good professional judgment constitutes at essential part of geotechnical engineeringand it comes only with practice.4.1 Definition of Soil Exploration

The design of a foundation, an earth dam, or a retaining wall cannot be made intelligently unless the designer has at least a reasonably accurate conception of the physical properties of the soils involved. The field and laboratory investigations required to obtain this essential information constitute the soil exploration. Until about the 1930s soil exploration was consistently inadequate because rational methods for soil investigation had not yet been developed. On the other hand, at the present time the amount of soil exploration and testing and the refinements in the techniques for performing the investigations are often quite out of proportion to the practical value of the results. To avoid either of these extremes, the exploratory program must be adapted to the soil conditions and to the size of the job.

4.2 Purposes of Soil Exploration Programme Selection of type and depth of foundation. Evaluation of the load-bearing capacity of the foundation. Estimation of the probable settlement of a structures. Determination of the potential foundation problem. Establishment of ground water table. Prediction of lateral earth pressure. Establishment of construction methods for changing subsoil condition.4.3 Soil Reconnaissance/Visual InspectionThe inspection is to obtain the following information: A General topography of the site; possible existence of drainage, creep of slopes and deep, wide shrinkage cracks and expansive soil. Soil stratification from deep cuts; highway and railroads. Type of vegetation at the site. 4 High-water marks on nearby buildings and bridge abutments. Ground water levels. Types of construction nearby or existence or cracks in walls.4.4 Objectives of Site InvestigationsThe objectives of site investigation have been defined by the various Codes of Practice (BS CP 2001:1950, 1957; BS 5930:1981). They can be summarized as providing data for the following.

i. Site selection. The construction of certain major projects, such as earth dams, is dependent on the availability of a suitable site. Clearly, if the plan is to build on the cheapest, most readily available land, geotechnical problems due to the high permeability of the sub-soil, or to slope instability may make the final cost of the construction prohibitive. Since the safety of lives and property are at stake, it is important to consider the geotechnical merits or demerits of various sites before the site is chosen for a project of such magnitude.

ii. Foundation and earthworks design. Generally, factors such as the availability of land at the right price, in a good location from the point of view of the eventual user, and with the planning consent for its proposed use are of over-riding importance. For medium-sized engineering works, such as motorways and multistorey structures, the geotechnical problems must be solved once the site is available, in order to allow a safe and economical design to be prepared.

iii. Temporary works design. The actual process of construction may often impose greater stress on the ground than the final structure. While excavating for foundations, steep side slopes may be used, and the in-flow of groundwater may cause severe problems and even collapse. These temporary difficulties, which may in extreme circumstances prevent the completion of a construction project, will not usually affect the design of the finished works. They must, however, be the object of serious investigation.

iv. The effects of the proposed project on its environment. The construction of an excavation may cause structural distress to neighbouring structures for a variety of reasons such as loss of ground, and lowering of the groundwater table. This will result in prompt legal action. On a wider scale, the extraction of water from the ground for drinking may cause pollution of the aquifer in coastal regions due to saline intrusion, and the construction of a major earth dam and lake may not only destroy agricultural land and game, but may introduce new diseases into large populations. These effects must be the subject of investigation.

v. Investigation of existing construction. The observation and recording of the conditions leading to failure of soils or structures are of primary importance to the advance of soil mechanics, but the investigation of existing works can also be particularly valuable for obtaining data for use in proposed works on similar soil conditions. The rate of settlement, the necessity for special types of structural solution, and the bulk strength of the sub-soil may all be obtained with more certainty from back-analysis of the records of existing works than from small scale laboratory tests.

vi. The design of remedial works. If structures are seen to have failed, or to be about to fail, then remedial measures must be designed. Site investigation methods must be used to obtain parameters for design.

vii. Safety checks. Major civil engineering works, such as earth dams, have been constructed over a sufficiently long period for the precise construction method and the present stability of early examples to be in doubt. Site investigations are used to provide data to allow their continued use.

4.5 Site InvestigationsThe Four Major Steps or Components of a Sites Investigation. For a major project (a tunnel, large bridge, tall building, etc., will require four phases for its site investigation:

Phase 1: Literature Search.

This phase collects all the existing information of the site and the structure. For the site, it involves aerial photos, surveys, previous geotechnical data, building codes and adjacent structures. For the structure, it requires all the major structural data of the building.

Phase II: Reconnaissance Sub-surface Exploration.

The site and the neighborhood is carefully studied. Test pits are excavated, soil borings and penetrometers are driven, samples of soil at each strata are taken, the ground water is established, percolation tess are performed and in-situ testing is completed.

Phase III: Laboratory Testing and Reports.

The samples are taken to the laboratory and engineering parameters are determined, in order to calculate bearing capacities, settlements and special solutions. All the data from these first three phases are summarized in a Geotechnical and Foundation Recommendations Report.

Phase IV: Detailed Site Investigation.

Very large projects will require an expansion of the three phases above.

The site investigation works consists of planning, making some test boreholes and collecting soil samples. It has been found that the best site investigations involve a considerable number of activities, some of which may become relatively unimportant in some cases, but should never be forgotten. An ideal order of events might be as shown in Table 1.1.

The sequence of geotechnical site investigation might be:1. preliminary desk study, or fact-finding survey;

2. air photograph interpretation;

3. site walk-over survey;

4. preliminary subsurface exploration;

5. soil classification by description and simple testing;

6. detailed subsurface exploration and field testing;

7. the physical survey (laboratory testing);

8. evaluation of data;

9. geotechnical design;

10. field trials; and

11. Liaison by geotechnical engineer with site staff during project construction.Requirements for Boring Layout and Depth

Required minimum depth of the borings should be predetermined.According to ASCE (American Society of Civil Engineers, 1972), the minimum depth of boring (Db) shall be determined by the following:- Net increase of stress ((() under a foundation (Figure 2)

Fig 2 :Determination of the minimum depth of boring

Estimate the variation of the vertical effective stress ((V') with depth.

Determine the depth (D) = D1. where . Where q is estimation of net stress on the foundation.

Determine the depth (D) = D2, where .

The smaller of the two depths is the approximate minimum depth of -boring required; unless bedrock is encountered.

# For hospitals and office building: For light steel or narrow concrete building;

Db(m) = 3S0.7 ....................(1)

Db(ft)-10S0.7.......................(2) For heavy steel or wide concrete building;

Db (m) = 6S0.7 .....................(3)

Db (ft) - 20S 0.7 ....................(4)# The depth of boring should be at least 1.5 times the depth of excavation.

# Spacing of boreholes can be increased or decreased depending on the subsoil

condition.

# While there are no set rules for boring depth and spacing is shown in Tables

1.2 and 1.3 give a guide to these requirements respectively.

The site investigation works sequence can also be presented in flow chart shown in Fig. 1.

Fig 1 : Flow chart for SI works

4.6INVESTIGATION AND BORING METHODS4.6.1IntroductionMany different techniques are available for site investigation. The method employed will depend on many factors such as depth required, area to be covered, ease of access, etc. On large jobs preliminary borings are used to furnish overall subsoil surveys followed by final borings so soil or rock profiles may be determined at the most useful orientations. In general, exploration

contracts should be open ended so that intermediate borings may be added in areas that prove to be critical.

4.6.2Soil DrillingA wide variety of equipment is available for performing borings and obtaining soil samples. The method used to advance the boring should be compatible with the soil and groundwater conditions to assure that soil samples of suitable quality are obtained. Particular care should be exercised to properly remove all slough or loose soil from the boring before sampling. Below the groundwater level, drilling fluids are often needed to stabilize the sidewalls and bottom of the boring in soft clays or cohesionless soils . Without stabilization, the bottom of the boring may heave or the sidewalls may contract, either disturbing the soil prior to sampling or preventing the sampler from reaching the bottom of the boring. In most geotechnical explorations, borings are usually advanced with solid stem continuous flight, hollow-stem augers, or rotary wash boring methods. These methods are often augmented by in-situ testing .Assuming access and utility clearances have been obtained and a survey base line has been established in the field, field explorations are begun based on the information gained during the previous steps. Many methods of field exploration exist; some of the more common are described below.

4.6.3 Test Pits and Trenches

These are the simplest methods of inspecting subsurface soils. They consist of excavations performed by hand, backhoe, or dozer. Hand excavations are often performed with posthole diggers or hand augers. They offer the advantages of speed and ready access for sampling. They are severely hampered by limitations of depth and by the fact they cannot be used in soft or loose soils or below the water table.

4.6.4 Boreholes

Borings are probably the most common method of exploration. They can

be advanced using a number of methods, as described below. Upon completion,

all borings should be backfilled and in many cases this will require grouting.

i.Hand Auger BoringsThe hand auger provides a light, portable method of sampling soft to stiff soils near the ground surface. At least six types of auger are readily available:

posthole or Iwan auger;

small helical auger (wood auger);

dutch auger;

gravel auger;

barrel auger; and

spiral auger.

Fig. Selection of hand-operated augers.

Hand augers are used by one or two men, who press down on the cross-bar as they rotate it thus advancing the hole. Once the auger is full, or has collected sufficient material, it is brought back to the surface and the soil removed. Although the method is cheap because of the simplicity of the equipment, it does suffer from several disadvantages.

The most commonly used auger for site investigation is the Iwan auger. This is normally used at diameters of between 100 and 200 mm. Small helical augers are quite effective in stiff clays, but become difficult to use once the water table is reached.

Barrel augers are now rarely seen, but were formerly used with the light percussion rig when progress through clays was made using a shell. They allowed the base of the borehole to be very effectively cleaned before sampling took place. Because they are heavy they require a tripod for raising and lowering them in the borehole. When lowered to the bottom of the hole they were turned by hand.

In stiff or very stiff clays, hand-auger progress will be very slow, and the depth of boring may have to be limited to about 5 m. When such clays contain gravel, cobbles or boulders it will not normally be possible to advance the hole at all. In uncemented sands or gravels, it will not be possible to advance the hole below the water table, since casing cannot be used and the hole will collapse either on top of the auger (which makes it difficult to recover the auger from the hole) or when the auger is being removed. Only samples of very limited size can be obtained from the hole. In addition, it will not be possible to carry out standard penetration tests without a frame to lift the trip hammer and weight, so that no idea of the relative density of granular deposits can be obtained.

Despite these difficulties, where access for machinery is impossible the hand auger may give valuable information.

ii.Auger Borings

Rotating an auger while simultaneously advancing it into the ground; the auger is advanced to the desired depth and then withdrawn. Samples of cuttings can be removed from the auger; however, the depth of the sample can only be approximated. These samples are disturbed and should be used only for material identification. This method is used to establish soil strata and water table elevations, or to advance to the desired stratum before Standard Penetration Testing (SPT) or undisturbed sampling is performed. However, it cannot be used effectively in soft or loose soils below the water table without casing or drilling mud to hold the hole open. See ASTM D 1452 (AASHTO T 203).

iii.Mechanical Auger

A large variety of size and type are available. Basic types are:

(a) Plate Auger. Used in strata which will stand unsupported. It is necessary to pull out every foot to examine cuttings. Depth limited by length of kelly bar (generally 6 m).

(b) Continuous Flight Auger. A spiral continuous flight is used to transfer the soil to the surface. Identification of strata changes is difficult. Useful in proving known strata.

(c) Hollow Flight Auger. A continuous spiral around a tube is used to transfer cuttings to the surface. A plug and spade auger device can be used to drill soil below the control tube, or a continuous sample can be taken in a central sampling barrel, or undisturbed samples driven ahead through the tube. SPT and undisturbed samples are obtained through the hollow drill stem, which acts like a casing to hold the hole open.

This is frequently a slow process, and due to the very great torque required to drive the auger may be uneconomic. This method is largely experimental at the moment.

(d) Bucket or Grab Auger. This type of auger drills a large diameter hole, with or without casing. A large plant is involved, and it is infrequently used in investigation work.

iv.Wash BoringsIn this method, the boring is advanced by a combination of the chopping action of a light bit and the jetting action of water flowing through the bit. This method of advancing the borehole is used only when precise soil information is not required between sample intervals. Borings can be made in most alluvial strata by a wash boring technique. A shopping bit on a string of rods is used inside a casing, soft strata being washed out below the casing and carried to the surface by a jet of water passing through the rods and bit, and returning inside the casing. Firmer materials are penetrated by chopping with the bit, and chopped particles being carried to the surface by the flow of water. The casing can usually be agitated down by turning, as boring proceeds, but it may be necessary to drive it. Samples can be obtained and in situ tests made through the casing from time to time. In this method of boring, unless continuous samples are taken, which defeats the main object of the technique, speed, the only evidence of the strata being penetrated is the very fine soil particles being carried to the surface by the flow of water. Wash borings are normally made using casing between 50 mm and 150 mm diameter, above this size, the pump unit required is generally too large. The technique is generally used as a fast and consequently cheap method of supplementing information obtained from a series of dry sample borings. It is particularly useful for obtaining samples or carrying out in situ tests at some depth in know strata, e.g. in a clay layer, below a sand stratum. Disturbance of the ground by the water jet may in some cases extend two feet or more below the casing, and care should be taken in sampling and testing to ensure that this is not carried out in the disturbed area. The use of wash boring without adequate dry sample boring control should be avoided.

4.7SAMPLINGThere are 2 types of soil samples, there are disturbed and undisturbed.4.7.1Disturbed Sampling

Disturbed samples are generally obtained to determine the soil type, gradation, classification, consistency, density, presence of contaminants, stratification, etc. The methods for obtaining disturbed samples vary from hand excavating of materials with picks and shovels to using truck mounted augers and other rotary drilling techniques. These samples are considered .disturbed. since the sampling process modifies their natural structure.

4.7.2Undisturbed Sampling

Undisturbed samples are used to determine the in place strength, compressibility (settlement), natural moisture content, unit weight, permeability, discontinuities, fractures and fissures of subsurface formations. Even though such samples are designated as .undisturbed, in reality they are disturbed to varying degrees. The degree of disturbance depends on the type of subsurface materials, type and condition of the sampling equipment used, the skill of the drillers, and the storage and transportation methods used.

4.7.3Common Types of Samplers

The cuttings or washings from exploratory drill holes are inadequate to furnish a satisfactory conception of the engineering characteristics of the soils encountered, or even of the thicknesses and depths of the various strata. On the contrary, such evidence more often than not is grossly misleading and has been responsible for many foundation failures. Proper identification of the subsurface materials requires that samples be recovered containing all the constituents of the materials in their proper proportions. Moreover, evaluation of the appropriate engineering properties, such as the strength, compressibility, or permeability, may require the performance of laboratory tests on fairly intact or even virtually undisturbed samples. The expenditure of time and money increases rapidly as the requirements become more stringent with respect to the degree of disturbance that can be tolerated and with increasing diameter of sample. Therefore, on small projects or in the initial exploratory stages of large or complex projects, it is usually preferable to obtain relatively inexpensive, fairly intact samples from the exploratory drill holes. On the basis of the information obtained from these samples, the necessity for more elaborate sampling procedures can be judged.

Types of Soil Sampler

A wide variety of samplers are available to obtain soil samples for geotechnical engineering projects.

These include standard sampling tools which are widely used as well as specialized types which may be unique to certain regions of the country to accommodate local conditions and preferences.

General guidelines to assist geotechnical engineers and field supervisors select appropriate samplers, but in many instances local practice will control.

Common types of samplers used.

i.Split Barrel Sampler

Used to obtain disturbed samples in all types of soils.

Typically used in conjunction with the Standard Penetration Test (SPT),

The sampler is driven with a 63.5-kg (140-lb) hammer dropping from a height of 760 mm (30 in). (AASHTO T206 and ASTM D1586),

Available in standard lengths of

457 mm (18 in) and 610 mm (24in)

Inside diameters ranging from 38.1 mm (1.5 in) to 114.3 mm (4.5 in) in 12.7 mm (0.5 in) increments (Figure 3-7a,b).

The 38.1 mm (1.5 in) inside diameter sampler is popular because correlations

High area ratio disturbs the natural characteristics of the soil being sampled, thus disturbed samples are obtained.

This corresponds to a relatively thick walled sampler with an area ratio

(Hvorslev, 1949).

As shown in Figure 3-8a, when the shoe and the sleeve of this type of sampler are unscrewed from the split barrel, the two halves of the barrel may be separated and the sample may be extracted easily.

The soil sample is removed from the split-barrel sampler it is either placed and sealed in a glass jar, sealed in a plastic bag, or sealed in a brass liner (Figure 3-8b).

Separate containers should be used if the sample contains different soil types.

Alternatively, liners may be placed inside the sampler with the same inside diameter as the cutting shoe (Figure 3-9a).

This allows samples to remain intact during transport to the laboratory.

In both cases, samples obtained with split barrels are disturbed and therefore are only suitable for soil identification and general classification tests.

ii.Thin Wall Sampler (Shelby)

To obtain relatively undisturbed samples of cohesive soils for strength and consolidation testing.

Commonly, it has a 76 mm (3.071in) outside diameter & a 73 mm (2.875 in) inside diameter,

Resulting in an area ratio of 9 percent. (Figures 3-10)

Vary in outside diameter between 51 mm (2.0 in) and 76 mm (3.0 in)

typically come in lengths from 700 mm (27.56 in) to 900 mm (35.43 in), (Figure 3-11).

Larger diameter sampler tubes used when higher quality samples are required and sampling disturbance must be reduced.

The thin-walled tubes are manufactured using carbon steel, galvanized-coated carbon steel, stainless steel,and brass.

Carbon steel tubes

the lowest cost tubes but are

unsuitable if the samples are to be stored in the tubes for more than a few days or if the inside of the tubes become rusty,

significantly increasing the friction between the tube and the soil sample.

Galvanized steel tubes

preferred in stiff soils

carbon steel is stronger,

less expensive

galvanizing provides additional resistance to corrosion.

Stainless Steel tubes

preferred for offshore bridge borings,

salt-water conditions, or

long storage times

Manufactured with a beveled front edge for cutting a reduced-diameter sample [commonly 72 mm (2.835 in) inside diameter] to reduce friction.

Can be pushed with a fixed head or piston head.

The following information should be written on the top half of the tube and on the top end cap: project number, boring number, sample number, and depth interval.

We should also write on the tube the project name and the date the sample was taken.

Near the upper end of the tube, the word "top" and an arrow pointing toward the top of the sample should be included.

Putting sample information on both the tube and the end cap facilitates retrieval of tubes from laboratory storage and helps prevent mix-ups in the laboratory when several tubes may have their end caps removed at the same time.

Both ends of the tube should then be sealed with at least a 25 mm (1 in) thick layer of microcrystalline (nonshrinking) wax after placing a plastic disk to protect the ends of the sample (Figure 3-12a).

iii.Piston Sampler

Also known as an Osterberg or Hvorslev sampler.

Particularly useful for sampling soft soils where sample recovery is often difficult although it can also be used in stiff soils.

The piston sampler (Figure 3-13) is basically a thin-wall tube sampler with a piston, rod, and a modified sampler head.

The quality of the samples obtained is excellent

Probability of obtaining a satisfactory sample is high.

Advantages are that the fixed piston helps prevent the entrance of excess soil at the beginning of sampling, thereby precluding recovery ratios greater than 100 %.

Helps the soil enter the sampler at a constant rate throughout the sampling push.

The head used acts creates a better vacuum which helps retain

the sample better than the ball valve in thin-walled tube (Shelby) samplers.

iv.Pitcher Tube Sampler

Used in stiff to hard clays and soft rocks, and is well adapted to sampling deposits consisting of alternately hard and soft layers.

This sampler is pictured in Figure 3-14 and the primary components shown in Figure 3-15a and these include :

an outer rotating core barrel with a bit and an inner stationary,

spring-loaded,

thin-wall sampling tube that leads or trails the outer barrel drilling bit, depending on the hardness of the material being penetrated.

v.Denison Sampler

Is similar to a pitcher sampler except that the projection of the sampler tube ahead of the outer rotating barrel is manually adjusted before commencement of sampling operations, rather than spring-controlled during sampler penetration.

The basic components of the sampler (Figure 3-16) are :

an outer rotating core barrel with a bit,

an inner stationary sample barrel with a cutting shoe,

inner and outer barrel heads,

an inner barrel liner, and

an optional basket-type core retainer.

The coring bit may either be a carbide insert bit or a hardened steel saw tooth bit.

The shoe of the inner barrel has a sharp cutting edge.

The cutting edge may be made to lead the bit by 12 mm (0.5 in) to 75 mm (3 in) through the use of coring bits of different lengths.

The longest lead is used in soft and loose soils because the shoe can easily penetrate these materials.

The minimum lead is used in hard materials or soils containing gravel.

Used primarily in stiff to hard cohesive soils and in sands, which are not easily sampled with thin-wall samplers owing to the large jacking force required for penetration.

The sampler is also suitable for sampling soft clays and silts.

vi.Block sampling

Block sampling has traditionally involved the careful hand excavation of soil around the sample position, and the trimming of a regular-shaped block. This block is then sealed with layers of muslin, wax and clingfilm, before being encased in a rigid container, and cut from the ground. The process is illustrated in Fig. 6.5. A similar process can be carried out in shafts and large-diameter auger holes.

Trial pits are normally only dug to shallow depths, and shafts and large-diameter auger holes tend to be expensive. Therefore block samples have not traditionally been available for testing from deep deposits of clay. In the past decade, however, there has been an increasing use of rotary coring methods to obtain such samples. When carried out carefully, without displacing the soil, rotary coring is capable of producing very good quality samples. When the blocks are cut by hand then obviously the pit will be air-filled, but when carried out in a borehole it will typically be full of drilling mud.

During the sampling process there is stress relief. At one stage or another the block of soil will normally experience zero total stress. This will lead to a large reduction in the pore pressures in the block. The soil forming the block will attempt to suck in water from its surroundings, during sampling, either from the soil to which it is attached, or from any fluid in the pit or borehole. This will result in a reduction in the effective stress in the block.

In addition, where block sampling occurs in air, negative pore pressures may lead to cavitations in any silt or sand layers which are in the sample. Cavitation in silt and sand layers releases water to be imbibed by the surrounding clay, and the effect will be a reduction in the average effective stress of the block.

Block sampling is an excellent method of ensuring that the soil remains unaffected by shear distortions during sampling, but samples obtained in this way may not (as a result of swelling) have effective stresses that are the same as those in the ground. Therefore the strength and compressibility of the soil may be changed. This should be allowed for either by using appropriate reconsolidation procedures, or by normalizing strength and stiffness, where appropriate, with effective stress.

Table 7 : Types of sampler generally used in Malaysia

4.8COMMON LABORATORY TEST FOR SITE INVESTIGATION 1. Soil Classification Tests: BS 1377: Part 2: 1990

Moisture content, Liquid limit, Plastic limit, Plasticity index, linear shrinkage, particle size distribution.

(These tests are from disturbed samples such as split spoon samplers (SPT), bulk samples, etc.).

2. Chemical & Electro-chemical Tests: BS 1377 Part 3: 1990

Organic matter content, Mass loss on ignition, Sulphate content of soil and ground water, Carbonate content, Chloride content, Total dissolved solids, pH value, 3. Compaction-related (tests from bulk samples) Tests: BS 1377: Part

3.1 Dry density - moisture relationship (2.5 kg/4.5 kg hammer)

- Soil with some coarse gravels

- vibrating method

3.2 Moisture condition value (MCV)

3.3 CBR tests

4. Compressibility, Permeability and Durability Tests: BS 1377: Part 5

4.1 1-D consolidation test

4.2 Swelling and collapse tests

4.3 Permeability by constant head

4.4 Dispersibility

5. Consolidation & Permeability Tests in Hydraulic Cells & with pore pressure measurements: BS 1377: Part 6

5.1 Consolidation Properties using hydraulic cell

5.2 Permeability in hydraulic consolidation cell

5.3 Isotropic consolidated properties using triaxial cell

5.4 Permeability in a triaxial cell

6. Shear Strength Tests (Total Stress) BS 1377: Part 7

6.1 Lab vane shear

6.2 Direct shear box (small)

6.3 Direct shear box (large)

6.4 Residual strength

6.5 Undrained shear strength (UU)

6.6 Undrained shear strength (multi loading)

7. Shear Strength Tests (Effective Stress) BS 1377: Part 8

7.1 CIU with pore pressure measurement

7.2 CD with pore pressure measurement

4.9IN-SITU TEST / FEILD TESTSeveral in-situ tests define the geostratigraphy and obtain direct measurements of soil properties and geotechnical parameters. The common tests include: standard penetration (SPT), cone penetration test (CPT), piezocone (CPTu), flat dilatometer (DMT), pressuremeter (PMT), and vane shear (VST). Each test applies different loading schemes to measure the corresponding soil response in an attempt to evaluate material characteristics, such as strength and/or stiffness. Figure 5-1 depicts these various devices and simplified procedures in graphical form. Details on these tests will be given in the subsequent sections.

Figure 5-1. Common In-Situ Tests for Geotechnical Site Characterization of Soils

Boreholes are required for conducting the SPT and normal versions of the PMT and VST. A rotary drilling rig and crew are essential for these tests. In the case of the CPT, CPTU, and DMT, no boreholes are needed, thus termed .direct-push. technologies. Specialized versions of the PMT (i.e., full-displacement type) and VST can be conducted without boreholes. As such, these may be conducted using either standard drill rigs or mobile hydraulic systems (cone trucks) in order to directly push the probes to the required test depths.

The truck-mounted and track-mounted systems commonly used for production penetration testing. The enclosed cabins permit the on-time scheduling of in-situ testing during any type of weather. A disadvantage of direct-push methods is that hard cemented layers and bedrock will prevent further penetration. In such cases, borehole methods prevail as they may advance by coring or noncoring techniques. An advantage of direct-push soundings is that no cuttings or spoil are generated.

Table below shows common field tests practice in Malaysia.

4.9.1 STANDARD PENETRATION TESTThe standard penetration test (SPT) is performed during the advancement of a soil boring to obtain an approximate measure of the dynamic soil resistance, as well as a disturbed drive sample (split barrel type).

The test was introduced by the Raymond Pile Company in 1902 and remains today as the most common in-situ test worldwide. The procedures for the SPT are detailed in ASTM D 1586 and AASHTO T-206.

The SPT involves the driving of a hollow thick-walled tube into the ground and measuring the number of blows to advance the split-barrel sampler a vertical distance of 300 mm (1 foot). A drop weight system is used for the pounding where a 63.5-kg (140-lb) hammer repeatedly falls from 0.76 m (30 inches) to achieve three successive increments of 150-mm (6-inches) each. The first increment is recorded as a .seating., while the number of blows to advance the second and third increments are summed to give the N-value ("blow count") or SPT-resistance (reported in blows/0.3 m or blows per foot). If the sampler cannot be driven 450 mm, the number of blows per each 150-mm increment and per each partial increment is recorded on the boring log. For partial increments, the depth of penetration is recorded in addition to the number of blows. The test can be performed in a wide variety of soil types, as well as weak rocks, yet is not particularly useful in the characterization of gravel deposits nor soft clays. The fact that the test provides both a sample and a number is useful, yet problematic, as one cannot do two things well at the same time.

4.9.2 VANE SHEAR TEST (VST)

The vane shear test (VST), or field vane (FV), is used to evaluate the inplace undrained shear strength (suv) of soft to stiff clays & silts at regular depth intervals of 1 meter (3.28 feet). The test consists of inserting a four-bladed vane into the clay and rotating the device about a vertical axis, per ASTM D 2573 guidelines. Limit equilibrium analysis is used to relate the measured peak torque to the calculated value of su. Both the peak and remolded strengths can be measured; their ratio is termed the sensitivity, St. A selection of vanes is available in terms of size, shape, and configuration, depending upon the consistency and strength characteristics of the soil. The standard vane has a rectangular geometry with a blade diameter D = 65 mm, height H = 130 mm (H/D =2), and blade thickness e = 2 mm.m The test is best performed when the vane is pushed beneath the bottom of an pre-drilled borehole.

For a borehole of diameter B, the top of the vane should pushed to a depth of insertion of at least df = 4B. Within 5 minutes after insertion, rotation should be made at a constant rate of 6/minute (0.1/s) with measurements of torque taken frequently. Figure 5-9 illustrates the general VST procedures. In very soft clays, a special protective housing that encases the vane is also available where no borehole is required and the vane can be installed by pushing the encasement to the desired test depth to deploy the vane. An alternative approach is to push two side-by-side soundings (one with the vane, the other with rods only).

Then, the latter rod friction results are subtracted from the former to obtain the vane readings. This alternate should be discouraged as the rod friction readings are variable, depend upon inclination and verticality of the rods, number of rotations, and thus produce unreliable and questionable data.

The general expression for all types of vanes including standard rectangular (Chandler, 1988), both ends tapered (Geonor in Norway), bottom taper only (Nilcon in Sweden), as well as rhomboidal shaped vanes for any end angles is given by:

where iT = angle of taper at top (with respect to horizontal) and iB = angle of bottom taper, as defined in Figure 5-11.

Vane ResultsA representative set of shear strength profiles in San Francisco Bay Mud derived from vane shear tests for the MUNI Metro Station Project are shown in Figure 5-12a. Peak strengths increase from suv = 20 kPa to 60 kPa with depth. The derived profile of sensitivity (ratio of peak to remolded strengths) is presented in Figure 5-12b and indicates 3 < St < 4.

4.9.3 FLAT PLATE DILATOMETER TEST (DMT)

The flat dilatometer test (DMT) uses pressure readings from an inserted plate to obtain stratigraphy and estimates of at-rest lateral stresses, elastic modulus, and shear strength of sands, silts, and clays. The device consists of a tapered stainless steel blade with 18 wedge tip that is pushed vertically into the ground at 200 mm depth intervals (or alternative 300-mm intevals) at a rate of 20 mm/s. The blade (approximately 240 mm long, 95 mm wide, and 15 mm thick) is connected to a readout pressure gauge at the ground surface via a special wire-tubing through drill rods or cone rods. A 60-mm diameter flexible steel membrane located on one side of the blade is inflated pneumatically to give two pressures: .A-reading. That is a lift-off or contact pressure where the membrane becomes flush with the blade face (* = 0); and .B-reading. That is an expansion pressure corresponding to * = 1.1 mm outward deflection at center of membrane. A tiny spring-loaded pin at the membrane center detects the movement and relays to a buzzer / galvanometer at the readout gauge. Normally, nitrogen gas is used for the test because of the low moisture content, although carbon dioxide or air can also be used. Reading .A. is obtained about 15 seconds after insertion and .B . is taken within 15 to 30 seconds later. Upon reaching .B. the membrane is quickly deflated and the blade is pushed to the next test depth. If the device cannot be pushed because of limited hydraulic pressure (such as dense sands), then it can be driven in place, but this is not normally recommended.

Procedures for the Flat Plate Dilatometer Test

Flat Plate Dilatometer Equipment

4.9.4Flat Plate Dilatometer ResuLts

The two DMT readings (po and p1) are utilized to provide three indices that can provide information on the stratigraphy, soil types, and the evaluation of soil parameters: Material Index: ID = (p1 - po)/(po - uo) Dilatometer Modulus: ED = 34.7(p1 - po) Horizontal Stress Index: KD = (p1 - po)/(vo

where uo = hydrostatic porewater pressure and (vo = effective vertical overburden stress. For soil behavioral classification, layers are interpreted as clay when ID < 0.6, silts within the range of 0.6 < ID < 1.8, and sands when ID >1.8.

Example results from a DMT conducted in Piedmont residual soils are presented in Figure 5-16, including the measured lift-off (p0) and expansion (p1) pressures, material index (ID), dilatometer modulus (ED), and horizontal stress index (KD) versus depth. The soils are fine sandy clays and sandy silts derived from the in place weathering of schistose and gneissic bedrock.

Figure 5-16. Example DMT Sounding in Piedmont residual soils (CL to ML) in Charlotte, NC.4.9.5 PRESSUREMETER TEST (PMT)

The pressuremeter test consists of a long cylindrical probe that is expanded radially into the surrounding ground. By tracking the amount of volume of fluid and pressure used in inflating the probe, the data can be interpreted to give a complete stress-strain-strength curve. In soils, the fluid medium is usually water (or gas), while in weathered and fractured rocks, hydraulic oil is used.

The original pressiometer was introduced by the French engineer Louis Menard in 1955. This prototype had a complex arrangement of water and air tubing and plumbing with pressure gauges and valves for testing. More recently, monocell designs facilitate the simple use of pressurized water using a screw pump. Procedures and calibrations are given by ASTM D 4719 with Figure 5-17 giving a brief synopsis. Standard probes range from 35 to 73 mm in diameter with length-to-diameter ratios varying from L/d = 4 to 6 depending upon the manufacturer.

There are four basic types of pressuremeter devices:

1. Prebored (Menard) type pressuremeter (MPMT)

is conducted in a borehole, usually after pushing and removing a thin-walled (Shelby) tube. The MPMT is depicted in Figure 5-17. The initial response reflects a recompression region as probe inflates to meet walls of boring and contact with soil.

2. Self-boring pressuremeter (SBP)

is a probe placed at the bottom of borehole and literally eats its way into the soil to minimize disturbance and preserve the Ko state of stress in the ground. Either cutter teeth or water jetting is used to advance the probe and cuttings are transmitted through its hollow center. The probe has three internal radial arms to directly measure cavity strain, ,c = dr/ro, where ro = initial probe radius and dr = radial change. Assuming the probe expands radially as a cylinder, volumetric strain is related to cavity strain by the expansion: ()V/Vo) = 1 - (1 + ,c)-2

3. Push-in pressuremeter (PIP)

consists of a hollow thick walled probe having an area ratio of about 40 percent. Faster than prebored and SBP above, but disturbance effects negate any meaningful Ko measurements.

4. Full-displacement type (FDP):

Similar to push-in type but complete displacement effects. Often incorporated with a conical point to form a cone pressuremeter (CPMT) or pressiocone.

Procedures for Pressure meter test

Pressure Meter Test Results

The pressuremeter provides four independent measurements with each test:

1. Lift off stress, corresponding to the total horizontal stress, Fho = Po;

2. An "elastic" region, interpreted in terms of an equivalent Young's modulus (EPMT) during the initial loading ramp. An unload-reload cycle removes some of the disturbance effects and provides a stiffer value of E. Traditionally, the elastic modulus is calculated from:

where : V = Vo + (V = current volume of probe, Vo = initial probe volume, (P = change in pressure in elastic region, (V = measured change in volume, and ( = Poissons ratio. Alternative procedures are available to directly interpret the shear modulus (G), as given in Clark (1989).

3. A "plastic" region, corresponding to the shear strength (i.e., an undrained shear strength, suPMT for clays and silts; or an effective friction angle ( for sands).

4. Limit pressure, PL (related to a measure of bearing capacity) which is an extrapolated value of pressure where the probe volume equals twice the initial volume (V = 2Vo). This is analogous to (V = Vo. Several graphical methods are proposed to determine PL from measured test data. One common extrapolation approach involves a log-log plot of pressure vs. volumetric strain ((V /Vo.) and when log((V /Vo.) = 0, then P = PL.

Figure 5-19 shows a representative curve of pressure versus volume from a PMT in Utah. The recompression, pseudo-elastic, and plastic regions are indicated, as are the corresponding interpreted values

of parameters.

Figure 5-19. Menard-type Pressure meter Results for Utah DOT Project.4.9.6 CONE PENETRATION TESTING (CPT)

The cone penetration test is quickly becoming the most popular type of in-situ test because it is fast, economical, and provides continuous profiling of geostratigraphy and soil properties evaluation. The test is performed according to ASTM D-3441 (mechanical systems) and ASTM D 5778 (electric and electronic systems) and consists of pushing a cylindrical steel probe into the ground at a constant rate of 20 mm/s and measuring the resistance to penetration. The standard penetrometer has a conical tip with 60 angle apex, 35.7-mm diameter body (10-cm2 projected area), and 150-cm2 friction sleeve. The measured point or tip resistance is designated qc and the measured side or sleeve resistance is fs. The ASTM standard also permits a larger 43.7-mm diameter shell (15-cm2 tip and 200-cm2 sleeve).

The CPT can be used in very soft clays to dense sands, yet is not particularly appropriate for gravels or rocky terrain. The pros and cons are listed below. As the test provides more accurate and reliable numbers for analysis, yet no soil sampling, it provides an excellent complement to the more conventional soil test boring with SPT measurements.

Most electric/electronic cones require a cable that is threaded through the rods to connect with the power supply and data acquistion system at the surface. An analog-digital converter and pentium notebook are sufficient for collecting data at approximate 1-sec intervals. Depths are monitored using either a potentiometer (wire-spooled LVDT), depth wheel that the cable passes through, or ultrasonics sensor. Systems can be powered by voltage using either generator (AC) or battery (DC), or alternatively run on current. New developments include: (1) the use of audio signals to transmit digital data up the rods without a cable and (2) memocone systems where a computer chip in the penetrometer stores the data throughout the sounding.

Figure 5-6. Geometry and Measurements Taken by Cone and Piezocone Penetrometers.Procedures for the Cone Penetration Test

Piezocone Results

4.10 GEOPHYSICAL METHODS

There are several kinds of geophysical tests that can be used for stratigraphic profiling and delineation of subsurface geometries. These include the measurement of mechanical waves (seismic refraction surveys, crosshole, downhole, and spectral analysis of surface wave tests), as well as electromagnetic techniques (resistivity, EM, magnetometer, and radar). Mechanical waves are additionally useful for the determination of elastic properties of subsurface media, primarily the small-strain shear modulus. Electromagnetic methods can help locate anomalous regions such as underground cavities, buried objects, and utility lines. The geophysical tests do not alter the soil conditions and therefore classify as nondestructive, and several are performed at the surface level (termed non-invasive).

4.10.1Seismic Refraction (SR)

Seismic refraction is generally used for determining the depth to very hard layers, such as bedrock. The seismic refraction method involves a mapping of Vp arrivals using a linear array of geophones across the site, as illustrated in Figures 5-22 and 5-23 for a two-layer stratification. In fact, a single geophone system can be used by moving the geophone position and repeating the source event. In the SR method, the upper layer velocity must be less than the velocity of the lower layer. An impact on a metal plate serves as a source rich in P-wave energy. Initially, the P- waves travel soley through the soil to arrive at geophones located away from the source. At some critical distance from the source, the P-wave can actually travel through soil-underlying rock-soil to arrive at the geophone and make a mark on the oscilloscope. This critical distance (xc) is used in the calculation of depth to rock. The SR data can also be useful to determine the degree of rippability of different rock materials using heavy construction equipment. Most recently, with improved electronics, the shear wave profiles may also be determined by SR.

The velocity of Pwave given by: where:E = modulus of elasticity of medium.

( = unit weight of the medium,

g = gravity accelaration.

( = Poisson's ratio.Need to determine the value of velocity (v) and the thickness of each layer (Zi). Procedures:(i) Times of first arrival; t1, t2, t3 ... at various points and x1, x2, x3, ... from point of impact, (ii) Plot the graph of time (t) vs distance (x).(iii) Determine slopes ab, be, cd ... by using slopes 1/v, and determine the v values.

( vi) note that the value of xc, Ti1 and Ti2 can be estimated from figure 1.21b

Fig. 1.21 : Seismic refraction surveyProcedures for the Seismic Refraction Survey

Fig. 1.22 : Example4.10.2Cross-Hole Seismic Survey

The velocity of shear waves created as the result of an impact to a given layer of soil can be effectively determined by the cross-hole seismic survey (Stokoe and Woods, 1972). The principle of this technique is illustrated in Figure 2.36, which shows two holes drilled into the ground a distance L apart. A vertical impulse is created at the bottom of one borehole by means of an impulse rod. The shear waves thus generated are recorded by a vertically sensitive transducer. The velocity of shear waves can be calculated as

Where t = travel time of the waves

The shear modulus Gs of the soil at the depth at which the test is taken can be determined from the relation or

Where vs = velocity of shear waves, ( = unit weight of soil ,

g = acceleration due to gravity

the shear modulus is useful in design of foundations to support vibrating machinery and the like.

Procedures for the Cross holes seismic survey

4.10.3Resistivity SurveyAnother geophysical method for subsoil exploration is the electrical resistivity survey. The electrical resistivity of any conducting material having a length L and an area of cross section A can be defined as where R = electrical resistanceThe unit of resistivity is the ohm-centimeter or ohm-meter. The resistivity! various soils depends primarily on their moisture content and also on the com: tration of dissolved ions in them. Saturated clays have a very low resistivity; drysci. and rocks have a high resistivity. The range of resistivity generally encountered: various soils and rocks is given in Table 2.9.

The most common procedure for measuring the electrical resistivity of a soil profile makes use of four electrodes driven into the ground and spaced equally along a straight line. The procedure is generally referred to as the Wenner method (Figure 2.37a). The two outside electrodes are used to send an electrical current I (usually a dc current with nonpolarizing potential electrodes) into the ground. The current is typically in the range of 50-100 milliamperes. The voltage drop, V, is measured between the two inside electrodes. If the soil profile is homogeneous, its electrical resistivity is

In most cases, the soil profile may consist of various layers with different rest tivities, and equation above will yield the apparent resistivity. To obtain the actual resistivity of various layers and their thicknesses, one may use an empirical method that involves conducting tests at various electrode spacings (i.e., d is changed). The sum of the apparent resistivities, Sp, is plotted against the spacing d, as shown in Figure 2.37b. The plot thus obtained has relatively straight segments, the slopes of which give the resistivity of individual layers. The thicknesses of various layers can be estimated as shown in Figure 2.37b.The resistivity survey is particularly useful in locating gravel deposits within a fine-grained soil. Figure 2.37 Electrical resistivity survey: (a) Wenner method; (b) empirical method for determining resistivity and thickness of each layer

Fig Resistivity Equipments

Fig Arrangement of electrode 4.12SITE INVESTIGATION REPORT

Information on subsurface conditions obtained from the boring operation is typically presented in the form of a boring log (boring record). A continuous record of the various soil or rock strata found at the boring is developed. Description or classification of the various soil and rock types encountered and changes in strata and water level data are considered the minimum information that should constitute a log. Any additional information that helps to indicate or define the features of the subsurface material should also appear on the log, Items such as soil consistency and strength or compressibility can be included. "Field" logs typically consist of the minimum informationclassification, stratum changes, and water level readings.

Information to be recorded on the borehole logs1.General informationThe essential information which needs to be recorded on the log is as follows:a.Borehole number: This should be unique to the site and kept as simple as possible without extraneous ciphers.b.Location: (i) Site, including project name, town country or state name where necessary(ii) Grid Reference which should always be stated to at least 1 Om accuracy. Appropriate local co-ordinate systems should be applied (iii) Elevation relative to C.O. for the ground level at the borehole site to an accuracy of 0.05m.(iv) Orientation of the borehole given as an angle to the horizontal (-ve upwards, +ve downwards) and azimuth (0 to 360 clockwise relative to Grid North).c.Drilling technique: The following should be stated(i) The method of penetration and flush system(ii) The make of machine with the model number(iii) The type of core barrel and bitd.Contract details: The following should be noted (with the agreement of the client)(i) Name of site investigation contractor(ii) Name of client or authority(iii) Job reference number(iv) Name and profession of loggere.Miscellaneous: There should be an opportunity for relevant miscellaneous information to be included in the log.2.Drilling progressThe following data need to be recorded:-a.Rate of drilling: The depth of the borehole at the completion of each day or shift and the limits of each run of the core barrel should be recorded. The actual penetration rate for each run or part of a run should be measured. Core diameter and changes of core size (recorded by reference to B.S. 4019 or as metric dimension). b.Casing: It is essential that the progress of installation of the casing be recorded relative to the depth of the borehole; the diameter of the casing need not be recorded except where relevant to interpretation of the data.c.Flush returns:The character and proportion of the circulation medium returning to the surface should be recorded.d.Standing water level: This should be recorded before and possibly after each drilling shift.3.Descriptive geologyThe following factors have to be incorporated in a log for adequate engineering geological description: -(i) systematic description(ii) alteration weathering state(iii) structure and discontinuities(iv) assessment of rock material strength(v) other features, including stratigraphy

Figure 10a : Example of boring log

1.2

1.3

Figure 2.5 Hollow Stem Auger.

Figure 3-7: Split-Barrel Samplers: (a) Lengths of 457 mm (18 in) and 610 mm (24 in); (b) Inside diameters from 38.1 mm (1.5 in) to 89 mm (3.5 in).

Figure 3-8: Split Barrel Sampler: (a) Open sampler with soil sample and cutting shoe; (b) Sample jar, split-spoon, shelby tube, and storage box for transport of jar samples.

Figure 3-9: Split Barrel Sampler.

(a) Stainless steel and brass retainer rings (b) Sample catchers.

Figure 3-12: Shelby Tube Sealing Methods.

(a) Microcrystalline wax (b) O-ring packer.

Figure 3-13: Piston Sampler.

(a) Picture with thin-walled tube cut-out to show piston, (b) Schematic (After ASTM D4700).

Figure 3-14: Pitcher Tube Sampler.

Figure 3-15: Pitcher Sampler. (a) Sampler Being Lowered into Drill Hole; (b) Sampler During Sampling of Soft Soils, (c) Sampler During Sampling of Stiff or Dense Soils. (Courtesy of Mobile Drilling, Inc.)

Figure 3-16: Denison Double-Tube Core Barrel Soil Sampler

(Courtesy of Sprague & Henwood, Inc.)

Fig. 5-11 : Definitions of Vane Geometries for Tapered & Rectangular Blades.

Fig. : Selection of Vane Shear Blades

Figure 10b : Example of Summary of laboratory test results

Figure 10c : Example of Summary of fieldwork performed

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