ch1 site investigation (sem2 200910)

8/20/2019 CH1 Site Investigation (SEM2 200910)

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SITE INVESTIGATION PRACTICE

1.0 Introduction

To design a foundation that will support a structure, an engineer mustunderstand the types of soil deposits that will support the foundation.

Moreover, foundation engineers must remember that soil at any sitefrequently is non-homogeneous; that is the soil profile may vary.

Soil mechanics theories involve idealized conditions, so the application ofthe theories to foundation engineering problems involves a well judgedevaluation of site conditions and soil parameters.

To do this requires some knowledge of the geological process by whichthe soil deposit at the site was formed, supplemented by subsurfaceexploration.

Good professional judgment constitutes at essential part of geotechnicalengineering—and it comes only with practice.

1.1 Definition of Soil Exploration

The design of a foundation, an earth dam, or a retaining wall cannot be madeintelligently unless the designer has at least a reasonably accurate conception ofthe physical properties of the soils involved. The field and laboratoryinvestigations required to obtain this essential information constitute the soi lexplorat ion. Until about the 1930s soil exploration was consistently inadequatebecause 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 andthe refinements in the techniques for performing the investigations are often quiteout of proportion to the practical value of the results. To avoid either of theseextremes the exploratory program must be adapted to the soil conditions and to


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extremes the exploratory program must be adapted to the soil conditions and to

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i. Site selection.

The construction of certain major projects, such as earth dams, is dependent onthe 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 permeabilityof the sub-soil, or to slope instability may make the final cost of the constructionprohibitive. Since the safety of lives and property are at stake, it is important toconsider the geotechnical merits or demerits of various sites before the site ischosen 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 goodlocation from the point of view of the eventual user, and with the planningconsent for its proposed use are of over-riding importance. For medium-sizedengineering works, such as motorways and multistorey structures, thegeotechnical problems must be solved once the site is available, in order to allowa safe and economical design to be prepared.

iii. Temporary works design.The actual process of construction may often impose greater stress on theground than the final structure. While excavating for foundations, steep sideslopes may be used, and the in-flow of groundwater may cause severe problemsand even collapse. These temporary difficulties, which may in extremecircumstances prevent the completion of a construction project, will not usuallyaffect 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


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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 obtainparameters for design.

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

1.5 Site Investigations

The Four Major Steps or Components of a Site’s Investigation. For a majorproject (a tunnel, large bridge, tall building, etc., will require four phases for itssite investigation:

Phase 1: Literature Search.This phase collects all the existing information of the site and the structure. Forthe site, it involves aerial photos, surveys, previous geotechnical data, buildingcodes and adjacent structures. For the structure, it requires all the majorstructural data of the building.

Phase II: Reconnaissance Sub-surface Exploration.

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


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1.6 INVESTIGATION AND BORING METHODS

1.6.1 Introduction

Many different techniques are available for site investigation. The methodemployed will depend on many factors such as depth required, area to becovered, ease of access, etc. On large jobs preliminary borings are used tofurnish overall subsoil surveys followed by final borings so soil or rock profilesmay be determined at the most useful orientations. In general, explorationcontracts should be open ended so that intermediate borings may be added in

areas that prove to be critical.

1.6.2 Soil Drilling

A wide variety of equipment is available for performing borings and obtaining soilsamples. The method used to advance the boring should be compatible with thesoil and groundwater conditions to assure that soil samples of suitable quality areobtained. Particular care should be exercised to properly remove all slough orloose soil from the boring before sampling. Below the groundwater level, drillingfluids are often needed to stabilize the sidewalls and bottom of the boring in softclays or cohesionless soils . Without stabilization, the bottom of the boring mayheave or the sidewalls may contract, either disturbing the soil prior to sampling orpreventing the sampler from reaching the bottom of the boring. In mostgeotechnical explorations, borings are usually advanced with solid stemcontinuous flight, hollow-stem augers, or rotary wash boring methods. These

methods are often augmented by in-situ testing .Assuming access and utilityclearances have been obtained and a survey base line has been established inthe field, field explorations are begun based on the information gained during theprevious steps. Many methods of field exploration exist; some of the more


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1.6.4 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.


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Fig 2 :Determination of the minimum depth of boring

# For hospitals and office building:

For light steel or narrow concrete building;Db(m) = 3S

0.7 ....................(1)Db(ft)-10S

0.7.......................(2)

For heavy steel or wide concrete building;Db (m) = 6S

0.7 .....................(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 subsoilcondition.


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i. Hand Auger Borings

The hand auger provides a light, portable method of sampling soft to stiff soilsnear 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

1.2

1.3


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Hand augers are used by one or two men, who press down on the cross-bar asthey 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 doessuffer from several disadvantages.

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

Barrel augers are now rarely seen, but were formerly used with the lightpercussion rig when progress through clays was made using a shell. Theyallowed the base of the borehole to be very effectively cleaned before samplingtook place. Because they are heavy they require a tripod for raising and loweringthem in the borehole. When lowered to the bottom of the hole they were turnedby hand.

In stiff or very stiff clays, hand-auger progress will be very slow, and the depth ofboring 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. Inuncemented sands or gravels, it will not be possible to advance the hole belowthe water table, since casing cannot be used and the hole will collapse either ontop of the auger (which makes it difficult to recover the auger from the hole) orwhen the auger is being removed. Only samples of very limited size can beobtained from the hole. In addition, it will not be possible to carry out standardpenetration tests without a frame to lift the trip hammer and weight, so that noidea of the relative density of granular deposits can be obtained.


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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 footto 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. Identificationof strata changes is difficult. Useful in proving known strata.


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SPT and undisturbed samples are obtained through the hollow drill stem, whichacts like a casing to hold the hole open.

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


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(d) Bucket or Grab Auger.

This type of auger drills a large diameter hole, with or without casing. A largeplant is involved, and it is infrequently used in investigation work.


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down by turning, as boring proceeds,but it may be necessary to drive it.

Samples can be obtained and in situtests made through the casing fromtime to time. In this method of boring,unless continuous samples aretaken, which defeats the main objectof the technique, speed, the onlyevidence of the strata beingpenetrated is the very fine soil

particles being carried to the surfaceby the flow of water. Wash boringsare normally made using casingbetween 50 mm and 150 mmdiameter, above this size, the pumpunit required is generally too large.The technique is generally used as afast and consequently cheap method

of supplementing informationobtained from a series of dry sampleborings. It is particularly useful forobtaining samples or carrying out insitu tests at some depth in knowstrata, e.g. in a clay layer, below asand stratum. Disturbance of the ground by the water jet may in some casesextend two feet or more below the casing, and care should be taken in samplingand testing to ensure that this is not carried out in the disturbed area. The use ofwash boring without adequate dry sample boring control should be avoided.


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1.7 SAMPLING

There are 2 types of soil samples there are disturbed and undisturbed


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materials, type and condition of the sampling equipment used, the skill of thedrillers, and the storage and transportation methods used.

1.7.3 Common Types of Samplers

The cuttings or washings from exploratory drill holes are inadequate to furnish asatisfactory conception of the engineering characteristics of the soilsencountered, or even of the thicknesses and depths of the various strata. On thecontrary, such evidence more often than not is grossly misleading and has beenresponsible for many foundation failures.

Proper identification of the subsurface materials requires that samples berecovered containing all the constituents of the materials in their properproportions. Moreover, evaluation of the appropriate engineering properties, suchas the strength, compressibility, or permeability, may require the performance oflaboratory 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 betolerated and with increasing diameter of sample. Therefore, on small projects orin the initial exploratory stages of large or complex projects, it is usuallypreferable to obtain relatively inexpensive, fairly intact samples from theexploratory drill holes.

On the basis of the information obtained from these samples, the necessity formore elaborate sampling procedures can be judged.Types of Soil Sampler

• A wide variety of samplers are available to obtain soil samples for


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i. Split Barrel Sampler

• Used to obtain disturbed samples in all types of soils


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• 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 barrelmay be separated and the sample may be extracted easily.

• The soil sample is removed from the split-barrel sampler it is either placedand 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 soiltypes.

• 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

th f l it bl f il id tifi ti d l l ifi ti

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


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ii. Thin Wall Sampler (Shelby)

• To obtain relatively undisturbed samples of cohesive soils for strength andconsolidation testing.

• Commonly, it has a 76 mm (3.071in) outside diameter & a 73 mm (2.875in) 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 arerequired and sampling disturbance must be reduced

Figure 3-9: Split Barrel Sampler.(a) Stainless steel and brass retainer rings (b) Sample catchers.


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• Manufactured with a beveled front edge for cutting a reduced-diametersample [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, anddepth interval.

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

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

• Putting sample information on both the tube and the end cap facilitatesretrieval of tubes from laboratory storage and helps prevent mix-ups in thelaboratory when several tubes may have their end caps removed at thesame 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 plasticdisk to protect the ends of the sample (Figure 3-12a).


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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.

Figure 3-13: Piston Sampler.(a) Picture with thin-walled tube cut-out to show piston, (b) Schematic


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iv. Pitcher Tube Sampler

• Used in stiff to hard clays andsoft rocks, and is well adapted tosampling deposits consisting ofalternately hard and soft layers.

• This sampler is pictured in Figure3-14 and the primarycomponents shown in Figure 3-15a and these include :

– an outer rotating corebarrel with a bit and aninner stationary,

– spring-loaded,

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

Figure 3-14: Pitcher Tube Sampler.


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v. Denison Sampler

• Is similar to a pitchersampler except that theprojection of the samplertube ahead of the outerrotating barrel is manuallyadjusted beforecommencement of samplingoperations, rather than

spring-controlled duringsampler penetration.

• The basic components of thesampler (Figure 3-16) are :

– an outer rotating corebarrel with a bit,

– an inner stationarysample 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 ahardened steel saw tooth bit.

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

Figure 3-16: Denison Double-TubeCore Barrel Soil Sampler

(Courtesy of Sprague & Henwood,

Inc.)


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vi. Block sampling

Block sampling has traditionally involved the careful hand excavation of soilaround the sample position, and the trimming of a regular-shaped block. Thisblock is then sealed with layers of muslin, wax and clingfilm, before beingencased in a rigid container, and cut from the ground. The process is illustratedin Fig. 6.5. A similar process can be carried out in shafts and large-diameterauger 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 traditionallybeen available for testing from deep deposits of clay. In the past decade,however, there has been an increasing use of rotary coring methods to obtainsuch samples. When carried out carefully, without displacing the soil, rotarycoring is capable of producing very good quality samples. When the blocks arecut by hand then obviously the pit will be air-filled, but when carried out in aborehole it will typically be full of drilling mud.

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

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


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Table 7 : Types of sampler generally used in Malaysia


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(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: Part3.1 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 54.1 1-D consolidation test4.2 Swelling and collapse tests4.3 Permeability by constant head4.4 Dispersibility

5. Consolidation & Permeability Tests in Hydraulic Cells & with pore pressuremeasurements: BS 1377: Part 65.1 Consolidation Properties using hydraulic cell5 2 Permeability in hydraulic consolidation cell


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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 thesubsequent sections.

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


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Table below shows common field tests practice in Malaysia.

1.9.1 STANDARD PENETRATION TEST

The standard penetration test (SPT) is performed during the advancement of asoil boring to obtain an approximate measure of the dynamic soil resistance, aswell as a disturbed drive sample (split barrel type).The test was introduced by the Raymond Pile Company in 1902 and remains


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gravel deposits nor soft clays. The fact that the test provides both a sample anda number is useful, yet problematic, as one cannot do two things well at the

same time.


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1.9.2 VANE SHEAR TEST (VST)

The vane shear test (VST), or field vane (FV), is used to evaluate the inplaceundrained shear strength (suv) of soft to stiff clays & silts at regular depthintervals of 1 meter (3.28 feet). The test consists of inserting a four-bladed vaneinto the clay and rotating the device about a vertical axis, per ASTM D 2573guidelines. Limit equilibrium analysis is used to relate the measured peak torqueto the calculated value of su. Both the peak and remolded strengths can be

measured; their ratio is termed the sensitivity, S t. A selection of vanes is availablein terms of size, shape, and configuration, depending upon the consistency andstrength characteristics of the soil. The standard vane has a rectangular


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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 isgiven by:

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


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Vane Results

A representative set of shear strength profiles in San Francisco Bay Mud derivedfrom 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. Thederived profile of sensitivity (ratio of peak to remolded strengths) is presented inFigure 5-12b and indicates 3 < St < 4.

Fig. 5-11 : Definitions of VaneGeometries for Tapered &

Rectangular Blades.

Fig.5-10 : Selection ofVane Shear Blades


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1.9.3 FLAT PLATE DILATOMETER TEST (DMT)

The flat dilatometer test (DMT) uses pressure readings from an inserted plate toobtain stratigraphy and estimates of at-rest lateral stresses, elastic modulus, andshear strength of sands, silts, and clays.

The device consists of a tapered stainless steel blade with 18° wedge tip that ispushed 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) isconnected 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 steelmembrane located on one side of the blade is inflated pneumatically to give twopressures: .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 correspondingto * = 1.1 mm outward deflection at center of membrane. A tiny spring-loaded pinat 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 about15 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 thenext test depth.


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Procedures for the Flat Plate Dilatometer Test

Flat Plate Dilatometer Equipment


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1.9.4 FLAT PLATE DILATOMETER RESULTS

The two DMT readings (po and p1) are utilized to provide three indices that canprovide information on the stratigraphy, soil types, and the evaluation of soilparameters:

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 verticaloverburden 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 presentedin 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 inplace weathering of schistose and gneissic bedrock.


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Example on dilatometer test

Notes :

From McCarthy (2007) :

Dilatometer modulus,

1

7.34 E

p E D ; where E – soil modulus of

elasticity and μ is Poisson ratio

Material index, 00 u p

p I D

; where u0 – pore water pressure at test

depth and 01 p p p ; Δp – the difference initial and final dilatometer

pressure.

Given :

Dilato testing is performed at a planned construction site as part ofthe subsurface insvestigation. The dilatometer instrument gaugeindicates pressure in bars.

At one location and depth, the corrected dilatometer test pressurereadings are :


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Use the following figure :

From the figure : with I =1 26 and E =225 55


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1.9.5 PRESSUREMETER TEST (PMT)

The pressuremeter test consists of a long cylindrical probe that is expandedradially into the surrounding ground. By tracking the amount of volume of fluidand pressure used in inflating the probe, the data can be interpreted to give acomplete 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 andplumbing with pressure gauges and valves for testing. More recently, monocelldesigns 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 abrief 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)


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3. Push-in pressuremeter (PIP)

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

4. Full-displacement type (FDP):Similar to push-in type but complete displacement effects. Oftenincorporated with a conical point to form a cone pressuremeter (CPMT) orpressiocone.

Procedures for Pressure meter test


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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 ofthe disturbance effects and provides a stiffer value of E. Traditionally, the elastic

modulus is calculated from:

h V V + V t l f b V i iti l b l P


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

Figure 5-19. Menard-type Pressure meter Results for Utah DOT Project.

1.9.6 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 ofgeostratigraphy and soil properties evaluation. The test is performed accordingto ASTM D-3441 (mechanical systems) and ASTM D 5778 (electric andelectronic systems) and consists of pushing a cylindrical steel probe into the


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Most electric/electronic cones require a cable that is threaded through the rods toconnect with the power supply and data acquistion system at the surface. An

analog-digital converter and pentium notebook are sufficient for collecting dataat 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 canbe powered by voltage using either generator (AC) or battery (DC), oralternatively run on current. New developments include: (1) the use of audiosignals to transmit digital data up the rods without a cable and (2) memocone

systems where a computer chip in the penetrometer stores the data throughoutthe sounding.


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Figure 5-6. Geometry and Measurements Taken by Cone and PiezoconePenetrometers.

Procedures for the Cone Penetration Test


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Piezocone Results

Example on cone penetrometer

Given :

The CPT reading at the depth of 8m is shown below. Groundwater isfound at 3m The soil unit weight is 16kN/m3 Geometrical cone


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Total overburden pressure,σt = 8m(16kN/m

3) = 128kPa

Effective overburden pressure,σ’ v = 3m(16kN/m

3) + 5m(16-9.81kN/m3) = 48 + 31=79kPa

Unit cone tip resistance,

kPakPa

auqq ct

5.62275.0190600

12

Parameter t t

q 622.5kPa – 128kPa = 494.5kPa

Normalized cone tip resistance,

25.679

5.494

'

kPaqQ

v

t t tipcone

Normalized friction ratio,

%1.6%100

5.494

30%100

kPa

kPa

q

f F

t t

sr

From Figure: (SBT)Fr=Type 3 (clay, silty clay)


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1.10.1 Seismic 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 Figures5-22 and 5-23 for a two-layer stratification. In fact, a single geophone systemcan 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 thelower layer. An impact on a metal plate serves as a source rich in P-waveenergy. 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 thegeophone and make a mark on the oscilloscope. This critical distance (xc) isused in the calculation of depth to rock. The SR data can also be useful todetermine the degree of rippability of different rock materials using heavyconstruction equipment. Most recently, with improved electronics, the shear waveprofiles may also be determined by SR.

Procedures for the Seismic Refraction Survey


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The velocity of P-wave given by :

1211

g

E v

Where : E – modulus of elasticity of medium

- unit weigh of the medium

g – gravity accelerationμ – Poisson’s ratio

Need to find the value of velocity, v and the thickness of each layer, Z i Procedures :

1. Times of first arrival; t1, t2, t3, …….. at various points x1, x2, x3,…….. from point of impact

2. Plot graph of time,t vs distance x

3. Determine slopes ab, bc, cd, ……. Using slopes = 1/v, find value of v

4. Determine thickness of top layer

Using thickness,c

xvv

vv Z

12

12

1

2

1

5. Determine thickness of second layer

Using2

2

2

3

23

13

2

1

2

3

122 22

1

vv

vv

vv

vv

Z T z i

6 Note that the value of x Ti1 and Ti2 can be estimated from


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Example 2.2

The results of a refraction survey at a site are given in the following table.Determine the P-wave velocities and the thickness of the material encountered.Table Example 2.2

Distance from thesource of disturbance (m)

Time of first arrival(sec x 10-3)

2.55

7.5101520253035

4050

11.223.3

33.542.450.957.264.468.671.1

72.175.5

The figure is time of first arrival vs distance (x)

The plot has three straight line; 0a, ab and bc.

Slope of each straight line is inverse velocity, 1/v.

.255

1023

t

1 3or

di

time


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m Z 94.35.102288.814

2288.814

2

11

Thickness of layer 2 :

With Ti2 = 65 x 10-3 sec (Figure Example 2.2)

22

22

3

2

8.8144214

8.8144214

2284214

228421494.321065

2

1

Z

= m66.1247.8300345.0065.021

Therefore rock layer lies at a depth of Z1 + Z2 = 3.94 + 12.66 =16.60 mmeasured from ground surface


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mV V

V V X Z Z 68.15

8.8144214

8.8144214

2

3094.385.0

285.0

23

23212

Table 1.7 Range of P-wave velocity in various soils and rocks

Type of soil or rockP-wave velocity

m/sec Ft/sec

Soil :

Sand, dry silt, and fine-grained top soil Alluvium

Compacted clays, clayey gravel, anddense clayey sandLoessRock :Slate and shale

SandstoneGraniteSound limestone

200-1,000500-2,000

1,000-2,500

250-750

2,500-5,000

1,500-5,0004,000-6,0005,000-10,000

650-3,3001,650-6,600

3,300-8,200

800-2,450

8,200-16,400

4,900-16,40013,100-19,70016,400-32,800

1.10.2 Cross-Hole Seismic Survey

The velocity of shear waves created as the result of an impact to a given layer ofsoil can be effectively determined by the cross-hole seismic survey (Stokoe andWoods, 1972). The principle of this technique is illustrated in Figure 2.36, which


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Procedures for the Cross holes seismic survey

1.10.3 Resistivity Survey

Another geophysical method for subsoil exploration is the electrical resistivitysurvey. The electrical resistivity of any conducting material having a length L andan area of cross section A can be defined as

where R = electrical resistance


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

In most cases, the soil profile may consist of various layers with different resttivities, and equation above will yield the apparent resistivity. To obtain the actualresistivity of various layers and their thicknesses, one may use an empiricalmethod that involves conducting tests at various electrode spacings (i.e., d ischanged). The sum of the apparent resistivities, Sp, is plotted against the

spacing d, as shown in Figure 2.37b. The plot thus obtained has relativelystraight segments, the slopes of which give the resistivity of individual layers. Thethicknesses of various layers can be estimated as shown in Figure 2.37b.The resistivity survey is particularly useful in locating gravel deposits within afine-grained soil.


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Fig Resistivity Equipments


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1. 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 withoutextraneous 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 Omaccuracy. Appropriate local co-ordinate systems should be applied

(iii) Elevation relative to C.O. for the ground level at the borehole site to anaccuracy of 0.05m.

(iv) Orientation of the borehole given as an angle to the horizontal (-veupwards, +ve downwards) and azimuth (0° to 360° clockwise relativeto 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 bit

d. 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


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b. Casing:It is essential that the progress of installation of the casing be recorded relative tothe depth of the borehole; the diameter of the casing need not be recordedexcept where relevant to interpretation of the data.

c. Flush returns:The character and proportion of the circulation medium returning to the surfaceshould be recorded.

d. Standing water level:

This should be recorded before and possibly after each drilling shift.

3. Descriptive geology

The following factors have to be incorporated in a log for adequate engineeringgeological description: -

(i) systematic description

(ii) alteration weathering state(iii) structure and discontinuities(iv) assessment of rock material strength(v) other features, including stratigraphy


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Tutorial 1

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QUIZ NO.1

Given a site that is proposed for the development of a housing areathat consist of two major types of soil :

Part A : hilly area that most soil are residual granite thatcontain mostly of granular type of soil.

Part B : valley part that mostly covered by marin clay

Suggest a list of procedures that would be practical to implement thesoil investigation in both parts.

(20 marks)

Vibro-Replacement extends the range of soils that can be improved by vibratory

techniques to include cohesive soils. Reinforcement of the soil with compacted granular

columns or "stone columns" is accomplished by the top-feed method

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MNMAR’07 - EDITED BY AKS AND PM IR AZIZAN SEM 2 200910 69

Figure 10b : Example of Summary of laboratory test results

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Figure 10c : Example of Summary of fieldwork performed

ch1 site investigation (sem2 200910)

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