unit_1-2 gEO tECHNICAL eNGINEERING ii cHAPTER 1 & 2

7/27/2019 unit_1-2 gEO tECHNICAL eNGINEERING ii cHAPTER 1 & 2

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-Pi.essuremetertest The bore hole Pressuremetertest is an insitu test developed around 1$56[lenard (1956)l to measure the.strengthand deformation draracleristics of the xfl. The Pressuremeter is used in Sub-soil lnvestigation work for finding theinsitu stress{eformation draraderistics of rock, grarrel, sand, silt and day deposits belour ground level, belovr orabore the ground water level. Witr the help of prcssuremeter. ontinuously tre stressdebrmation dtarac{erislicsareobtainedfromthenatural stateofsoil undergradualll increasingradial stress.

Before starlirq lhe actual pressurenieter test, proper planning b needed to decjde about he,location of the tests.ftre test is dde at difiercnt thpths in a treshli dritteO'Uorehote with the help of a prcssurcmeter wtrictr consists of' an elpandable probe wi$r a measuring cell at the centre and two guard cells at lhe top and bottom. The probe isinsertld OS 1.9) in a pre-bored hole and is eryanded in volume either by liquid or air pressure untilthe soilfails orthe etganded volume of the measuring cell readres twice the volume of the cavity. The guard cells are used lominimlie the end efecl on the measuring cell. To prevent caving in the borehole, M.S casing can be provided. Thebottom of the casing is kept at least lm above the desired test depth. Depending upon the soil condition it is alsopossible to ddll the hole 2m b 5m belour the caslng and do successive pressuremeter test. The typical dimensionsof the probe and the *"Y:olf t'ffi:.T'#- of pressuremeer probe and Borehore

Holedesignation Diameter ofprobe{mm} L.(ml L(m) Borehole diarnete r(mrn)Nominal MaximumAX 44 36 66 46 52BX 58 21 42 60 66NX 70 25 50 72 48

Fig: Menard type Pressuremeter..

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Ahifisls of test results

i']il:affrtflffi oblahect by conduding pressuremebrtests at mrious depths, typicat pressure fietds curve for stress vs. deformation is

(1) !:i:q"qbftshtng phase. tom the origin to point A; (2)end{3) theplastic phase, from point B to -point C. eier me3,ry,"Ftts,'#H,fPfr $h]n1f *";fi f, :lH&ffi [i """'l[ilit whidr tre vorume of rhe borehore cavity tras n ilv-rui",n"d'i6'iti'initi"rposition' and is given lhe ooordinares' n!, o'. The pseydo+hsric phase, the straighl-line portion of lherrve between points A and B, is dubbed so uecause'or ns resemuia-nce-ti'ttre etastic behavior of steel oroncrete. Point B is the point

atrvhk$ cneep pressure has beenreac*red, and is g]::l tF coordinates, vt , ?1.m"pbstic phase begins! point B and extends to point c, which'ia ".ni'ptoti" to tfi'fiirft*uL. poinr c, which is given thecoordinales vr, Pl. is defned as- the point where the pressure remains consliant despite increasing'volume' The limit pressure is defi ed as tte pilssuie ,.idrir!,JiJiri;il'il measuring celr by an amountutbeyond the votume requlred to inf,ate the pressurem",", (V") and to push the borehole wall back to itsoriginal position ( y"). the pressuremeter can be used to aid in the design of foundations for all types ofoils' Induding residuat soits. Ttre seniemens or f*;"G; L"'il'irltirated usiqg a deformationodulus, E , which can be derived torrr n" pseuoo*lastic p-rras" lii silignt-rine portion) of the loadeformatlon diagram.

Fig. Pressuremeter Field Curye

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where,( vr, F*;and1vf, ,FITarelhevolumeandthepressureatthepointAandBrespecrivelyu^d 4=volume of measuring cell in its natural state (535 ml).The value of lldepends on the size of the borehole. The injected volume at the limit pressure h) ," rnrs

4* u'= zvc + Ycwhete, vo=volume required to inflate pressuremeter and push soil to its origina!trposition; and rr= initial volume of the measuring cell. The allowable bearing capaqity of clayey soils forshallow and deep foundations is generally delermined from the pressure test results by the empirical andsemFempirical methods. For shallow foundations, the illowable bearing capacity may be considered as,

for deep foundations, the allowable fractional resistance may be taken as

t *hr'* zgThus, the pressuremeter gives in-situ lateral stresses in the ground, the stress strain behavior and the strengthof the soil at different depths. The test takes only 1G15 minutes after drilling operation. Since the results areavailable within a short time it is possible to arrive al quick conclusions regarding the suitability of the site to beadoptei.

Plate loadtest i'he plate lv-.?d trisl is a semi-direct meth,.-d to measure the allo+rable pressure of soil to induce a gi\ren amount ofsettleriiel'li. piaies, rounci or square, varying in sizes, from 30 to 60 cm ald thickness of about 2.5 cm are employed forthe test,The load on the plate is applied by making use of a hydraulic jack. The reaction of the jack load is taken by a crossbeam oi a steel truss anchored suitrably at both lhe ends. The settlement of the plate is m-easured by a set of ihree dialWySgs o-f sensitivity 0.02mm placed at 12Oo aparL The dial gauges are fixed to independent supports which do not getdisturbed during the test. Fig shows the anangement for the plate load test.Procedure:The method of performing the test is essentially as follows:Excavate a pit of size not less than 5 times the size of the plate. The bottom of the pit coinciCes with levei of thefoundation.lf water table is abol'e the level of foundation, pump out the water carefully and it should be kept just at the level of thefoundation.A suitable sizc of the plate is selected forthe teSt. Normally a plate of size 30crn is used in sandy soils and bigger sizein clay soils. The ground should be leveled and the plate is seated over the ground.A. seating load of "oor, 7081r/fH2i" firrt placed and released after sometime. A higher load is next placed on theplate and setllements are recorded by means of the dial gauges.Observations ol gvery load increment shall be taken until the rate of setilement is less than 0.25mm per hour. Loadincr.ements shall be approximately one-fifth oJ lhe estimated safe bearing capacity of the soil. The average of thesettlements recorded by 2 or 3 dial gauges tbken as the settlements of the plate for each of the load incrlment.The test should continue until a total settlement of 2.5cm or the seftlement at which the soil fails, whichever is earlier, isobtained. After the loailis increased, the elastic rebound of the soil should be recorded.

vr=Yr*

4a-+

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LimitationsSince a plate load test is of short duration, consolidation settlements cannot be predicted. Thetest gives the value of immediate settlements only. lf the underlying soil is sandy in natureimmediate settlement can be traken as total settlement. lf the soil is of clayey type, the immediatesettlement is only a part of the total settlement. Load tests, therefore do not have muchsignificance in clayey s'oits to determine allowable pressure on the basis of seftlement criterion.Plate load test results should be used with caution and the present pradige is not to rcly too much on thistest. lf the soil is not homogenous to a great depth, plate load tests give very misleading results.Plate load tests is not at all recommended in soils which are not homogenous at least to a depth equal to 1.5to 2 times lhe width of the prototype foundation.Plate load lests should not be relied on to determine the ultimate bearing capacity_ of .sandy soils. as thescale effeds give misleading results. However, when the tests are carried on clay soils, the. ultimate bearingcapacrty as ttltermined by the test may be taken as equal lo that of the foundation since the bearingcapacity of clay is essentially lndependent of the footing size'The plate load test is possibly the only way of determining lhe allowable bearing pressures in -gravelly soildeposits. For tests on iuctr sdil deposits th6 size of the plate should be bigger lo eliminate lhe effect of grainsize.

t.l Planning For Subsurface ExplorationThe plannilg of the site exploration program involves localion and deplh of borings, test pits or other methods to be used, andmethods of iampling and tests to be carried oul. The purpose of the exploratiolr program is to determine, within praclical limits,the stratificaiion-and engineering properties of the soils underlying the 'site. The principal properties of interest will be thestrength, defc'mution, anC trydraulic charac{eristics. The program should be planned so t!a! the maximum amount ofinfoniation can be obtained it minimum cost. ln the earlier stages of an investigation, the information available is ofteninadequate to allow a firm and detailed plan to be made. The investigation is lherefore performed in the following phases:

1. Facl finding and geological survey. Reconnaissance

1. Prelimirary exploration2. Detrailecl exploration3. Special exploration

1. Factfinding and geological surveyAssemble all infcrmation on dirhensions, column spacing, type and Use of structu're, basement requircments, and any specialarchitec{ural considerations of the proposed building. Foundation regulations in the local building code should be consulted forany special requirements. For bridges the soil engineer should have access to type and span lengths as well as pier loadings.This information will indicate any settlement limitations, and can be used to estimate foundation loads.

2. Reconnaissance :This may be in the form of a field trip to the site which can reveal information on lhe type and behavior of adjacent sites andstrudures such as cracks, noticeable sags, and possibly sticking doors and windows. The type of local existing structure mayinfluence, to a considerable extent, lhe exploration program and the best foundation type for the proposed adjacent structure.Since nearby existing structures must be maintained, excavations or vibrations will have to be carefully controlled. Erosion inexisting cuts 1or ditchls) may also be observed. For highways, run off palterns , as well as soil stratification to the depth of theerosioi cut , iray be oUierveO. Rock outciops may give an indication of the presence orthe depth of bedrock'

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unit_1-2 gEO tECHNICAL eNGINEERING ii cHAPTER 1 & 2