4.1 INTRODUCTION -4.2 THEORY. Objectives After studying this unit, you should be able to determine. Structure, P = failure load, and

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1 -- UNIT 4 UNCONFINED COMPRESSION TEST Structure, 4.1 Introduction Objectives 4.2 Theory 4.3 Applications 4.4 Compression Machine with Proving Ring as Load Measuring Device Apparatus Prwcedure Observations and Calculations Results Precautions 4.5 Screw Jack with Spring Compression as Load Measuring Device Apparatus Pmedure Obseivations and Calculations Results Precautions 4.6 Reference 4.1 INTRODUCTION The unconfined compression test is the simplest and quickest test devised to determine the shear strength of a soil specimen whether undisturbed or remoulded. The specimen in the form of a cylinder 3.8 cm in diameter and 7.6 cm in height is subjected to direct compression, till the specimen fails either by shearing on a diagonal plane or by lateral bulging. Since no lateral support is provided to the specimen, only cohesive soils can be tested. Bbcause no drainage is necessary, it is essentially a quick test. Objectives After studying this unit, you should be able to determine -4.2 THEORY the unconfined compression strength of a cohesive soil sample, the sensitivity of the soil sample, and shear parameters of the soil sample. a) The unconfined compressive strength is defined as the ratio of failure load to the cross-sectional area of the soil specimen if it is not subjected to any lateral pressure, where, qu = unconfined compressive strength, P = failure load, and A, = corrected area at failure. Now,

2 Strengtb d Materids and Soil Mcrhanica where, A, = initial area, and 6 = strain. where, M = change in length, and L, = initial length of the specimen. This test is undrained, since the rate of applying load is so fast that no &re water is allowed to drain and pore water pressure does not get dissipated. Sensitivity is defmed as the ratio of unconfined compressive strength of undisturbed soil sample to the unconfined compressive strength of remoulded sample at constant moisture content. Sensitivity = ( 9u tmdis- ( qu ) remoulded Cohesion of the soil specimen may be computed by the relation: ol= a3tan2a+2ctana... (4.5) where, a1 = major principal stress at failure, 03 = minor principal stress at failure, a = failure angle with major principal plane = 45 + Z 2. (4 = angle of internal friction) c = cohesion. In unconfined compression test 03 = 0, at = qa. Hence, Therefore, If the soil specimen is fully saturated and no drainage is permitted, then. $ = 0, therefore, c = - 9u 2 '... (4.8) The shear sfrength of the soil is estimated from Coulomb's relation : TJ = c +a,~ tan*... (4.9) where, zf = shear resistance, and a,ff= effective nofinal stress.

3 4.3 APPLICATIONS Unconfieed Compression Test This is the simplest and quickest method for determination of cohesion and shear strength on cohesive soils. These values are used for checking the short term stability of foundations and slopes, where the rate of loading is fast but drainage is very slow. Soil consistency can easily be known from the value of unconfined compressive strength from Table 4.1. Table 4.1 : Soil consistency qu, wan2 (-3 < 2.5 < (0.25) ( ) ( ) ( ) ( ) > 40.0 > (4.0) ~ oconsistency i ~ Very soft Soft Medium Stiff Very stiff Hard L t Sensitivity is a very useful factor to know the effect of remoulding on shear strength of cohesive soils. Remoulding of soils is very common during excavation and pile driving. Generally, soils having sensitivity less than 4 are considered good for construction purposes. Soils are designated as follows with respect to sensitivity. Table 4.2 : Sensitivity Sensitivity > 15 Designation Normal Sensitive Extra sensitive Quick I I The following two methods are described to determine the compressive strength of soils. 1) Compression machine with proving ring as load measuring device. 2) Screw jack with spring compression as load measuring device. 4.4 COMPRESSION MACHINE WITH PROVING RING AS LOAD MEASURING DEVICE In this case the measuring device will he a proving ring. - Figure 4.1 : Hand Operated Compression Machine with Proving Ring as h d Measuring Device

4 Strength of Materials and S d Mechanics Apparatus The apparatus include some general and some special ones. n ey include the following : General 1) Stop watch 2) Vernier callipers and scale 3) Knife 4) Balance.with an accuracy of 0.1 g 5) Grease or oil. Special 1) Compression machine 2) Proving ring of capacity 500 N and 1000 N with a least count of 1.0 and 0.2 N respectively 3) Dial gauge of least count 0.01 mm 4) Split mould of internal diameter 38 mm and length 76 mm 5) Sampling tube df internal diameter 38 nlm and length 200 mm 6) Sample extractor Procedure 1) For undisturbed sample, push the sampling tube into the clay sample. Remove the sampling tube along with the soil. 2) Saturate the specimen in sampling tube by a suitable method, if possible. 3) Coat the inside of the split mould with a thin layer of grease or oil to prevent adhesion of the soil. 4) Exttude the specimen from the sampling tube to the split mould with the help of sample extractor and knife. 5) Trim the two ends of the specimen. 6) Weigh the soil specimen and the mould. 7) kemove the specimen from the mould by splitting the mould into two parts. 8) Measure the length and the diameter of the specimen. 9) Place the specimen on the bottom plate of the compression machine. 10) Raise the bottom plate of the machine to make mtact of the specimen with the upper plate. 11) Adjust strain dial gauge and proving ring dial gauge to read zero. 12) Apply the compression,load by raising the bottom of the machine to produce axial strain at a rate of 0.5 to 2 % per minute. 13) Record the strain and proving ring dial gauges readings at 30 sec intervals. 14) Compress the specimen till 20 % vertical deformation is reached or it fails, whichever is earlier. 15) Note the least count of strain dial gauge in mmldiv and total load dial gauge in kgtdiv. 16) Msasure the failure angle from horizontal is possible, especially if the soil specimen is not fully saturated. 17) Determine the moisture content of the specimen Observations and Calculations 1) Use Tables 4.3 and 4.4 for recording observations and calculating strain and stress as shawn.

5 Date Table 43 : Soil Properties Soii Spedmen No. UmconPmed Compression Test Items Type of Specimen Undlshrrbed Remoulded 1) Initial length of specimen, Lo (mm) 2) Initial diameter of specimen (mm) 3) Initail area of specimen (cm2) 4) Initial volume of specimen, Vo (cm3) 5) Mass of the specimen + mould (g) 6) Mass of the mould (g) 7) Mass of the specimen. W = (5)-(6) (g) 8) Density of the specimen. 3 = (7 ) l(4) (glcc) 9) Crucible No. 10) Mass of crucible (g) 11) Mass of crucible + wet specimen (g) 12) Mass of crucible + dry specimen (g) 13) Watercontenfw = -xloo (%) (12) - (10) Gs(l+w)~-l 14) Void ratio, eo = GSWX ) Degree of sahuation, S = Date v eo Table 4.4 : Compressive Stress (For Compression Machine with Proving Ring) a)?).pe of specimen : UndisturbedfRemoulded - b) Least count of deformation dial gauge (rnmfdiv.), LC = C) Proving ring constant (Nldiv.), PRC - Soil Specimen No. SLNo Time Elapsed Deformation ( L) (DIV.) mm (3) x LC Strain AL 6=- LO Corrected Area, Ao Ar = Load OU) (WO = (7) x PRC Compressive Stress (1) (2) (3) (4) (5) (6) (7) (8) (9) Failure angle = degrees. Note : Use as many tables as the number of specimens.

6 ~a~+akads~il 2) Pld the stress-strain curve on the graph taking strain along the x -axis and the stress along the y -axis (Figure 4.2). biol stroin,61/~~ ( "/.I Figure4.2 : Graph for Compression Machine witb Proving Ring 3) The maximum value of stress from this plot gives the value of the unconfined strength. Where no maximum is observed, stress at 20 % strain will give the unconfined compressive strength. 4) Compute the sensitivity of the specimen as ( qu ) undisturbed Sensitivity = ( qu ) remoulded - 5). If failure angle ( a ) has been measured, compute the angle of internal friction'as 6) Compute the value of cohesion, c as if the specimen is fully saturated, or if the specimen is partially saturated. 7) Read soil consistency and designation from Tables 4.1 and Results Summarise the results in Table 4.5, from Figure 4.2. Date Table 4.5 Unconfined compressive strength q, (~lcm~) - Strain at failure (76) - Cohesion c (~lcm~) - Angle of internal friction g ' - Soil consistency - Sensitivity and designalion - Soil Specimen No. Note : Use as many tables (Table 4.5) as the number of specimens.

7 4.4.5 Precautions Unc~nAned Gmpressien Test 1) The specimen should be handled with great care to prevent disturbances, changes in density or loss of moisture. Loss of moisture during the testing may be checked by sealing the specimen with rubber membranes. 2) The two ends of the specimen should be perpendicular to the long axis of the specimen. 3) The seating of the specimen should be proper on the upper and lower plates. 4) The loading of the specimen should be at constant rate. 5) Remoulded specimen should be prepared at the same moisture content and density as of the undisturbed specimen. 6) If degree of saturation is less than 100 5% then the failure angle must be measured. 7) The specimen should always be pushed in the sampling tube or the mould along the same direction in which it enters the tube in the field. 8) The least count and observations of deformation dial gauge should not be interchanged with those of the proving ring dial gauge. 4.5 SCREW JACK WITH SPRING COMPRESSION AS LOAD MEASURING DEVICE In this experiment, the load measuring device is a screw jack with spring compression Apparatus The apparatus include some general and some special ones. They are as follows : General 1) Stop watch 2) Scale and knife 3) Vernier callipers 4) Balance with an accuracy of 0.1 g 5) Grease or oil 6) Drying crucibles 7) Drying oven 8) Desiccator. Special 1) Unconfined compression apparatus: screw jack with spring as load measuring device (Figure 4.3). Figure A3 : Unconfined Compression Apparatus with Spring as LoPd Measuring Device 2) Sampling tube of initial diameter 38 mrn and length 200 mrn. 3) Sample extractor.

8 sgength of Materidsand SOU 4) Split mould of internal diameter 38 mm and length 76 mm. MeCannics 5) Coning tool Procedure 1) For undishnbed specimen, push the sampling tube into the clay sample. Remove the sampling tube along with the soil. 2) Saturate the soil specimen in the sampling tube by a suitable method, if possible. 3) Coat the inside of the split mould with a thin layer of grease or oil to prevent the adhesion of the soil. 4) Extrude the specimen from the sampling tube to the split modd with the help of sample extractor and knife. 5) Trim the two ends of the specimen. 6) Detennine the weight of the soil specimen inside the mould by weighing the mould with the specimen and empty. 7) Use the coning tool to form cones on the two ends of the specimen. 8) Remove the specimen from the mould by splitting the mould into two parts. 9) Measure the length and the diameter of the specimen. 10) Place the specimen in the compression apparatus. 11) Fix the graph paper and pencil (Figure 4.4). Figure A4 : Gmph for Screw Jack with Spring 12) Apply compressive load by giving about half a turn of the handle per second till the sample fails or 20 % strain occurs. 13) Measure the angle of failure plane from the horizontal. 14) Detennine the moisture content of the soil specimen.

9 4.5.3 Observations and Calculations 1) Use Tables 4.6 and 4.7 for recording the observations and calculations. Date Table 4.6 Soil Specimen No. Unconfined Compression Test Items 1) Initial length of specimen (nun) Type of Specimen Undisturbed Remoulded 2) Initial diameter of specimen (nun) 3) Initail area of specimen (cm2) 4) Initial volume of specimen, VO (cm3) 5) Mass of the specimen + mould (g) 6) Mass of the mould (8) 7) Mass of the specimen, W = (5)-(6) (g) 8) Density of the specimen, % = (7 ) l(4) (gkc) 9) Crucible No. 10) Mass of crucible (g) 11) Mass of crucible + wet specimen (g) 12) Mass of crucible + dry specimen (g) 13) Watercontenfw = -x1m (%) (12)- (10) ~,(I+w)T-~ 14) void ratio, e, = 71 G,wxlOo 15) Degree of saturation, S = e~ Date Table 4.7 (For $crew Jack with Spring Compdon) Soil Specimen No. a) Type of specimen - Undisturbed/Remoulded - b) Spring constant, Nlbm - (1) Deformation L (em) L (2) Strain 8 = - L Points on 6be &we Undisturbed Remoulded A0 (3) Corrected area, A, = - (1 - q,) 'cm2 ) (4) Spring extension, (cm) (5) Load = (4) X Spring Constant, (N) (6) Compressive stress, q '= (5)/(3), (~lcm') (7) Failure angle ( ) (8) Compressive strength

10 dmateri* a* Soil 2) Note the point of maximum spring extension on the graph paper (Figure 4.4) if the M W specimen has failed. Select any three points on the right hand side and one on the left hand side of the point of maximum extension. Read the spring extensions and deformations corresponding to these points. 3) Compute the compressive stresses for these points. Maximum stress will give the unconfiied compressive strength. 4) If specimen does not fail, stress corresponding to 20 % strain is taken as unconfined compressive strength. 5) Compute the sensitivity of the specimen as Sensitivity = ( q,, ) undisturbed ( 4, mnoulded 6) Compute the angle of internal friction $ if failure angle a has been measured for pattially saturated specimen, as $ = (a.-45)x2 7) Compute the value of cohesion, c as if the specimen is fully saturated, or q, c = - 2 ' if the specimen is partially saturated. q, c,, = - 2 tan a' Results Use Table 4.8 to summarise the results. Date Table AS Soil Specimen No. Unconfined compressive strength q, (N/cm2) - Strain at failure (9%) - Cohesion c (N/cm2) - Angle of internal friction $ - Soil consistency - Sensitivity and designation - Note : Use as many tables as the number of specimens. SAQ 1 i) How would you define unconfined compressive strength of a soil? ii) iii) iv j Is it pwible to determine the unconfined compressive strength of all types of soils'? How is corrected area defined and determid?. How are shear parameters defined7 How car1 you obtain these from an unco~~fined compression test?

11 SLSQ 2 il ii) iii) How is sensitivity of a soil defined asid determined? Wllerc are ths uncvtlrined compressive strength and sensitivity of soiis,-,pplicd? What is the size of the specimen in an urlcor~fined conipression test anif Ihi! length to diameter ratio'? i ~ ) Eow aretfle loads and deformations rplcasured in an unconfined comprcssicc tcst? Unconfined Compression Test Precautions 1) The specimen should be handled with great care to prevent disturbances, changes in density or loss of moisture. Loss of moisture during the testing may be checked by sealing the specimen with rubber membranes. 2) The two ends of the specimen should be perpendicular to the long axis of the specimen. 3) The seating of the specimen should be proper on the u@er and lower conical ends. 4) The specimen and mould should be weighed before using the coning tool. 5) The pencil s'hould be fixed with care so as to press the paper and draw a smooth curve. 6) The horizontal arm of the lever must rest on the adjustable stop when the load is applied. 7) The loading of the specimen should be at a constant rate. 8) Remoulded specimen should be prepared at the same moisture content and density as of the undisturbed specimen. 9) If degree of saturation is less than 100 % then the faiiure angle must be measured. 10) The specimen should always be pushed in the sampling tube or the mould along the same direction in which it enters the tube in the field. 4.6 REFERENCE Prakash, S.; Jain, P.K. (1988): Engineering Soil Testing, Nem Chand & Bros, Roorkee.

SHEAR STRENGTH OF SOIL UNCONFINED COMPRESSION TEST

SHEAR STRENGTH OF SOIL UNCONFINED COMPRESSION TEST SHEAR STRENGTH OF SOIL DEFINITION The shear strength of the soil mass is the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it. INTRODUCTION