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Direct Shear Testing

MOH. SOFIAN ASMIRZA

Fakultas Teknik Sipil Universitas Sumatera Utara

INTRODUCTION

The mainly warm, wet climatic conditions in tropical countries have result in varying depths of weathering of a wide range of igneous, metamorphic and indurated sedimentary rocks, to give profiles which grade from residual soils at the surface trough to unweathered bedrock at depth.

It is for this reason that the direct shear testing is important to determine shear strength of soil, which specifically uses tropical residual soils obtained in the surroundings of Kuala Lumpur, Malaysia as soil samples. Before performing the test, it is however, very important to understand the principle of shear box test, definitions, theory and testing programme. A BRIEF HISTORY OF THE DIRECT SHEAR BOX TEST

The direct shear box test is a conceptually simple test that apparently was used for soil testing as early as 1776 by Coulomb (Lambe & Whitman, 1969) and was featured prominently by French engineer Alexandre Collin in 1846 (Skempton, 1984). He used a split box, 350 mm long, in which a sample of clay 40 x 40 mm section was subjected to double shear under a load applied by hanging weights (Fig 4.1).

Fig. 1 : Shearbox apparatus devised by Collin (1846) : (a) general arrangement, (b) forces on sheared

portion of sample

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In Britain, Bell (1915) made the earliest measurements who constructed a device which was to be the prototype for subsequent developments of the shearbox. Bell was the first to carry out and publish result practical of shear tests on various types of soil (Skempton, 1958).

A simple shear box with a single plane of shear was designed in 1934, using the 'stress control' principle where the load was applied in increments by progressively adding weights to a pan (fig. 2). This required considerable care and judgement on the part of the operator in order to ascertain the load at which failure accured.

Fig. 2 Principle of early type of controlled-stress shear box A modern shear box was designed by A.Casagrande at Havard (USA) in 1932.

four years later, Gilboy at MIT, developed a constant rate of displacement machine which applies the 'strain control' principle, using a fixed speed motor. In 1946, Bishop at imperial College introduced the improvements of design using this principle in details. Most commercial shear box machines are still based on the displacement control principle as shown in Fig. 3 These machine provide a wide range of displacement speeds, from a few millimetres per minute to about 10000 times slower. The stress-control method has certain advantages in some long-term tests in which increments of stress must be applied very slowly, and in tests for the study of the effect of 'creep' under constant shear stress. However, for routine testing applications the displacement-control method is the one now normally used.

Fig. 3 : Arrangement of conventional 60 mm shear box apparatus with displacement control


PRINCIPLE OF SHEAR BOX TEST The shear box test is the oldest and simplest form of shear test arrangement.

Basicalliy, the testing procedure is very straightforward. The test has ben used for measuring the ‘immediate’ or short-term shear strength of soils in terms of total stresses. In principle, the shear box is an ‘angel of friction’ test, in which one portion of soil is made to slide along another by the action of steadily increasing horizontal shearing force, while a constant load is appiled normal to the plane of relative movement. To perform the test, a soil specimen is placed in a rigid square metal box, consisting of two halves. The lower half of the box can slide relative to the uppear half when pushed or pulled by a motorised drive unit, while a yoke supporting a load hanger provides the normal pressure. The principle is shown in Fig. 4.

Fig. 4 : Principle of shear box test: (a) start of test, (b) during relative displacement

In order to obtain a load/displacement curve, the relative displacement of the two portion of the specimen and the applied shearing force are both measured during the shearing process. The change in the height of the specimen (and thus the volume change of the specimen) can be obtained from the radings of a dial gauge that measures the vertical movement and this will enable changes in density and voids ratio during shear to be evaluated.

The direct shear apparatus is particularly suited to consolidated drained testing because the drainage paths within the the specimen are short and excess pore pressures can rapidly dissipate. Generally, the shape of the specimen used has been one of the factors that contributes to its suitability.

DEFINITIONS FORCE : That influence which causes a change of state of motion of a body. Force = mass x acceleration NORMAL FORCE or DIRECT FORCE. A force which acts normal (perpenicular) to a plane of section SHEAR FORCE : A force which acts tangential to plane of section. STRESS: Intensity of force, i.e force per unit area. STRAIN (linear) : Change in length per unit length due to a stress measured in the direction of the stress.


SHEAR STRESS: Shear force per unit area SHEAR STRAIN: Angular distortion, measured in radians due to the action of shear stresses. DISPLACEMENT: Horizaontal movement of one portion of specimen relative to the other along the surface of sliding and in the direction of the applied force, in a direct shear test. SHEAR RESISTANCE (of a soil) : The resistance offered (by a soil) to deformation when it is subjected to a shear stress. SHEAR STRENGTH: The maximum shear resistance which a soil can offer under defined conditions of effective pressure and drainage. (Often used synonymously with peak strength). UNDRAINED SHEAR STRENGTH: The shear strength of a soil under undrained conditions, i.e. immediately after the application of stress and before drainage of water can take place. APPARENT COHESION (cu) : The shear strength of a soil when subjected to zero nomal stress or confining pressure. ANGEL OF SHEAR RESISTANCE (cp) : The component of shear strength of a soil which is due to friction between the particles. DILATANCY: Expansion of a soil when subjected to shear stress. FREE DRAIN1NG SOIL: A soil in which water can move easily through the void spaces so hat no excess pore pressure or suction develops as a result of the application of stress or deformation. CRITICAL VOIDS RATIO: The voids ratio at which a granular soil neither contracts nor dilates when subjected to shear. PEAK STRENGTH: (See shear strength). RESIDUAL STRENGTH: The shear resistance which a soil can maintanwhen subject to large shear displacement after the peak strength has been mobilised FAILURE: The poin at which continued shear deformation under a constant or decreasing shear stress begins. COULOMB'S LAW: The relationship between the shear stress Tf and normal stress an σn a plane of failure, expressed by the empirical equation

Tf = Cu + σn tan Φ

THEORY Shear in soils The principle of shear box is shown in Fig. 4 (a). A vertical s1ress an is produced when a normal load is applied to the soil where σn = N/L2, and L is the length of side of the square box. A steadily increasing displacement, which causes an increasing force F, is applied to one half of the sample in horizontal direction, while the other half is restrained by the load-measuring device. The shear stress induced on the pre-determined slip plane is equal to F/L2. While the force F is increasing, a horizontal displacement (Fig.4.4(b)) of the soil in the bottom half of the box relative to that in the top half takes place gradually. This phenomenon is shown in Fig 5 by OA in the load/displacement graph. At point B, that is the maximum shear s1ress (Tf) which can be sustained on the surface of sliding is


offered by the soil. This shear stress is the shear mess is the shear Strength of the soil under the particular normal stress cm and the point B is know as the 'peak' of the shear stress displacement curves. Shear failure of the soil occurred when the shear resistance falls off the peak as indicated by Bc. Several tests, usually three, can be carried out on specimens of the same soil under different normal loads, (denoled here by NI, N2, N3), giving three different values of the normal stress σn. fig. 6 shows each stress/displacement curve are plotted as in Fig. 4.7. this graph generally approximates to a straight libne, its inclination to the horizontal axis being to the angle of shearing resistance soil, Φ, and its intercept on the vertical (shear stress) axis being the apparent cohesion, denoted by Cn.

Fig. 5. Relationship between sher stress and displacement in shear box test


Fig. 6. Shear stress/displacement curves for specimens tested under 2 different normal

Fig. 7. Maximum shear stress related to normal stress from shear box tests (Coulomb envelope) Colomb's law

The general relationship between maximum shearing resistance, tf, and normal stress σn, for soil, shown graphically in Fig. 7. can be represented is known as the 'failure envelope'. Coulomb equation as states above is applicable only to total stresses. When the results are to be expressed in terms of effective stresses, this equation has to be modified to take account of pore water pressures. Thus, the new equation will be: tf = Cu' + σn’ tan Φ’ =Cu' + (O'n-u) tan Φ’ where c' = effective cohesion of soil

Φ’ = effective angle of shearing resistance of soil u = pore water pressure


The shearing resistance of soils represent by the Coumomb's equation is generally made up two components. (1) Friction (denoted by tan Φ), which is due to the interlocking of particles and the friction between them when subjected to normal stress. (2) Cohesion (denoted by cu), which is due to internal forces holding soil particles together in a solid mass.

The friction component increasws with increasing normal stress but the cohesion component remains conctant. The friction disappears if there is no normal stress.

Soils may be divided into three categories on the basic their total strength properties. (1) Frictional, cohesionless (granular) soils, such as dry sands or completely submerged

free-draining sands. These soils possess no cohesion (cu = o), but derive their shear strength entirely from intergranular friction (the tan Φ component). The failure envelope passes through the origin.

(2) Cohesive soils, such as virtual saturated clays. They exhibit cohesion, but if no change in water content occurs during the test appear to possess no friction (Φ = 0). The failure envelope is virtual horizontal such as shown in Fig. l8.

(3) Intennediate soil types, which possess both cohesion and friction (c, Φ soils).

Fig. 8 : Failure envelope is virtually horizontal (Φ = o) Shear strength of Dry and Saturated Sand

The shear strength of a dry send depends upon several factors, such as the mineralogical composition of the grains; their size, surface texture and grading; the soil structure, i.e. packing of the grains; and the moisture content. For a particular sample of dry sand, the only variable that governs the test is the state of packing, which has an important influence on the shear strength. The state of packing can be expressed in the tenDS of relative density or void ratio, or porosity, or dry density. Experience has shown that shear strength result obtained on saturated sand are very similar to those for dry sand, provided that thesand remains satunlted and the drainage takes place freely during shear. Section (1), (2) 1 and (3) below relate equally to dry sand, or to fully saturated free draining sand.


Dense Sand Fig 9 (a) represent the syaye of packing the grains in adense sand with low voids

ratio. If the sand is sheared along a plane X-x, and ifit is assumed that distortion and crushing of individual grains does not occur, grains lying just above the surface XX will be forced to ride up and over those lying just below when relative movement occurs. As a result, expansion occurs which can be measured by observing the upward displacement of the top surface of the sand. The resulting increase in volume is known as dilantancy and in the case of free-draining submerged sands, additional water will enter the soil structure.

Fig. 9 : Effect of shear on grain structure in sands.

The shear stress/displacement curve is of the fonn marked (D) in Fig. 4.10 (a), and the corresponding volume change relationship with displacement is marked (D) in Fig. 10 (b). The small initial contraction is due to some bedding down of grains when shearing begins. It can be seen that the stress curve rises quite sharply to a peak value then falls off to a lower value than the peak. The excess of the peak over the final value, denoted by E, represent the extra work which has to be put in to produce the vertical movement due to dilantancy. After shearing, the grains adjacent to the shear surface are in a less dense state of packing than they were initially.

Fig. .10: Shear characteristics of dense and loose sand; (a) shear stress, (b) volume change, (c) voids ratio change; all plotted against displacement

Loose Sand A loose state of packing of grains is shown in Fig. 9 (b). If the sand is sheared along a plane Y-Y, grains will be forced to move downwards into void spaces, resulting in a collapse of the relatively open structure. This cause a volume decreases (contraction),


which can be measured as a downward movement of the top surface, and in free-draining submerged sands result in water being expelled from the soil structure.

Fig. : 10 (a) shows the resulting shear stress/displacement curve, marked (L), is less steep than curve (D) and does not have a pronounced peak. The curve (L) shown in Fig. 4.1O(b) represent the corresponding volume change relation ship with displacement. After shearing, the grains adjacent to the shear surface are in a denser state of packing than they were initially. Comparison Of Dense And Loose Sands Volume changes during shear for both states of packing are represented in Fig. 10 (c) in terms of void ratio. Initial voids ratio are denoted by eD (dense) and eL (loose). At the end of the shearing displacement the voids ratio in each case approaches a common value known as the critical voids ratio, ecr.

The relative density (RD) can be used to related any voids ratio e to the limiting

voids ratios (emax dan emin), which is defined by the following equation: emax – e

RD = emax – emin and is stated in terms of percentage. The value of RD equals to unity when the soil is in the densest state of packing of grains (e= emin) and equals to zero when the soil is in the loosest state of packing of grains (e = emax).

If three are carried aout under three different normal stresses for each state of packing, the shear strength/normal stress relationships can be plotted as shown in Fig. 4.11 (a). The angle of shear resistance(ΦD) for the dense state is grater than that (ΦL) for the loose state. A relation ship between Φ and relative density can be obtained by perfonning additional tests at intermediate voids ratio or realive densities, as shown in Fig. 4.11 (b). The corresponding change of voids ratio up to point of failure (i.e. maximum shear stress) can also be plotted against relative density (Fig. 11 (c)). The interception with the horizontal axis gives the relative density corresponding to the critical voids ratio, because this is the density at which no change in voids ratio occurs due to shear.


Fig. 11 : Effect of initial voids ratio on shear resistance of a sand; (a) Coulomb envelopes for dense and loose states, (b) value of Φ related to relative density, (c) voids ratio change during shear related to initial relative density. Shear Strength of Clays

The shear strength of clays depends not only the soil type and composition, but also on factors related to the mineralogy, grain size and shape, adsorbed water, and water chemistry of the clay minerals present. Initial moisture content of the clay, and the rate at which the soil structure can expel or take in water during a test, also influence the shear strength of clay.

The three factors which are the greatest signification on shear strength testing of a particular type of clay are : 1. the water content (expressed in terms of liquidity index); 2. the degree of saturation-whether fully saturated or partially saturated; 3. the rate of shear displacement in relation to the permeability of the soil-whether quick (no drainage allowed), or slow (full drainage allowed, i.e. the dissipation of any excess pore water pressure set up during shear).

Compression tests are usually used to determine the undrained shear strength of

saturated plastic clays because the shear box test is less satisfactory for these soils. However their shear strength can be measured directly by the vane apparatus.

Quick (Undrained) Tests On Saturated Clay Test Conditions

For a quick shear test, virtual no drainage takes place from a clay during te short period (usually up to a maximum of20 min) due to the low permeability of clay.

If the porewater pressure is not taken into account, and only total stresses are measured, the undrained shear strength of saturate clay is independent of the applied


normal stress. From Fig. 12 (a), a failure envelope is obtained by performing a series of quick shear tests under different normal pressures in which the line is virtually horizontal (i.e.Φ is practically zero), and the intercept on the shear stress axis is equal to the undrained shear strength, or apparent cohesion, denoted by cu. under these conditions, the clay derives its ahear strength entirely from the cohesion component.

For an overconsolidated clay, a quick test gives a slightly curved envellope as indicated in Fig. 12 (b).

Intermediate Soil Types

Generally, soils which are intermediate between sands and c1aya, such as sandy clay or silt, have some cohesion as well as internal mction. The value of Φ obtained from quick undrained tests is less than for sands. These soil types exhibit a failure envelope such as shown in Fig. 12 Fig. 12 : Representative Coulom envelopes from quick shear tests: (a) saturated clay, (b) overconsolidated clay, (c) sandy clay or silt. 4.7 Slow (Drained) Tests On ClayAnd Silts Principle

A set specimens each at a different pressure is tested. The soil is first allowed to consolidate under the selected normal pressure, until consolidation is completed and there virtually no excess of pore pressure remaining. Dissipation of any further pore water pressure (whether positive or negative) which may develop due to shear, is allowed by shearing the soil slowly. The rate displacement is determined from the consolidation stage. Under these conditions the effective stresses are equal to the applied stresses The


shear strength envelope for a normally-consolidated clay from a set of tests is usually of the form shown in Fig. 3(a). The envelope is approximately a linier and paased through the origin as does that of a dry or saturated sand. The inclination of the envelope gives the value of the angle of shear resistance in the drained condition, designated Φd The envelope of an over consolidated clay may be slightly curved as shown in Fig. 13 (b). it intercepts with the shear stress axis to give a value, known as cohesion intercept, cd.

This type of test is reffered to as a consolidated-drained (CD) shear box test. The parameters, cd and Φ d reffered to above differ only slightly from the effective shear strength parameters c', Φ' obtained from undrained tests in which pore water pressures are measured and usually in many cases and for many purposes the two sets of parameters can be considered to be equal.

Fig. 13: Representative Coulomb envelopes from slow (drained) shear tests: (a) normally consolidated clay, (b) over consolidated clay

Shear Box Size

In general, there is no significant scale effect in shear testing of cohesionless soils due to the size, soils with larger particles obviously requiring larger shear boxes. ASTM (1985) gives two guideniles for direct shear testing :

a) specimen thickness to be at least six times the maximum gram diameter of the soil;

b) specimen diameter (or width) to be at least twice the thickness.

However, both ASTM guidelines may not be suitable for some granitic soils with maximum grain diameters of 8 mm or more. It is therefore not convenient to use small shear box for such routine investigations.

Extensive investigation on the shear box size effect in gravel testing have also been carried out in china. The Chinese Specification for Geotechnica1 Tests compiled for


use in the hydroelectricity and hydraulic bureaus (HBPRC, 1980 gives two guideniles for testing sandy gravel in direct shear : a) specimen thickness to be between four to eight tiIries the maximum grain diameter of the soil; b) specimen diameter to be between eight and twelve times the maximum grain diameter.

These guideniles are largely based on the result of a study using both 500 mm diameter by 300 mm thick and 500 mm diameter by 500 mm thick shear boxes on alluviwn with maximum particle sizes from 20 to 120 mm. Shear Rate

The shear rate is the principle factor that determines whether the test is drained or undrained for saturated soils tested in the ordinary direct shear test. It is impossible in theory to attain either a fully drained or fully undrained condition in a constand rate of shear test, but in practice it is often possible to select a shear rate so that the deviation from ideal conditions is not significant.

In a drained test, the rate of displacement at which the specimen should be sheared depends upon the drainage charateristic, i.e. the permeability of the soil and the thickness of the sample. Since permeability is related to coefficient of consolidation, the consolidation stage of the test can provide the data for estimating a suitable time to failure.

Gibson & Henkel (1954)derived an expession for the time to failure (tf) in a drained direct shear test necessary to attain a specified degree of dissipation of pore pressure at the centre of the specimen, namely:

H2 tf =----------------------------------(1)

2Hb (1-Uc) Where H = half thickness of specimen Cb = soil consolidation coefficient Uc = degree of dissipation of pore pressure at the centre of the specimen.

Equation (1) is based on the assumption that the uppear and lower surface of the direct shear specimen are allowed to drain freely, and that the rotation of principle stresses during the test can be neglected.

Under the applied normal stress, the consolidation of the specimen would give a curve of settlement against square-root-time (minutes), of the form indicated in Fig. 4.14. Atangent is drawn to the early straight line portion of the curve and is extended to intersect the horizontal line representing 100% consolidation, which often corresponds to the 24 hour reading. The point of intersection gives the value of t100 (Fig. 4.14), which when multiplied by it self gives the time intercept t100 (min) as defined by Bishop and Henkel (1962). The time required to failure, tf is related to t100 by the empirical equation:


Tf = 12.7 x t100 min ------------------------- (2)

Equation (2) is based on 95% pore pressure dissipation, but it has an advantage over equation (1) in that it utilises more readily available input data. Catering for 98% pore pressure dissipation, this expression would transform to :

tf = 30 x t100 min ------------------------- (3) ASTM (1985) recommends : tf = 50 x t50 ------------------------- (4)

where t50 primary consolidation. This equation gives essentially the same time to failure as equation (2), and would therefore represent 95% pore pressure dissipation.

After an appropriate time to failure has been determined using one of the equations above, the limiting rate of shear for a drained direct shear test can be estimated as follows:

Rate of shear ≤ δ f/tf ---------------(5) Where δ f is the horizontal displacement of the shear box at peak strength (failure). However, δ f can only be estimated prior to the test it self.

The coefficient of consolidation, cv, can be calculated ftom the equation : Cv = 0.l03H3 m2/year----------(6)

t100 Where H is the specimen thickeness (mm) and t100 is in minutes. For a standard specimen of height H = 20 mm, Equation (6) becoms: Cv = 41 m2 /year - - - - - - - - - -(7) t100

A difficulty aries with this method if the consolidation if the consolidation curve does not resemble the theoretical curve in which there is no straight line in the initial up to about 50% consolidation. This phenomenon may be due to the effect of bedding of the grid plate, or to the presence of air in the voids of the soil (i.e. partial saturation).

Binnie and Partners (1968) came up with a method which gives a reasonable estimate of t100, from which cv and the time to failure may be derrived, as illusttated in Fig. 15. It requires a number of settlement readings to be taken in the later stages of consolidation.

Find the point C which is the earliest at which consolidation is substantially complete, i.e. beyond which the curve virtually flattens out. Make AB = 1/2 AC, and read off the value of t100 at the point B. Values of t100,tf and cv are then calculated as described above.


STRENGTH ANISOTROPY

Some residual soils often had anisotropic strength properties because of the parallel orientation of their flaky grains, particular micas and koalinite clays. They were weaker in shear parallel to the grain, faces than perpendicular to them.

Strength anisotropy also existed in both undisturbed and compacted granitic soils in Japan (Onitsuka et al, 1985). The shear strength of vertical specimens was found to be greater than that of horizontal specimens of undisturbed soils. For static and impact compacted (remoulded)soils, the COTresponding strength anisotropy was found to be higher too. RESIDUAL SHEAR STRENGTH

If shearing a dense sand is continued after the peak point to the maximum displacement of the shear. box, a curve of the type shown in Fig. 16 (a) is obtained. At first, the shear strength decreases rapidly from the peak value, but eventually it reaches a a steady state (ultimate) value which is maintained as the displacement increaches. No expansion at the critical value.

When subjected to large shearing displacement under fully drained conditions, over consolidated clays behave in a similar manner to dense sands. This requires shear strength to be measured in terms of effective stress, not total stress. The shear strength which the the clay ultimately reaches is known as the 'residual strength', which is often appreciably lower than the maximum or 'peak strenth'.


Peak and Residual Envelopes Mohr-Coulomb envelopes for both the peak and the residual strength conditions

can drawn after the result of a set of residual strength test on three or more identical specimens are obtained, as shown in Fig. 16 (b). Peak shear strength is represented by the equation

Tf = c' + σ’ tan φ' and residual shear strength by the equation

Tf = cr + σ’ tan φr’ fn these equations the dashes indicate effective strength values as determined from drained tests.

The value of cr is often very small, and the residual strength envelope can be assumed to pass through the origin and be represented by the equation

Tr = σ’ tan φr’ Fig. 16: (a) Peak and residual shear strength, (b) Coulomb envelopes for peak and residual conditions (after Skempton, 1964) Effect of Stress History

The influence of overconsolidation on the shear strength/displacement relationship is illus1rated in Fig. 17 (a), whih represent direct shear testsextended to large displacements for a normally consolidated clay (NC) and for an idencital day which has been overconsolidated (OC). For the OC clay, the preconsolidation effective stress is appreciably hegher than the applied normal stress, which is the same for both tests.

The peak and residual strengths of the NC clay exhibit only a slight difference, but for clays of higher plasticity index this difference tends to be greater. The OC clay shows a much higher peak strength at a smaller displacement, compared with the NC clay, followed by a marked decrease in strength to a residual value which is the same as that of the NC clay.

Change in volume and in voids ratio during shear test are shown respectively in Fig. 17 (c), and (d). Coulomb envelopes for the NC and OC peak strengths are shown by the full lines in Fig. 17 (b), and the residual strength envelope is shown by the broken line. This envelope is usually found to be slightly curved, implying that φr is dependent on stress level.


A drained shear test on a fully remoulded specimen of the same clay requires a large displacement to reach the residual strength without first reaching a peak value as indicated by the dashed curve in Fig. 7 (a).

Fig. 17 : Influence of overconsolidation and remoulding on peak and residual shear strengths in a clay (after Skempton 1964) : (a) shear stress/displacement, (b) Coulomb envelopes, (c) volume change during shear, (dalam) voids ratio change during shear APPLICA TIONS Applications of Shear Strength Parameters

Many soil stability problems are concerned with a limiting condition in which the mechanism of failure involves the sliding of a body of soil relative to the main soil mass. The surface of slip along which relative movement is assumed to take place may be plane or curved and it is assumed that the soil along the whole of the slip surface is at a state of failure, i.e. its maximum shear strength has been mobilised. It is very important to ensure that this condition will never occur for practical purpose. Thus, to limit all types of deformations within the tolerable limits, A suitable factor of safety is generally applied to soil structmes so that the shear stress in the soil is nowhere greater than a certain proportion of its maximum shear strength.

If the water content of the soil does not change under load, an analysis in term of total stresses, based on immediate undrained shear strength, can be applied. Examples are indicated in simple terms below (Terzaghi and Peck (1967), Art.26-32).


(1) Bearing capacity of footing and foundations for structmes built on saturated homogeneous clays, immediately after construction. The soil beneath a foundation, if loaded to fuilure, is assumed to fail by shear in the manner indicated in Fig. 4.18 (a).

(2) Earth pressure on a retaining wall, for the conditions prevailing immediately after construction as shown in Fig. 18 (b).

(3) Earth pressure againts bracing in temporary excavations in clay [Fig 18 (c)]. (4) Safeguard againts heave of the bottom of temporary oven excavations in clay [Fig.

4.18 (d)]. (5) Stability of the side slopes of cutting immediately after excavation [Fig. 4.18 (e)]. (6) Short-term stability of embankments and earth dams during construction [Fig.

4.18(f)].

In short-term stability problems, the value of undrained shear strength or apparent cohesion, cu is applicable. The angle of shear resistance φ, is required to determine either earth pressure coefficients or bearing capacity coefficients.

In long-term stability problems such as retaining walls, embankments, and earth dams the drained shear strength parameters c', φ ' are required. The long-term stability of slopes and cutting in overconsolidated clays is based on the residual shear strength parameters cr', φ r'. Use of Standard Shear Box Originally, the shear box was developed to obtain the angle of shear resistance, φ, of recompacted sands. It provides the most direct means of relating φ to the voids ratio, e, and of determining the critical voids ratio of dry sands which do not contain fine material in suffici~t quantity to impair the drainage characteristic.

One of the main application of shear box testing in recent years has been the measurement of the residual shear strength of overconsolidated clays as an extension to the procedure for measuring peak drained strength.


Fig. 18 : Simplifed examples of mechanisms of failure in soils : (a) foundation, (b) retaining wall, (c) bracing in excavation, (dalam) deep excavation, (e) cutting, (f) embankment or earth dam Application to Miscellaneous materials

The shear box apparatus can be used for the measurement of fiictional resistance of other engineering materials such as :

• Friction between soil and rock. • Friction on a joint surface in rock. • Bond strength of adhesives and cementing agenys. • Friction between soil and manufactured materials such as concrete, fabric matting,

reinforcing materials used in reinforced earth construction, components of ground anchor systems.

• Friction betWeen materials and components used in laboratory testing, e.g. latex rubber and silicone grease on stainless steel.

In most of the above application the property measured is the angle of friction, or

coefficient of friction. The strength of a bonding or cementing agent would show u as an apparent cohesion. LIMITATIONS AND ADVANTAGES OF THE SHEARBOX TEST Limitations (1) Drainage conditions cannot be controlled, except by varying the rate of shear

displacement. (2) Shear stress on the failure plane is not uniform, failure occuring progressively from

the edges toward the centre of the specimen. (3) Pore water pressure cannot be measured. (4) The actual stress pattern is complex and the direction of the planes of principal

stresses rotate as the shear strain in increased. (5) The specimen is constrained to fail along a predetermined shear plane. (6) The deformation which can be applied to the soil is limited by the maximum length of

travel of the apparatus. (7) The area under the shear and vertical loads does not remain constant throughout the

test, i.e. area decreases as the test proceeds. A correction to allow for this was proposed by Petley (1966), but its effect is small. It affects the shear stress and normal stress in equal proportion, and the effect on the Coulomb envelope is usually negligible.

Advantages (1) The test is relatively quick and simple to carry out. (2) The basic principle is easily understood. (3) The principle can be extended to gravelly soils and another materials containing large

particles, which would be more expensive to test by another means. (4) Preparation ofrecompacted test specimens is not difficult.


(5) Friction between rocks and the angle of fiiction between soils and many other engineering materials can be measured.

(6) The apparatus can be used for drained tests and for the measurement of residual shear strength by the multi-reversal process.

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Dewan Bahasa dan Pustaka, Kuala Lumpur. R.F. RF. Craig., 1992. Soil Mechanics, 5th ed., Chapman & Hall. London. A Century of soil Mechanics., 1969. The Institution of Civil of London. Proceeding of the Sepcially Session on engineering Properties of Lateritic Soils. Vo1.1

august 1969. sian Institue of Technology, Bangkok. AS. Balasubramaniam. D.T.Bergado % S. Chandra, 1985. Geotechnical engineering in

southeast. Asia Insitutte ofTechnilogi Bangkok. Braja, M. Das. 1994. Principles of Geotechnical Engineering, 3rd. PWS. Publishing

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