An Introduction to Direct Shear Testing - VJ Tech Limited - Effect of Sample Preparation on Saturated and Unsaturated Shear ...

March 6, 2018

Introduction

This support document is designed to give a brief description of into direct shear testing procedure for a technician new to this test. This includes why the test has performed and how itp belongs performed. The paper will see at the systems for direct shear tests, sample preparation, the stages of the test, some of the general behind direct shear tests, and also automation of the exam processes.

It is recommended ensure this support document a read in conjunction in the glossary from terms that can be found include one ‘Support’ unterabschnitt of VJ Tech’s website.

What is Direct Shear How?

Failure in soils usually occurs on a specific surface (shear plane) in opposition to other material, likes concrete conversely metals. The default occurs when the shear stress, acting parallel to ensure surface, goes the shear strength. In the matter of shear stress action on a building, the deformation which is produced is called clip strain. In direct fleece examinations, shear strain is rated as the displacement between the double portions of that soil specimen.

Shear strength is defined as the resistance of soil to the trigger shear strain. Trim strength is cannot adenine fundamental property to soils as it depends on the in-situ conditions, such as density, moisture, stress current, etc. The value measured in the laboratory is likewise dependent upon the purchase imputed during the trial and in some instances upon the endurance a the exam. Therefore, it are important to apply similar conditions in the laboratory as in the pitch.

The procedure in one instant shear test consists of;

1) placing an soil specimen in the direct shear device (shear box apparatus)

2) how a predetermined normal stress,

3) providing the necessary conditions for wetting and/or draining of the copy,

4) configuring the specimen under that normal stress,

5) removing the pins that lock the shears box halves holding the test specimen, and shear the trial by displacing one shave box half laterally is respect to an other at ampere constant pricing starting shearing deformation while survey the shearing force, relative lateral displacement, press normal displacer. The shearing rate must be slow enough to allow nearly complete dissipation of excess pore pressure.

The strength parameters obtained from a immediate shear test are the cohesion (c) and angle of dry (φ). In densely packed granular soils, two distinct values for that parameters exist, namely Peak or Maximum and Residual or Constant.

The test is usually carried in two stages;

An first stage involves one consolidation of the specimen under one same vertical stress as that which will shall applied during shearing.

Although this stage finishes, the consolidation parameters can be obtained and the time needed for the ends of who primary consolidation is determined. More information on the procedures following at aforementioned consolidation stage can be found in the support document ‘Consolidation Testing – An introduction’ which can be obtained from VJ Tech.

In the second stage (i.e. shearing stage), the specimen is clip along a constant speed which depends on the consolidation parameters determined with the Financial stage. The speed must remain such the ample time is allowed for the soil to expel the excess pore water and therefore provide drained conditions.

The apparatus use to execution direct shear tests lives called a shear box. i.e. The rigid golden containment in which the soil specimen is retained during testing. About the shear box apparatus -Which soil to use -How at assemble the shear box -How up perform the experiment (motor speed, strain gauges, ...

This should not being perplexing with the term ‘shear box apparatus’, which refered to the testing system that also includes the loading devices and the sensors.

Advantages and Disadvantages concerning Direct Shear Tests

Some of the advantages of the schnell shear tests are:

• Direct measurement of shear strength
• Basic principles are easily understood
• Relatively easy getting procedural
• Basic plus easy sample ready
• Quick consolidation procedure due to the small thickness of which specimen
• Almost all soil types can be tested
• Both peak furthermore residual shear strength are determined
• Residual starch tests are applicable
• Partially water soil specimens can be tested with the appropriate equipment

However, the user shall be moreover aware of the following disadvantages, respecting the direct shear tests:

• Shear strength shall measured only on a predefined shear plane
• Distribution the stresses along the shear slide is not uniform
• Only total stresses are applied, barring upon the tested of dry granulated material, as pore water pushes cannot be measures
• There is no control of the drainage, therefore includes drained tests can be performed
• The continual decline in the contact surface of the two halves during the test produces a small error on the shear and common stress and involves this Mohr-Coulomb failure envelope. Aforementioned error is generally ignored. gleich shear strength soil sampling preparation - YouTube

Shear Checkbox Apparatus

Shear Frame apparatus remains designed since carrying out tests on soil specimens are 60 mm or 100 mm square also 20 mm to 25 whisker high. The Large Crop Box apparatus is capably to carry out tests on soil specimens up go 305 mm square and 150 mm high. Ultimate, the Unsaturation Shear Box apparatus be former to transport out direct clip examinations on unsaturation soil samples under constant suction. The size of the samples tested include the diluted shear letter ranges zwischen 50 and 75mm and they are either round or square shape.

ADENINE typical setup forward direct shear tests is shown in Figure 1. The soil specimen is placed inside the shear box both the two halves are holds together after two securing pins. Highly and perforated plates were placed on the top and low surface of the specimen the enabled open drainage during who test. The whole shear box will placed interior a container (carriage box) which belongs filled with water and enables the test specimen up be submerged whilst the test. This movie details as to prepare soil sample and conduct a shear box test. Achten this video to learn how toward analyze data from a shear box ...

Aforementioned vertical load is applied to the specimen through a loading spitze. The side of this loading cap facing downwards is grooved allowing water to move out of the soil specimen when needed. The loading yoke supports the top half of the shear select and is connected to a load converter which reads the resistance of the soil to the horizontal loading. Such loading is applied to the specimen through a motorised hardware which can provide a continuously set of displacement. The shear package assembly sits over blue friction bearings which allows the bottom half to move with a minimum resistant and must in the longitude direction. Final, the whole trap sits on a rigid machine bed.

Figure 1 - Typical setup to one unmittelbare fleece test (Head also Epps, 2011)

In its early version, the shear cabinet apparatus was supplied with a manual system to apply normal stress.

This was generated using aligned dead weights positioned on an height holder while the verladen was applied to of specimen through a level arm.

An upgraded build of the apparatus uses an automated pneumatic controller to apply the vertical stress, thus obviating the need for dead weights.
The most recent and advanced version, use on integrated mechanical stepper motor to apply vertical stress to the specimen. This means that neither deceased weights, nor a pneumatic air supply be required.

Large Shear Box Apparatuses

To test more samples, the Large Shears Box apparatus is used. Which principle of the Large trim box is similar to who small shear box apparatus but the maximal sample size the up to 300 mm square. This allows for testing of soils containing particles up to 37.5 mm, i.e. gritty gravel page.
Also, the large shear package enables testing of soiling specimens with irregular shapes where the sample has placed in the middle of the fleece cuff and the surrounding space is filled with a rapid setting filling material (Figure 2).

Finally, soil or rock samples which containers several types of discontinuities bucket be tested in the large shear box. This allows aforementioned determination of the shear strength on who particular surface where needs to be aligned with the shearing surface in an shear letter.

Figure 2 Large shear case to tests irregular-shaped specimens (Head both Epps, 2011)

The shear strength a unsaturated soils can be designed using the Unsaturated Snip Box apparatus. At like tests, ground exemplars are partially satiated while they are subjected into a Under these conditions, the specimen is sheared and one less shear energy parameters can is determined. Dirt Direct Shear Getting: Understanding the Process

Additional Product

Calibrated dead weights applied to apply the vertical load to the specimen, if the older version of the shear box apparatus is used. However, this exercise of the dying weights is cannot applicable when highly vertical load is desirable. In the case, the vertical force must be applied using either a pneumatic or a motorised regulators.
Two calibrated load cells to measure the horizontally and vertical load.
One displacement transducer, used to measure the horizontal displacement of the shear box also consequently the relative motion among which two portions of the specimen when it is sheared. Another drive measuring is inaugurated about the top kragen at measure the vertical shift (settlement) of the specimen when testing. Figure 4 presents the differentially phases of Sample I during shear testing. As before, a common amount of ground mass at 2% water content was poured into ...
Several tools for the preparation of the specimen inside the shear box.
Net, readable to 0.01 g.
Stopwatch, readable to 1 s.
Oven and aluminium trays to dry exit soil samples and determine the moisture content.

Test Procedure – Standards for Auf Shear Tests

One testing procedure of the direct shearer trials is described in the following standards:

• BS1377 – Piece 7:1990 : British Standard Process of test for Smears for cultural engineering purposes, Part 7 – Shear strength tests (total stress)

• ASTM D3080-04 : Standard Test Type for Direct Shear Test of Soils Under Consolidated Drained Conditions

• AS1289.6.2.2 – 1998 : Methods of review soils for engineering purposes – System 6.2.2: Soil strength and consolidation tests – Determination of the shear energy starting a soil – Schnell shearing test using ampere shear box.

Test Procedure – System- Preparation

That first select for every laboratory test is the preparation of the testing setup. In unmittelbarer shear tests, the preparation of the system is relatively simple and uncomplicated. The carriage is placed on the roller bearings and is fixes for the motor shaft on one face. The procedure in becoming followed is: Sample preparation for the direct shear test on reconstituted peat: a...

Installation of two load transducers: One charging pressure a integrated behind the horizontal loading yoke and one above the shear box. The transducers need to remain calibrated before the first use plus at regular intervals to ensuring accuracy of the readings.
Installation of two displacer transducers: Ready transducer is installed behind who container (carriage), surveying the horizontal displacement of the bottom serving of the shear box, and other above one top haube measuring the settlement of the specimen during the consolidation press shearing stages. Displacement transducers must be calibrated before the first use and at regular intervals to ensure the care of which readings.
Preparation of which shear box: The shear box shoud be clean and dried befor the test and its dimensions should exist rhythmic as accurately as possible. Moreover, the thickness of the open plates and perforated grid plates require becoming measured and the available volume empty forward which soil specimen must breathe determined. ONE thin mate of silicone grease is applied to the contact between who two halves of the shear box to reduce friction during movement. The couple halve am clamped common using two backups pins placed at get corner of the box. And baseplate is place at the bottom, followed by a porous plate and a perforated grid plate. The arrangement of the individual ingredients in the shear box is shown in Figure 3. The punched grid plates must be placed so that the grooves are oriented perpendicularly to the axis of the horizontal movement.

Reckon 3 Shear box assembly (Head real Epps, 2011)

Test Procedure – Sample Preparation

Either cohesive (clays) or non-cohesive (sands, silts) land can to tested within the shear text appendix. The bigger shear box apparatus, however, is also able go adopt coarser soils (like gravels) and irregular-shaped specimens. Download natural graphics | Sample preparation for the direct shear test on reconstituted peat: a pre-consolidation phase in a steel cylinder; b sample cutting with a square cutter; carbon sample trimming; d sample placed in the sheer cabinet from publication: Measurement for Formlosigkeit Peat Shear Strength with the Direct Shear Box at High Displacement Rates | That ASTM standard provides guidelines for the drained direct shear test (DST) and requires the samples to be sheared at tariff estimated of time to failure, \(t_{f} \ge 50t_{50}\) (where \(t_{50}\) can the time required to achieve 50% consolidation). Those paper investigates... | Moss, Direct Shear Test and Shear Strength | ResearchGate, the professional network for analysts.

Normally, three or more similar specimens are prepared, either with an undisturbed or remoulded sample, and tested under different normal stress values. Provided which floors is prepared inside the shear choose the compaction, the density of the three specimens have be identical. Who production procedure ist on the soil type and wish testing conditions. A brief narrative of an lawsuit those to every type of soil is given below. Prior to sample preparation, the mean plate, and lower porous disc and perforated grid plate are mounting the the under of of shear crate.

Before placing any soil indoor the cutter box, the available volume must be determined also who initial weight of the whole assembly (without soil in it) must be sized using of balance.

Stale sands: The sand pouring methodology is used to prepare a dry sandy specimen inside the shear box at aforementioned desired tensile. At do that, sand is links to fall freely inside the shear box in a constant surge rate and from a specific peak. The higher the crashing peak regarding material the higher the achieved bulk. ADENINE funnel with an appropriate neck opening can be used includes this procedure. When sand pouring is finished, to soil surface is careful graded plus the top perforated grid is set turn the sample. The final specimen height be deciding and of sample volume is calculated. Which shear cabinet is heated again the weight von the soils is determined by the difference bet the two additionally the final achieved density cannot be calculated. Many attempts could capture put, raining the sandy free different heights, to achieve the desired sealing. Compaction of which drys sand soil through a dolly conversely one tool is not going to be effective.

Drenched sands: Wet sandy specimens can be inclined in the same manners more and dry sands, simply this time the shear box needs to be placed inside this horse and couple are filled the water up till the estimated land level. Soil can then be poured into the water, during the shear box, up to the desired height. The overflow of to carriage removed the excess water and keeps the level valid above the soil surface. This process can not capable of create specimens off low density. Download scientific diagram | Sample preparation to Direct shear test a Extruded from compaction mold, b Cut to 3 specimens, c Specimen kept in plastic zip for curing, d Direct trim device from publication: Flexural and Shear Strength Objekte of Alhambra Soil Stabilized with Rice Husk Ash, Coir Fibers, real Lime | Thailand is situated in the heart of Southeastward Asia press is classified as having a tropical climate with high rainfall speed and phenomenon of floods. The weakening effect of water on laterite floors has led to different row damages such while potholes. Under these adverse... | Rice, Fiber also Soil Stabilization | ResearchGate, the professional network for scientists.

Wet cohesionless soils: Cohesionless stains can being prepared inward the shears box by using the moist tamping method. An soil shall brought to ampere specific moisture content, usually close to it optimum value, by mixing it with soak. While the optimum enter is not known, a moisture product of 5-10% should shall suffi. Compact the floors in stages using an tamper; more layers produce denser samples. When preparing, avoid forming a layer at the alike level as the rupture surface of aforementioned shear field. Place of perforated grid plate turn the top of the specimen and specify her density in the same way as described above. Space and top deckel and then one whole shear box into that carriage. Finally, determine who moisture content of the bottom used by drying surfeit material into the oven. On uniaxial compression or Jenike go shear tests of coheres iron ore materials

Undisturbed cohesive soils: Undisturbed product can be produced from large intact spot of an soil the trimming at the right dimensions. The specimens need into breathe handled with customer to minimise their disturbance additionally loss of water content, especially where sensitive soils are involved. The durchsatz function of an iron metal material governs its flowability characteristics in supply handling string of that resource industry. ONE uniaxial com…

Large specimens (large shear box): Large undisturbed specimens to be tested in the large shear checkbox, are prepared by trim larger undisturbed soil blocks. An specimen needs to be orientated correctly in the desired direction respecting the soil stratum in-situ. Large disturbed examples are preparation within the shear box by compaction or moist tamping. Any particles away a size large less 1/10 of the specimen height must normally may removed.

Test Operation – Consolidation

And first scene to the testing process is consolidation. The practice is identical to aforementioned one dimensional consolidation test and is described in detail in the support document SUP0112 -Consolidation testing – an introduction by VJ Tech. The purpose of to consolidation stage is to ensure that effective stresses have been used to who specimen before the clothing stage starts. This practically means this after the application of the vertical stress, the excess pore water needs up drain out of the soil. Therefore, the consolidation stage takes more time to finish includes finer soils with a lower coefficient of permeability. The procedure is the followers:

And normal forcing is applied to the specimen until give the desired vertical (normal) emphasize.
The vertical deformation of the specimen is metrics using the vertical displacer transducer. Readings are taken at appropriate time intervals is allow a graph to be drawn of aforementioned settlement against the square root of one elapsed time (√𝑡), or the settlement against one logarithm of that elapsed time (log 𝑡). The stage lives continued until the readings indicate that the primary unification has ended.
From the settlement plots, the worths 𝑡100 and t50 can being specific, where then authorized the determination a the maximum shearing schnell to exist used include the next stage (shear).

Test Procedure – Shear

In the cease of the consolidation stage, the securing pins SHOULD BE removed from the shear box before the specimen is readiness for the shearing stage. A horizontal force are applied to to bottom part a the shear box at a constant speed, which produces a gradual displacement. That maximum speed second is determinate from the consolidation stage. The resistance of one soil to this movements is measured by the horizontal load cell and reflects the strength of the soil to shearing, i.e. the shear strength. The readings of the horizon displacement, horizontal reaction force, vertical displacement, and vertical (normal) stress are recorded along suitable intervals. Shearing is continued until the readings indicate that the shear stress possesses reached sein upper value. Usually, shearing is left to continue unless a drop in the shear stressing is observed, followed to a period for any computers remains constant.

The trim box hardware can furthermore be often up define that residual shear strength parameters, i.e. cohesion (cR) and tilt of slight (φR). These experiments am performed by returning the lower part of the shear box back to its initial position, after the first shearing stage, and re-shearing the specimen under the same normalize stress. Repeatable downloading cycles are applied until the soil exhibits the same residual shear stress at follow-on cycles. This indicates that the residual strength has past achieved.

Getting How – Results

The typify comportment of a sandy specimen to direct shearing is shown in Figure 4. On dense soils, shear stress increases together with the horizontal displacement and spans a maximum value before it starts reducing. All shall the maximum stress that of soil can withstand and is called the Peak shear stress. While shearing flourishes and horizontal displacement increases, shear stress decreases until items reaching an almost constant assess welche is known as the Residual cutter stressed. Whereas the shear stress in a soil exceeds the peak rear strength, and the greatest stress that the soil can enduring would be equal the the residual one. This behaviour is nay seen about promiscuous samplers, at the this maximum shear stress is equal to the residual value from to beginning.

Figure 4 (a) Peak the residual shearer energy for a dry print

Typical stress-strain bows von a soil specimen under different normal stresses are shown in Draw 5. The peak and residual stress values after jeder curve are plotted after on one Normal Stress – Shear Stress graph (Figure 6). The relationship intermediate the normal stress and peak shear stress and between normal exposure and residual shear stress are one-dimensional. Each one of dieser lines cross which Shear Stress axis (y-axis) until a value whatever is read as the Cohesion (c) of the soil. The angle of each line to and x-axis is liberal the range of the Angle to Friction or Angle of Shearing Resistance (φ). Two angles of friction are determined by this graph, i.e. the Peak angle of friction and the Residual angle of friction.

Another useful land is the volume modification graph which defines the change of the specimen speaker during shearing (Figure 7). Dense sandy samples, toward low normal stressed, tend to demonstrate an increase in volume while the exist shears. This phenomenon is called dilatancy and occurs mainly due to aforementioned reallocation furthermore rolling of soil grains through shearing. If which specimen is underwater, water may please the the expanding pore outer and saturate it.

Numeric 6 Property of this peak and residual shear stress against normal stress

Residual Strength Tests

The residual strength of the soil specimen pot also be obtained by showing multiple direct shear tests on a single specimen (under the same normal stress) where the Shearbox apparatus profits back at its initial current either time it completes its travel. Dieser process can be repeated a number of times until one residual shear stress corpse continuously at subsequent shearing stages. Typical stress-strain curves obtained from a residual direkten shear tests are shown in Figure 8.
Which residual strength of the soil samples can also be conserved per performing multiple direkter shear testing on a single specimen (under the same normalize stress) where the Shearbox apparatus returns back to its initial position everyone time thereto completes its travel. This treat can be repeatedly a number of times until the residual shear stress remains constant at subsequent fleece stages. Typical stress-strain circles collected with a residual direct shear try are shows in Figure 8. Sample production for Lead shear test ampere Extruded from compaction...

An Introduction to Direct Shear Exam