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 1. The shear strength of a granular soil is made of two components :- a) Mineral to mineral friction due to sliding and rolling. b) Degree of interlocking. 2. The shear strength of cohesive soil is cohesion due to the bonding between particles. 3. The component (c) of the shear strength is known as cohesion. Cohesion holds the particles of the soil together in a soil mass. The angle (Φ) is called the angle of internal friction. It represents the fractional resistance between the particles. UNCONFINED COMPRESSION TEST DEFINITION The unconfined compression test is used to measure the unconfined shear strength of fine-grained soils, which is an approximate value of their undrained shear strength. INTRODUCTION In the unconfined compression test, the cylindrical soil specimen loaded axially (compressive axial stress ) without a lateral support, which is mean that the minor principle stress & (confining pressure) is zero as shown in Fig.(1). Figure 1: A small cubical element undergoes a compressive axial stress only. The Mohr circle can be drawn for stress condition at failure. As the minor principle stress is zero, the Mohr circle passes through the origin as shown in Fig. (2). The failure envelope is horizontal (Φ = 0). The cohesion intercept is equal to the radius of the circle. 1
1 S u c u 2 q u 2 Each point on the Mohr s circle gives the stresses σ and τ on a particular plane. A Axial stress Su = cu Failure plane 45 Failure envelope Φ = 0 A Axial strain Є Pole σ1 = qu (a) (b) Figure 2: (a) A typical stress-strain response during the unconfined compression test, (b) The corresponding evolution of the Mohr circle in the total stress. The unconfined compression test may be either strain-controlled or stress-controlled. Height-to-diameter ratio: The length diameter ratio of the test should be long enough to a void interference of potential 45 failure planes of Fig. (3) and short enough that we do not obtain a "column" failure. The L/d ratio to satisfy these criteria is: 2 < L o/d o < 3 D o D o When L o/d o < 2, potential failure zones overlap When L o/d o > 2, no overlap of failure zones 45 o L o < 2D o When L o/d o > 3, specimen may act as column with bulging failure Figure 3: L o/d o ratios for any soil compression test (unconfined, Triaxial or other). 2
Types of failure Three types of failure are recognized 1- Plastic Failure: in which the specimen bulges laterally into a barrel shape without splitting as in Fig. (4.a). 2- Brittle Failure: in which the specimen shears along one or more well defined surfaces as in Fig. (4.b). 3- Semi-plastic Failure: failure in a manner intermediate between (a) and (b) as in Fig.(4.c). (a) Plastic failure (barreling) (b) Brittle failure (shear plane) (c) semi- plastic failure (intermediate). Figure 4: Modes of failure in compression test specimens Area correction: It is essential to correct area for each reading during testing which is explained in Fig.(5). Figure 5: Barreling deformation of compression specimen. 3
PURPOSES The unconfined compression test is widely used for a quick economical means of obtaining the approximate shear strength of a cohesive soil. APPARATUS 1. Mechanical load frame, either hand operated or machine driven. 2. Device for Load and deformation measurements (dial gauges or transducer). 3. Apparatus for extruding and trimming specimens (Split mould 38mm dia., 83mm long). 4. Vernier calipers. PROCEDURE a) Preparation of apparatus: 1) Ensure that the load frame stands firmly on a solid level support. 2) Check that the load and deformation measurement device (transducer) are connected with transducer read-out. 3) If a motorized unit is used, select the gear position which with give a platen speed of between 1 and 2% of the specimen length per minute. b) Preparation of specimens: The method of preparation depends upon the type of sample available, the most usual being as follows: 1- Undisturbed specimens from sample tube. Extrude specimen from the base end of the sample tube which is taken in-situ by extruder devices. 2- Remolded (disturbed) specimens. It can be prepared by compacting a soil in a cylindrical mold (compaction mold), then it is extrude from the mold by extruder devices. c) Test procedure 1. Measure the length and diameter of the specimen. 2. Place the specimen centeraly on the lower platen on the machine and check that the specimen axis is vertical. 3. Adjust the stress and strain gauges (transducer read out) to read zero. 4. Start the test and record the reading of load and deformation from transducer read out every (20 sec or 0.5mm). 5. Continue loading and taking reading until it is certain that failure has occurred according to one of the following criteria: a) Load decreases on sample significantly. b) Load holds constant for 4 readings. c) Deformation is significantly past 20% strain (very plastic clay). 4
CALCULATION 1. Compute the axial strain (Є), the corrected area (A c) and the compressive axial stress for all readings to define stress strain curve. L Є * 100 L 0, A c A 0 1 and q u = P A c So the undrained shear strength (S u) of the soil is: where: S u q u 2 axial strain, ΔL= change in length. L o= initial length of specimen. A o initial area of the specimen, P axial load at failure, A c corrected area q u = The axial stress ( ) at which the specimen fails. 2. Plot stress versus strain show q u on the graph, to obtain the corrected max. value of q u, place a ruler along the vertical axis and a set-squire against it so that a horizontal line can be drawn in just touch the peak of the curve as shown in Fig.(6). 3. Draw the corresponding Mohr circle and show the value of the soil cohesion (c u) as shown in Fig. (2.b). 4. Compute the second modulus of elasticity (E = ) of soil for stress of 0.75qu only. Figure 6: Plot of unconfined compression test data to obtain a stress-strain modulus and best value of qu. 5
DISCUSSION 1. How can you determine the sensitivity of a soil by UCT? Explain. 2. In which type of soil, deformation during UCT exceeds 20% strain? 3. According to relative consistency (very soft, soft, firm (stiff), hard and very hard), what is your soil consistency? 6