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ENCE 361 Soil Mechanics Unconfined Compression Test
Overview The unconfined compression test is used to measure the unconfined compressive strength of a cohesive soil. The unconfined compression test is applicable only to coherent materials such as saturated clays or cemented soils that retain intrinsic strength after removal of confining pressure; it is not a substitute for the Q test.
Overview In this test, a laterally unsupported cylindrical specimen is subjected to a gradually increased axial compression load until failure occurs. The unconfined compression test is a form of triaxial test in which the major principal stress is equal to the applied axial stress, and the intermediate and minor principal stresses are equal to zero.
Overview The unconfined compressive strength, q u is defined as the maximum unit axial compressive stress at failure or at 15 percent strain, whichever occurs first. The undrained shear strength, s u, is assumed equal to one-half the unconfined compressive strength.
Apparatus 1 Specimen Preparation Equipment Similar to Triaxial Tests Loading Device Measuring equipment, such as dial indicators and callipers. Timing device, either a watch or clock with second hand. Balances, sensitive to 0.1 g. Apparatus necessary to determine water content and specific gravity.
Loading Device Modern Ancient
Preparation of Specimen Similar to Triaxial Tests Two Possibilities of Specimen Preparation Undisturbed Specimens Disturbed Specimens Testing of Disturbed Specimens necessary to determine sensitivity of soils
Undisturbed Specimens Generally, undisturbed specimens are prepared from undisturbed tube or chunk samples of a larger size than the test specimen. Specimens must be handled carefully to prevent remoulding, changes in cross section, or loss of moisture. To minimise disturbance caused by skin friction between samples and metal sampling tubes, the tubes should be cut into short lengths before ejecting the samples.
Undisturbed Specimens Specimen preparation must be dimensionally accurate. As with triaxial specimens, a membrane is placed over the specimen before loading. Sample ejection should be accomplished with a smooth continuous and rapid motion in the same direction that the sample entered the tube. All specimens shall be prepared in a humid room to prevent evaporation of moisture.
Undisturbed Specimens If the specimen is not tested immediately after preparation, precautions must be taken to prevent drying and consequent development of capillary stresses. When drying before or during the test is anticipated, the specimen may be covered with a thin coating of grease such as petrolatum. Part of the undisturbed soil should be laid aside for a water content test.
Remoulded Specimens Remoulded specimens usually are prepared in conjunction with tests made on undisturbed specimens after the latter has been tested to failure. The remoulded specimens are tested to determine the effects of remoulding on the shear strength of the soil. The remoulded specimen should have the same water content as the undisturbed specimen in order to permit a comparison of the results of the tests on the two specimens.
Remoulded Specimens Place the failed undisturbed specimen in a rubber membrane and knead it thoroughly with the fingers to assure complete remoulding of the specimen. Remove the soil from the membrane and compact it in a cylindrical mould with inside dimensions identical with those of the undisturbed specimen.
Remoulded Specimens Carefully remove the specimen from the mould, preferably by means of a close fitting piston, and plane off the top of the specimen. The specimen is then ready for testing. Weigh, measure and load the specimen in the same manner as the undisturbed specimens.
Procedure Record all identifying information for the sample such as project, boring number, visual classification, and other pertinent data on the data sheet. The data sheet is also used for recording test observations described below.
Procedure Place the specimen in the loading device so that it is centred on the bottom platen; then adjust the loading device carefully so that the loading ram or upper platen barely is in contact with the specimen. Record the initial reading of the dial indicator on the data sheet. Test the specimen at an axial strain rate of about 1 percent/minute.
Procedure Observe and record the resulting load corresponding to increments of 0.3 percent strain for the first 3 percent of strain and in increments of 1 or 2 percent of strain thereafter. Stop the test when the axial load remains constant or when 20 percent axial strain has been produced.
Procedure Record the duration of the test, in minutes, to peak strength (time to failure), type of failure (shear or bulge), and a sketch of specimen after failure on the data sheet. After the test, place the entire specimen or a representative portion thereof in a container of known weight and determine the water content of the specimen.
Computations Index Properties Water Content Volume of Solids Void Ratio Degree of Saturation Dry Density Stress-Strain Properties Axial Strain Corrected Area of Specimen Compressive Stress All of these quantities should be computed at each load increment during the test
Presentation of Results Stress-strain curve(s) should be plotted for each Unconfined compressive strength q u is the compressive stress at 15% strain When remoulded specimens are tested, the sensitivity ratio is S t q u undisturbed q uremoulded
Possible Errors Test not appropriate to type of soil. Specimen disturbed while trimming. Loss of initial water content. A small change in water content can cause a larger change in the strength of clay, so it is essential that every care be taken to protect the specimen against evaporation while trimming and measuring, during the test, and when remoulding a specimen to determine the sensitivity. Rate of strain or rate of loading too fast.
Final Exam Schedule Last Lecture Session 26 November 2001 Final Review Session 28 November 2001 Final in Two Parts Take Home Part - 50% Given out 28 November Due 3 December In-class Part = 50% Done In Class 3 December Last Homework Day 3 December 2001
Stability of Slopes
Slope Stability One of the most important topics in the development of geotechnical engineering Also an important topic in actual practice Unsupported slopes are found in a wide variety of projects Highways Elevation changes on sites
Development of Theory The need for angled slopes has been understood since antiquity The theory necessary to analyse these slopes dates from the years during and after World War I The subject was especially important in Scandinavia, where sensitive clays were very subject to catastrophic failure
Gothenburg Harbour Failure 5 March 1916
Gothenburg Harbour Failure 5 March 1916 Soft clay deposit, 150' deep 50' was dredged out and replaced by sand fill; piles were driven to stabilise the quay Several hundred feet of wall slid seaward as shown
Appointment of Swedish Commission In 1913, the Swedish State Railroad Administration appointed a special Commission the first so titled to study these types of failures and to recommend a solution Its chairman, Wolmar Fellenius, developed the basic methods for analysing rotational failures of slopes which, with improvement, we use today
Types of Slope Failure Falls Topples Slides Spreads Flows Falls Slope failures consisting of soil or rock fragments that drop rapidly down a slope Most often occur in steep rock slopes Usually triggered by water pressure or seismic activity
Topples and Slides Topples Similar to a fall, except that it begins with a mass of rock of stiff clay rotating away from a vertical joint Slides Slope failures that involve one or more blocks of earth that move downslope by shearing along welldefined surfaces or thin shear zones
Types of Slides Rotational slides Most often occur in homogenous materials, such as fills or soft clays Translational slides Move along planar shear surfaces Compound slides Complex and composite slides
Spreads and Flows Spreads Similar to translational slides, except that the blocks separate and move apart as they also move outward Can be very destructive Flows Downslope movement of earth where the earth resembles a viscous fluid Mudflow can start with a snow avalanche, or be in conjunction with flooding
Homework Set 7 Textbook Readings Problems (cont'd) Chapter 15 (pp. 612-668) No Laboratory Soils Testing experiments Problems 15-8: Use Fellenius Method (hand or spreadsheet) 15-10 (use Simplified Bishop or Slope-W) 15-14 ((a) and (b) only; use Taylor charts or Slope-W) 15-29 (use Morganstern's rapid drawdown curves) Borrow volume problem
Borrow Volume Problem A 3.0 ft deep cut is to be made across an entire 2.5 acre site. The average unit weight of the soil is 118 pcf, and the average moisture content is 9.6%. It also has a Proctor maximum dry unit weight of 122 pcf and an optimum moisture content of 11.1%. The excavated soil will be placed on a nearby site and compacted to an average relative compaction of 93%. Compute the volume of fill that will be produced, and express your answer in cubic yards. Due Date: 3 December 2001
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