This document downloaded from

vulcanhammer.net

since 1997,your source for engineering informationfor the deep foundation and marineconstruction industries, and the historicalsite for Vulcan Iron Works Inc.

Use subject to the fine print to theright.

Dont forget to visit our companion site http://www.vulcanhammer.org

All of the information, data and computer software("information") presented on this web site is forgeneral information only. While every effort willbe made to insure its accuracy, this informationshould not be used or relied on for any specificapplication without independent, competentprofessional examination and verification of itsaccuracy, suitability and applicability by a licensedprofessional. Anyone making use of thisinformation does so at his or her own risk andassumes any and all liability resulting from suchuse. The entire risk as to quality or usability of theinformation contained within is with the reader. Inno event will this web page or webmaster be heldliable, nor does this web page or its webmasterprovide insurance against liability, for anydamages including lost profits, lost savings or anyother incidental or consequential damages arisingfrom the use or inability to use the informationcontained within.

This site is not an official site of Prentice-Hall, theUniversity of Tennessee at Chattanooga, VulcanFoundation Equipment or Vulcan Iron Works Inc.(Tennessee Corporation). All references tosources of equipment, parts, service or repairs donot constitute an endorsement.

ENCE 361Soil 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,

qu is defined as the maximum unit

axial compressive stress at failure or at 15 percent strain, whichever occurs first.

The undrained shear strength, su, 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

Ancient

Modern

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 tquundisturbedquremoulded

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 Failure5 March 1916

Gothenburg Harbour Failure5 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 well-defined 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 Chapter 15 (pp. 612-

668) No Laboratory Soils

Testing experiments Problems 15-8: Use Fellenius

Method (hand or spreadsheet)

Problems (cont'd) 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

Questions?