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Estimating Foundation Settlement by One-Dimensional Consolidation Tests

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Estimating Foundation Settlement by One-Dimensional Consolidation Tests ‘ENGINEERING MONOGRAPH No. 13 United States Department of the Interior BUREAU OF RECLAMATION ESTIMATING FOUNDATION SETTLEMENT BY ONE-DIMENSIONAL CONSOLIDATION TESTS Denver, Colorado March 1953 Engineering Monograph No. 13 ESTIMATING FOUNDATION SETTLEMENT BY ONE-DIMENSIONAL CONSOLIDATION TESTS by Harold J. Gibbs Engineering Laboratories Branch Design and Construction Division Technical Information Office Denver Federal Center Denver, Colorado “,” .-._._.* .___ ENGINEERING MONOGRAPHS are published in limited editions for the technical staff of the Bureau of Reclamation and interested technical circles in government and private agencies. Their purpose is to record developments, inno- vations, and progress in the engineering and scientific techniques and practices that are em- ployed in the planning, design, construction, and operation of Reclamation structures and equip- ment Copies may be obtained from the Bureau of Reclamation, Denver Federal Center, Denver, Colorado, and Washington, D. C. CONTENTS Page INTR9DUCTION.. ......................................... 1 DESCRIPTION OF EQUIPMENT, PROCEDURE, AND DATA .............. 3 Equipment and Procedure .................................. 3 Information Obtained from the Test ............................ 3 THEORETICAL INTERPRETATION FOR THE APPLICATION OF TESTDATA ........................................... 5 Load Consolidation ....................................... 5 ’ Time Consolidation ....................................... 7 LIMITATIONS OF THE ONE-DIMENSIONAL CONSOLIDATION TEST ........ 12 DETERMINATION OF THE PRESSURE DISTRIBUTION BELOW A LOADEDAREA . 13 EXAMPLE OF SETTLEMENT ANALYSIS . , . , . 15 EXAMPLE OF TIME OF CCNSOLIDATION ANALYSIS . , . 16 CLOSING DISCUSSION . , . , . , . 17 LIST OF FIGURES Number Page 1. The one -dimensional consolidometer ....................... 2 2. Load-consolidation test curve for a moist clay ................ 3 3. Time-consolidation test data for each increment of load application. .* ...................................... 4. Method of determining the Compression Index, Cc .............. 5. Determination of the Compression Index for the typical example ..... 6. Procedure for determining the Coefficient of Consolidation, C, ..... 7. Determination of the Coefficient of Consolidation, C,, for the typical example .................................... 9 8. Time factor curves for Cases No. 1, 2, and 3 .... ............. 10 9. Time factor curves for Cases No. 4 and 5 ................... 11 10. Movements caused by loading ............................ 12 11. Pressure distribution by Boussinesq’s Equation ............... 19 12. Pressure distribution by Newmark’s Chart ................... 20 13. Pressure distribution by Newmark’s Table ................... 21 14. Settlement determination by change in void ratio method .......... 22 15. Settlement determination by compression index method ........... 23 16. Time of consolidation determination ....................... 24 LIST OF TABLES Number Page 1 Summary of One-Dimensional Consolidation Test Results . 5 ii INTRODUCTION This monograph demonstrates the ap- 2. Rate of consolidation plication of one-dimensional consolidation test data to a foundation settlement analysis. 3. Influence of saturation on Soil samples are tested in the laboratory to consolidation determine the settlement characteristics of the soil under load. These characteris- 4 Permeability of the material tics are used to estimate the amount of while under load. settlement of a structure which would result In addition to giving information on these from the consolidation of its earth foundation items, the testing equipment has been applied because of the structure load. The test is to such specialized problems as soil-expan- also used to determine the settlements that sion studies and the estimating of pore- will occur within dams and earth embank- pressure development. The main purpose ments. of the test and the reason for its develop- The consolidation characteristics of a ment, however, are to permit rational esti- soil mass are influenced by numerous fac- mates of structure settlement through the de- tors. Some of these are size and shape of termination of consolidation characteristics. the soil particles, moisture content, per- meability, initial density, and physical and The discussions of consolidation and chemical properties of the soil. Because settlement in this monograph include the de- these factors are so numerous it is usually scription of the standard data obtained by the not possible to describe the consolidation Bureau’s method of test, and general appli- characteristics with a high degree of con- cations oi the results to settlement problems. fidence by means of judgment and simple Much of the information has been obtained index values. For structures that are crit- from a research study of many publications ical regarding settlement and those whose dealing with consolidation problems in the cost would justify such tests, it is advisable design of various structures; and throughout to analyze settlement from consolidation this paper footnote references are given for tests on the actual foundation material. the purpose of further study in this subject by the reader when desired. These refer- The one-dimensional consolidation test- ences are to publications issued prior to ing equipment and procedures used in the August 1951, when the manuscript for this Bureau of Reclamation laboratories are publication was completed. similar to those developed by Casagrande. ’ The testing procedures now conform quite consistently with the procedures presented The reader should bear in mind that, in popular soil mechanics publications. 2~3~4 while this monograph is principally con- As conducted by the Bureau the standard cerned with one-dimensional test data, the test5 provides four main items of informa- possibility of shear failure must not be left tion: unobserved, Thus in the design of any foun- dation it is equally important that (1) the 1. Magnitude of consolidation for bearing capacity or criterion of shear fail- various loads ure and (2) the settlement be studied. 1 Casagrande, A., “The Structure of 3 Taylor, D. W., Fundamentals of Soil, Clay and Its Importance in Foundation En- Mechanics John Wiley & Sons, New York, 1948, pp. 212-215. 4 Lambe, T. W., Soil Testing for En- Sk&n Wiley & Sons, New York, 1951, . -. 2 Casagrande, A., and Fadum, R. E., Dotes on Soil Te t-for Enpineermg Pur- 5 “One-dimensional Consolidation Test poses, Gradua: School of Engineering, Designation E- 13 ,” Earth Manual, Bureau .Iarvard University, 1940, pp. 37-49 of Reclamation, Denver, 1951. ,-;;:-.~ rf~ -..1: i ;, ~j-- *H1 JII ,-La ad plates., ~ i i j I '; II i I t--':"r , " , 'C i ;; ;" k , ?c, Base plate ID -.004 FIGURE 1 -The one-dimensional consolidometer . DESCRIPTION OF EQUIPMENT, PROCEDURE, AND DATA Equipment and Procedure at frequent time intervals until consolidation is complete. The specimen is allowed to A photograph and an elevation drawing of consolidate fully under each increment of the standard consolidation test equipment load so that a final magnitude of consolida- (one-dimensional consolidometer) are shown tion may be observed. (This generally re- in Figure 1. The soil specimen is confined quires from 5 to 24 hours.) laterally by a rigid ring 4-l/4 inches inside diameter by l-1/4 inches in depth, and is A permeameter tube, attached to the loaded and drained in the vertical direction. base of the container and leading directly Porous plates at the top and bottom allow into the bottom porous plate, is provided to moisture and air movements into or out of saturate the specimen and measure its per- the specimen. The top porous plate is free meability. The head of water in the tube to move downward when a load is applied, provides the pressure that causes flow and the amount of settlement of the speci- through the soil, and the amount of water men is read on a dial gage graduated to flowing through the specimen in any given l/10,000 of an inch. time interval is measured by the drop in head in the tube. From these data the co- The load is applied in a series of four efficient of permeability of the soil may be or more increments, usually 12.5, 25, 50, computed for the density or void ratio con- and 100 percent of the maximum load. In- dition at the time of the test. crements of 1.5, 3, 6, 12.5, 25, 50, 100, and 200 percent are recommended when a greater number of increments are desired. Information Obtained From the Test The intensities of load to be used depend on the weight of the structure and the over- The general method of plotting test re- burden pressures that occur in the mate- sults is shown in Figures 2 and 3. The erial, and should be of such values as to curves in these figures are plotted on the include the maximum anticipated pressure basis of consolidation in percent of initial on the foundation Loading the test speci- volume. They show accurately the con- men is performed expeditiously and as ac- solidation of the test specimen, and give a curately as possible to secure readings at general indication of the magnitude and rate such early time intervals as 4, 10, and 20 of settlement which may be expected in the seconds. The rate of consolidation is ob- foundation material represented by the tained by observing the amount of movement specimen. FIGURE 2 - Load-con- solidation test curve for a moiet c2ey. Addition of water after applica- tion of final load does not effect con- solid.ation. 0 20 40 60 60 100 120 140 160 LOAD - PSI TIME
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