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CHARACTERISTICS of SILT Ulider LOADING

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CHARACTERISTICS of SILT Ulider LOADING A L~BORATORY STUDY OF SETTLEMENT CHARACTERISTICS OF SILT UliDER LOADING by Mumtaz H. Rehman A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Engineering. Department of Civil Engineering and Applied Mechanics, McGill University, Non treal. April, 1961. ACKNOWLEDGMENT The author ~dshes to express his gratitude to Dr. Raymond N. Y. Yong for his support and direction of this study; and also to Professer V. W. G. Wilson for his assistance in the erection of the loading frame. C 0 N T E N T S Page No. HJTRODUCTION 1 THE ORY 3 P!lli"""VIOUS HORK 11 EXPER:OVLENTAL \~TORK 19 EFFSCTIVE STRESSES 34 l'iODULUS OF ITOLUHE CHANGE 40 cmrBU.fllW PRESSURE AND VALU.:!: OF 1 E 1 42 TEST RESULTS 46 SETTLEl'ŒNT ANALYSIS AriD DISCUSSION 66 PRACTICAL AP~:lLIGATION 80 CONCLUSIONS 81 SUGGESTIONS FOR FUTULŒ RESEAR.CH 83 BIBLIOGRAPHY 84 APPE~'DIX 92 - i - I L L U S T R A T I 0 N S GRAPHS Page No. Graph No. l Settlement vs. width of pla te (after Goerner and Press) 12 Il !! 2 Grain size distribution - diameter vs. percent finer by weight 20 Il " 3 Deviator stress vs. percent strain (cell press. 15 psi) 47 Il " 4 Deviator stress vs. percent strain (cell press. 30 psi) 40 !! ,, 5 Deviator stress vs. percent strain (cell press. 45 psi) 49 " Il 6 Deviator stress vs. percent strain (cell press. 60 psi) 50 " Il 7 Devia tor stress vs. percent strain (Unconfined) 52 Il rr 8 Pressure vs. void ratio (initial porosity 36.3%) 53 rr Tl 9 Il If Il Il ( 11 Il 37.4%) 54 Il Il 10 Il Tl " Il ( " Il 38.8/b) 55 Il Il ll 1[ " Il Il ( Tl " 43.4%) 56 Il !! Il Il 4::) . '') " 12 " " " ( :_). ?~b 57 fi " 13 " >1 Il " ( tl tl 5L~.l%) 58 Il Il 14 Pressure vs. modulus of volume change 59 Il tl 15 E~astic modulus vs. porosity 60 Il rr 16 Pressure vs. settlement 65 li rr 17 Elastic modulus vs. confining pressure 68 -ii - Page No. Gr a ph No.l8 Comparison of theoretical and actual settle- ment - Loose state 75 fr fr 19 Comparison of theoretical and actual settle- ment - Medium state 76 fr Il 20 Comparison of theoretical and actual settle- ment - Dense state 77 PLATES Plate No. l Sho1>.."'ing Triaxial Compression Apparatus 22 Il Il 2 fr Il Il Il 23 " " 3 Showing Consolidation Apparatus 26 Il fr 4 Showing Plate loading Test Ap:paratus 31 FIGURES Fig. No. l Pore pressure coefficients A and B (after Skempton 1954) .3 fr Il 2 Pla te loading test apparatus 30 Il Il 3 Mohr circles 43 Il 11 4 Pressure bulb und er a footing 44 fr Il 5 Hairline cracks 64 - iii - APPENI!IX TABLES Page No. Table No. 3 Calculation of confining pressure 93 Il Il 4 Pore pressure coefficient 'A' 94 If Il 5 Values of .,( 95 li li 6 Modulus of volume change - Loose state 96 Il Il 7 If Il Il Il - l'ledium state 97 Il Il 8 Il Il Il Il - Dense state 98 Il Il 9 Settlement calculatians - Loose state 99 Il Il 10 Il If - l'!:edium state lOO Tl Il 11 - Dense state 101 - i v - S Y N 0 P S I S A LABORATORY STUDY OF S:E.'TTLEI-îENT CHAR.ACTE.:l.ISTICS o:F' SILT UND1.ill LVAJING Settlement characteristics of silt have been studied in ~elation to those of sand and clay. This involved investigation of the immed­ iate elastic settlement due to deformation occurring under constant volume, and consolida.tion as a function oi' pore pressure dissipation. Oedometer and Triaxial compression tests (undrained and consoli­ dated-undrained) vlith pore pressure measurements vmre performed to obtain the values of the parameters my, the modulus of volume change, E, the elastic modulus, and pore pressure coefficient 1A1 • Plate-load tests on dry silt using a 611 square plate were conducted to check its behaviour pattern. For silt in dry state, it -..ras found that settlements could be predicted according to the classical elastic equation for immediate elastic settlement only, and that consolidation did not play any part. However, i t is expe cted, that consoli dation "-Tiich is dependent on the pore pressures set up under the load, may greatly influence the total settlement if the silt -.;v-ere saturated or nearJy so. Previous studies on madel plate loading tests on silt have been critically examined i\'ith respect to the re sul ts obtained from this study. In conclusion, suggesti ons are made to gui de further research on the subject. I N T R 0 D U C T I 0 N Settlement studies of structures on sand and clay have received considerable attention in the past. Little, however, is kno~n about settlement behaviour of silt under loading. In the case of settlement, ~ of footings on sand, Terzaghi's empirical equation b 2 q ·(-) .c (Terzaghi 1943, and Yong 1959) L b+l can be used, where C is a constant. The settlement of footings on clay is composed of three parts:- i) Immediate elastic settlement ~nich takes place without change in water content corresponding to the change in stresses. ii) A primary consolidation settlement which corresponds to change in stresses caused by dissipation of pore water pressure. iii) A secondary compression generally kno~n as secondary time effect. (This is not very well understood at present status of our knowledge.) The object of this work was to study settlement characteristics of silt under loading. This entailed determination of settlement parameters E and mv and their variation with porosity and pressure. The behaviour of cohesive soils can be explained by electro- chemical theory (Lambe 1958, 1960, Rosenqvist 1959, Harkentin and Yong 1960, and Yong 1960) and that of frictional soils by combined stress theory, but since silt falls in between, there is a need for more data for evaluation of its behaviour. The theory of 3ettlements on sand and clay ~~ll be examined and applied to silt. A check of the theory will be maàe by loading a moàel f ooting on silt. It will also be necessary to examine the previous work done on compression of silt under mod.el footings. - 2 - T H E 0 R Y The deformation characteristics of soil are best understood by visualizing it as a compressible skeleton of solid particles enclosing voids which in saturated sail, are filled •~:L th vJater, or, in partly saturated soil, ~~th both air and water. The normal stress on any plane is in general, the algebraic sum of t-v;o components - the stress carried hy the solid particles and the pressure in the fluid in the void space called the pore pressure. To obtain a clear picture of how the pore pressure responds to the different combinations of applied stress, the concept of pore pressure coeff icients is f ound to be conven- ient. (Skempton 1954 and Bishop and Henkel 1957) It is assu~ed that the compressible skeleton of sail particles behaves as an elastic isotropie materi al and t he fluid in the pore space shows a l:i.near relationship between volume change and stress. l~ l à&"3 ~ t.s-,- Atrl> ~ ~+tocs-, DL + o~ + D = o_!>·b ... àu. bu 1\ àu.J_ Fig. No. 1 (after Skempton 1954) If an element of soil is originally in equilibriurn under an all-round effective pressure p, then the application of stresses t.cr, and A~>'!tcan be considered as t aking pl ace i n t~o stages. Firstly, t he element is subjected to an equal all-round increment A~0 and secondly i t is subjected - 3 - to a deviator stress ( àSj- ACSS). Corresponding to each of these stages, be Au. and àud. there •,;'ill pore pressure changes 4 t."here àu. The increase in effective stress in a partially saturated sail is ::. If Cc is the compressibility of the sail structure, then the volume change is ••• (1) where V is the original volume. If Cv is the compressibility of the fluid (air and water) in the voids and n is the porosity of the soil, then the change in volume in the void space is ••• ( 2) But these two changes in volume are identical and hence from equations (1) and (2) àu.o. Il. = 1;1 = ••• (3) In saturated soils (zero air voids) Cv = o, since the compressibility Cc of water is negligible compared to that of soil structure; ••• B = 1 for fully saturated sail. For dry sail Cv approaches i nfinity since the compressibility of air is Cc far greater than that of soil structure, hence for dry soils, B = o. At any time -vmen the increment of deviator stress is ( b6",-A~), t he pore pressure due to t his increment is .6ud. and the correspond.ing - h - changes in the principal effective stresses are and à~• As it is assumed that the soil particles behave as an elastic material, therefore the volume change of the soil structure under the increment of deviator stress is AVe a c. v. .L ( t.i; ~ '2.6~3) - c ~ or c . v. .1. l (~e-,-.Ar )-3tuAd} ••• (4) h~ =- - c. ~ 3 And the volume change in the void space is 1::.'11 = n . ~~ ~ Il - c"" . v. ••• (5) But these two volume changes are identical and hence from equations (4) and (5) = As the sail does not behave as truly elastic material, the above expression should be ~Titten as Au.
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