Experimental Determination of Pressure Distribution Under a Combined Footing Citation Johnson, Samuel Yorks and Scott, Homer Judkins (1934) Experimental Determination of Pressure Distribution Under a Combined Footing. Master's thesis, California Institute of Technology. doi:10.7907/37jm-wc42. https://resolver.caltech.edu/CaltechTHESIS:09272024-182827005 Abstract No abstract. Item Type: Thesis (Master's thesis) Subject Keywords: (Civil Engineering) Degree Grantor: California Institute of Technology Division: Engineering and Applied Science Major Option: Civil Engineering Thesis Availability: Public (worldwide access) Research Advisor(s): Converse, Frederick James Thesis Committee: Unknown, Unknown Defense Date: 1 January 1934 Record Number: CaltechTHESIS:09272024-182827005 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:09272024-182827005 DOI: 10.7907/37jm-wc42 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16764 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 27 Sep 2024 22:03 Last Modified: 27 Sep 2024 22:04 Thesis Files PDF - Final Version See Usage Policy. 21MB Repository Staff Only: item control page

EXP:H:RIMEUTAL Dl~TERMINATION OF PRESSURE DISTRIBUTION UNDE?R A cmmINED FOOTING Theeis by Samuel Y. Johnson Homer J. Scott In Partial Fulfillment of the Requirements for the Degree of Master of Science California Institute of Technology Pasadena, California 1934 TABLE OF CONTENTS Page Acknowledgement -1- Purpose Introduction - 3- Description of Apparatus The P ressure Cell - 6- The Loading Apparatus - 8- The Footing - l l- Ca libration of the P ressure Cells -1 5- Measurement of Pressure Distribution Field Procedure - l6- Results and Discussi on -19- Conclusi ons - 30- Ap pendix F ield Da ta Sheets - 32 - Pressure Cell Calibration Curves -42- I. ACIGTOW~EDGEMgNT The authors wish to express their appreciation to Professor Frederick J. Converse who suggested the subject for thi8 thesis, and who has rendered valuable advice and assistance in its performanceo They also wish to express their thanks to Profess or Franklin Thomas for his assistance in promoting the project, and to the firm of Labarre & Converse, and to Mr . R. w. Binder of the McClintic Marshall Corporation for the loan of equipment. 2 PURPOSE The object of this experimental research was to determine the general distribution of pressure under a reinforced concrete spread footing of the "combined" type. INTRODUCTION The science of foundations at the present time has little available data concerning the way in which pressure distributes itself directly upon the under surface of loaded footings of the rigid slab type. Knowledge of the distri- bution of pressure in the soil for some distance beneath footings has been experimentally and theoretically investigated, and has resulted in the discovery of the so- ~alled pressure bulb, and the development of the classic Boussineeq theory for its application to determining bearing values of soils. This theory offers a solution for the problem of the stresses which dev~lop within the soil under a loaded footing. The next step in the sequence is a solution of the problem of the stresses which develop in the footing itself . These stresses are due to the direct soil loading on the footing, the point magnitude and distribution of which (lannot "be determined by the Boussin~sq theory, since the Boussinesq theory is bas~d upon an assumed point or distributed load , and it is the assumed distributed load that is desired in this case . Customary procedure up to date has been designing footings upon the basis of a uniform distribution of soil pressure across the under side of the entire footing ar~a. However according to experiments carried on by M. L. Enger, Frede<rick J. Converse, and others, the distribution of the soil reactions over the base of a rigid slab is by no means uniform .. Dr. Charles Terzaghi in his article 0 The Science l. of Foundations" says in regard to th is question. "Thus faI' all attempts to deal theoretically with the problem of stres s distribu tion have failed, and there is 11 ttle hope f or success within the near future. Considering the importance ~f the possible error involved in assuming uniform stress distribution, the need of a more exhaustive experimental investigation of this phase of the foundation problem becomes obvious ." Previous experiments performed for the purpose of determini ng pressure distribution under a foot i ng have used 2. round plates wi.th uniform and concentrated loads, or square 3. blocks , r:entrally loaded. J.n practically all cases the pressure variation found has approximated a parabolic distribution, th.e maximum pressure often being as great as twice the average pres~ure over the footing area. That ~uch var- iation can cause critical stresses within foundations is shown by Dr . Terzaghi in 11 The Science of Foundations". His analysis shows stresses caused by a parabolic pressure dietribution to be double those caused by a uniform distribution of the same total magnitude . of stress was reversed . In some cases even the direction There is reason to believe, with such a variance from uniform distribution with square and round footings, that even greater variations might be expected in the case of a combined footing, with corresponding critical moment and shear stresses in the foundation. !.Trans. Am. Soc .. C.E. 2. "Der Grundbau" 3. 11 Vo l. 93 p. 27? Brennecke & Lohmeyer Vol. 3 pp. 1-13 Dietribution of Pressure Under a Square Footing" F. J. Converse Civil EngineP.ring April 1933 It was to determine the type and magnitude of the pressure variation actually existing beneath a combined footing that this e xperiment was undertaken. With this pressure distribution known, the degree of error involved in current design practice may be evaluated, and the desirability of modification to fit the experimental data determined. 6 DESCRIPTION OF APPARATUS The P ressure Cell The pressure measuring device used in the investi gation is sketched in Fig. 1. Fundamentally, its action depended upon the bending of a thin circular plate simply supported around its entire circumference. The interior, or "working" portion consisted of a disc of 12 gauge mild steel 3-3/16 inches in diameter to which two rods (14 inch sections of 1/8 inch black pipe) were welded normal to the plane of the disc. These rods were placed l½ inches apart, symmetrically with respect to the c-enter of the plate. As the disc deformed due to the pressure of the soil, the tops of these rods moved apa rt, magnifying the deformation of the disc- to a degree such that it could be measured with a micrometer. To aid in the measurement, insulated contact points were set in the upper ends of the rods and connected to a battery and small light. When measurements were taken, the exact instant that contact was made between the micrometer screw and the points of the vertical rods, wa s indicated by the flashing of the light. The outer portion of the device consisted simply of a 13 inch section of 3 inch standard wrought iron pipe. A groove waw· machined in the lower end of this pipe to form a . sup port for the interior unit. The disc of the interior unit fitted loosely in this groove, with the bottom of the disc flush with the b ottom of the pipe so that a level surfa ce wa s maintained over the entire area of the cell. The contact 7, surfac·es of both the groove and the disc were machined to give an even bearing. The vertical deflection at the center of the thin disc amounted to about 0 .. 004 inches under a pressure of 5000 pounds per square foot. This movement was magnified approximately 20 times by the vertical rods .. Figure 1 8 The Loading Apparatus In order to load the footing with the desired total load of 60,000 lbs. (5000# /ft. 2 for 12 ft.~), a choice of two methods was available. The footing could be loaded directly vvi th an external load of the des ired amount, or it could be loaded by jacking against some fixed object with load-reading jacks. The latter method was selected a s the most pra.cticable, and loading was accom p lished by jacking against the anchored beam and pile system shown in the full page figure. As shown in the figure the load was transmitted from the two jacks to a heavy 18"-52# BG beam. At the ends of this beam the load was transmitted to t wo transverse 12 11 -25# I beams by means of the bolted plate connections s h own. Each transverse beam distributed its load equally to four 8 11 cast in place concrete piles by me a ns of 3/4 11 square reinforcing rod straps embedded in the piles. These straps ran the full length of a pile, passed over the end of the transverse beam and ran the full length of a corresponding pile on the other side of the beam as shown in the figure. From the piles the load was trans- mitted to the soil by means of skin friction~ The heavy beams used were for the purpose of getting the piles far enough away from the loaded footing so that the zone of influence of the distribution of pile loads to the soil would not extend appreciably to the soil under the footing. In t h e desi gn of the piles, it was decided after proper inves~ tigation and consultation, that the soil at the experiment site would safely give 5oo#/ft. 2 skin friction on vertical cast in place concrete piles. However, due to t h e fact that the distri- ~ I , _..., // 1.... I _.. / ._ I I t6 '-o" l ,___01...2_: _~ I I I I I I I I I I ,; / 16 - sz B_G Beam ,, ¾ '' °Remf Rod ----j I ' I ' I I rJ"les rr-'6 /0 'Lon9 (~-a"Reinr. Cone. ~ I I I I I - - f I I I I I - / / I , _ ___ .,.. / M oxlin um 17 = 30 lons lofal Lood on foofin9 LOAD/NG AP84RA TUS ,_ : Jo11N : 'f 6 1 jA(,/' ii I ..0 /0 bution of load in t h e soil would probably be i mpaired by the proximity of the test h ole a nd t h e nearness of t h e p iles to eac h oth er, the a ctual design of t h e piles was on a b asis o f a maximum 360if/ft.2 skin friction with an 8 inch pile 10 f e et long. The h oles were dug with an 8" soil auger and the piles were poured with a 2000# concrete. The beam sections we r e selected on the basis of a limiting bending stress of 18000#/in. 2 with the maximum 30 ton load applied to the footing. The bolted plate connections were desi g ned on the basis of a limitin g shear stress of 10,00011/in.2 in the threads, while the 3/4" reinforcing rod straps in the piles develop a tensile stress of 13,400# /in. ~ at max imum load. The apparatus performed satisfactorily durin g the whole course of experiments, and was loaded up to maximum desi gn load. The Footing The design of the footing was affected by several practical considerations. Its size was limited by the load- ing facilities available. The problem was not so much in finding jacks of sufficient capacity, but in providing a reaction for them to work against. The system adopted was capable of resisting a force of 30 tons with a reasonable factor of safety. ~ince the foundation Load desired was 2½ tons per square foot, 1~ square feet. the area of the footing was fixed at The desirability of providing symmetry for ease in checking results led to the selection of a rectangular shape and a symmetrical loading arrangement. The dimensions of the footing were therefore taken as l'-6" by 8'-0", and the pressure cells were arranged s o that equal loads could be ap plied 10 inches from each end. According to an investigation of previous foundation experiments, this size of footing would probably give differen t settlements than a lar g er footing with the same unit load, but at the same time the general distribution of pressure under this footing should be the same as that of a footing of proportionally larger size. In order to avoid excessive depth of footing and to maintain proportions similar to those of full size foundations, it was necessary to reinforce the concrete block for compressive as well as tensile stresses and to provide an unusually large amount of stirrup steel. These measures made possible a total thickness of the block of 12 inches and a distance of 10 inches bet ween compressive and tensile steel. The reinforcement as it appeared just before being placed is shown in Figure 3. ,2 In the design of the footing the following unit stresses were taken as allowable: Tensile strength of steel 18000 lbs. per sq. in. Compressive strength of concrete 800 It II It II Longitudinal shear in concrete 45 II II II II Vertical shear in conc-rete 120 II II II II Bond between steel and concrete 100 II II II II On account of the use of a water-cement ratio cal cu l a ted to produce concrete with an ultimate crushing strength of about 3000 lbs. per square inch, the comparatively high allowable stresses were not considered excessive. That this assumption was justified was shown by the fact that no cracks were observed in the concrete under any condition of loading. At points where pressure cells were placed, the heavy pipe was considered to have strength as an arch equal to the strength of the concrete displaced. Since the actual soil pressure was unknown at the time the footing was designed, the customary assumption of uniform pressure over the base was made. It was thought desirable to obtain further info r mation concerning the p roportion of load tr a nsmitted from the footing to the earth by friction between the sides of the foundation block and the soil. for the concrete. For this reason the earth was used as forms The sides of the excavation were accurately trimmed to size and care was taken to make them vertical. An attempt was made in arranging the pressure cell s in the footing to provide at the same time as much information and as many chances for comparison of results as was possible with the nurnber of cells available .. One lon(?;itudinal section down Figure 3 Figure 4 · Figure 4a the center of the footing and three crosa-sections in a transversi direction are provided, and except for one case each cell may be ~hecked against another in a similar location else where in the footing. The reinforcing steel and p ressure cells are shown in place in the excavation in Figures 4 and 4a. :b,igure 5 shows the footing dimensions and the location and numbering of the pressure c·ells .. IS Calibration of the Pressure Cells In order to determine the earth pressure in the field from the cell deflections measured there, it was first necessary to obtain a load-deflection curve for . each individual cell. These curves were determined by calibrating the cells before they were placed in the footing. The calibration was done on a small Fairbanks-Morse platform scales and readings were taken to an accuracy of one pound~ Between the cell and the platform, a celotex pad was placed to approximate the effect of the upward pressure of earth. In o rder to apply and h old small incTements of load, the scales were placed on the wei gh ing table of the 150,000# Tinius-Olsen testing machine, and increments of load applied to the t o p of the pipe by means of the movable head of the testing machine. Calibration readings of deflection were taken at every 50 lbs. of load. The desired l oad was hung on the balance arm of the scales, the machine run at low-Low speed until t he arm came up to the bal a nce point, and the load held at t ha t point until deflections were read by t h e micrometer method given in t h e description of the cells. Three calibration runs were run on each cell with 25 lb. intervals from 0-100 lbs., and 50 lb. intervals from 100-400 lbs. The results of t hes e runs were t ~ en used to plot t h e calibration curves of the cells. l<o M:EASUB.EM:I~NT OF THE PRESSURE DISTRIBU'rION Field Procedure The test runs in the field were made with tw o fifty ton pressure-~ecording gas jacks loaned by Converse & La barre Co. Th ese jacks were put in place on the f o oting, centered, and ca refully aligned vertically and horizontally. in p lace is shown in figure 6. One of the jacks The gas atta chments were connected up in parallel so that the two jacks always gave the same lo a d, the Load bei ng measured from the known area of the jack piston and the guage-recorded gas pressure in the jacks. In o rder to check elevations on the footing, a be n ch mark wa s established far enough away from the footing to be unaffected by adjace~set t lement, and r od readings were taken on the footing by a dumpy level. The elevations taken were points on the tops of the outer end cells on the north end, points on the outer middle cells, and points on t h e t wo corners of the south end. In order to read the settlement to t h e de gr ee of accuracy desired, a steel scale reading to 1/100 of an inch was clamped to t h e leve l rod, and level readings t a ken on this scale after ch ecking it on the bench mark. A set of zero readings was taken on the pressure cells in the manner described in t h e description of the cell, and the zero rod readings were made. The footing was then loaded with 6 ton jack loads (2000#/ft. 2 ) and the pressure cell and level rod readings taken. This procedure was repeated for the whole series of test runs. The test runs made with the earth sides intact, in their consecutive order was O, 2000#/ft. 2 , o, 30oo#/ft. 2 , 40oo#/ft. 2 , and 50oo#/ft. 2 18 Pictures taken during the course of these runs are shown in figures 7 and 8. The readings taken are recorded in the accompanying data sheets. Foilowing the completion of the above test runs with the e~rthen sides intact, the earth was dug away around the four sidea of the footing, so that the footing received no support from the side walls. Another series of test runs was then made, recording the same readings of the pressure cells and rod readings. Figure 9 shows the footing with the side walls removed during the course of the second series of runs. The test runs made in their consecutive order with no side wall support were O, 3000#/ft.2, 4000#/ft. 2 , and 50oo#/ft. 2 The readings taken are recorded in the accompanying data sheets. -Figure 9 Discussion of Res ults The results obtained for the pressure distribution undBr the footing are sho wn in the two pages of pressure distribution curves g iven, one for the earth walls intact, and the other for the earthen walls dug away. The pressure distribution obtained from these curves at first seems to be entirely without rhyme or reason. This is particularly true in vie.w of the fac-t that corresponding cells on opposite ends of the footing fail to c·heck each other as regards t.o absolute :) ressures. Also because the transverse rows of cells at the north end of the footing and the center, show maximum pressures near the edges of the footing in a transverse direction, while the transverse row of cells on the s outh part of t he footing shows maximum pressure at the midd le of the footing. There is also considerable difference in the distribution of pressure down the center of the footing as re ga rds the north and s ou th half. If the cells used were reasonably accurate, the difference bet ween pr essure distribution over the two ends of the footing can be exp lained by only one consideration; namely, a difference in the soil conditions at the two ends. ence existed. Actually such a d iffer- Shortly after the 3 foot main excavation f o r the footing was dug, a heavy rainstorm oc curred. Although provision had been made for drainage in the excavation, the s outh end which was the low end, soaked up enough moisture to make it qu ite soft and mushy as compared with the north end. Due to an impervious clay layer about seven feet below the surface and the extreme shadiness of the southern end of the excavation, the mo isture did not drai n out of the soil. When the footing 20 "' C: ,,,"'"':z. 0 . 5,., "',., % 0 0 (') :,:: 0 ~ "';, 0 ,., ~ -< 0 "'.,. :z. ~ ~ "' '' () ,I ...... ~ II ....... ~~ ~-~ 2~ ((~ ~~ L ~ ~! ~ :S , ~'.3 ~ -.J h: ~ I ~~ ~I t cs ;l:R ::..... - ~ ·- :::5 V') <I) 1 I ~ I ~:<:~ -- ' -- -'I--. -............... r f - ....... ---,- 21 ,., c; "',.,,.,z ,.,....~ ~ ....0 ,., z ~ 0 ? s ~ r~ 0 ::c 0 ,. 0 0 z "'~ -< 0 :,:, '?' z ? .;"' u u ~~ _.__ ~~ I ~~ ~I L ~ ~ ~ w -~.ti~~ ~ ~ ~ - ~- ~~ t __ _ :..7d 9NO.L N/ _,_ .d'.a'/75"',k...7& _,___ _,____~--"----'-----'--~-'---'---' was poured several weeks later, the soil at the south end of the ear then footing form wa.s so water-soaked that tamping squeezed water out of it, and pushing a finger into it caused bulging over a diameter of several inc-hes. other end was firm and strong. The soil at the Unfortunately no moisture read- ings were taken at the time. A possible significance of the effect of this water is made with the aid of "Der Grundbau", a German book on foundations written by Brennicke and Lohmeyer. In the first chapter of this book a comparison is made between the extremely different actions of a cohesive soil and a c-ohesionless soil under the same conditions of loading. A cohesive soil acts as a continuous elastic body, and transmits stress by shear. Consequently, considering the case of a rigid body placed upon a perfect continuous elastic body and loaded, the deflection curve is as shown in Fi g ure A. F ig. A The pressure on the rigid body acts entirely at the edges of it. Actually a cohesive soil has partially c6ntinuous elastic qualities, and with the qualities it has, the theoretical pressure on the bottom of a rigid footing as worked out by Brennicke and Lohmeyer is shown in Figure B. The pressure is a maxi mum at the edge region and decreases rapidly toward the center. With a non cohesive soil pressure is transmitted by grain to grain contact, a nd there tends to be a "flow" of particles away from the source of load application. Under such action the deflection of the soil is shovm in Figure C. The greatest pressure is at the c-enter 23 where the resistance to 11 flow" is the greatest, or according _J__L_ Fig. D Fig. C to Boussinesq's theory, where the effect of a series of loads approximating the uniform load, is the greatest. the actual qualities of a non-cohesive soil, Based upon the theoretical pressure distribution over the bottom of a rigid body is shown in Figure D. The pressure is a maximum at the center, and de- creases toward the edges in a parabol ic distribution~ According to experiments covered by Brennicke and Lohmeyer, cohesive loams and clays with active capillary water have shear strengths great enough to cause them to act like a .continuous .e last i .5! body similar to rock when loaded. However when the ca p illary water effect is inactive, the cohesion of the soil breaks down, and the clay or loam acts as a cohesionless material. This break-· do wn in cohesion occurs when the soil becomes water-soaked. Adapting thi s explanation to the results obtained, it is found that the transverse pressure distribution at the north end and center of the footing is similar to the theoretical pressure distribution for a rigid body on a cohesive soil, wh ile the transverse pressure distribution at the south end i s similar to the theoretical distribution for a rigid body on a cohesionless soil. The actual soil in place is very cohesive, so that an explanation of the differences in pressure distribution between the t wo ends, seems to be due to the fact that ere th:w; was a breakdown in the cohesive qualities of the soil under the south end of the footing, because of a high moisture content 24 at that point. A very good verification of' this explanation is obtained from the curves of longitudinal pressure distribution with side walls intact. From these curves it is seen that the pressures under the footing near the south end average about double the corresponding pressures at the north end. This additional bottom pressure indicates that the side wa ll s at the north end are taking considerably more pressure than the side walls at the south end. Since the amount of load taken by the side wa lls is a function of the shear resistance or cohesion of the soil, a considerable decrease in the cohesion of the side walls at the s outh end is indicated by these curves. The pressure distribution obtained in a lon g itudinal direction was the series of waves shown in the curves. The ex- planation of this distribution of pressure does not seem to be one that can be very easily arrived at. However, hazarding an explanation, if a rigid elastic footing is assumed acting upon a continuous elastic foundation, the deflection of the elastic foundation would be somewhat as shown in Figure E. - . ••. .::...: - Fig. E a b This def lection curve with its series of waves is obtained from Timoshenko's "Strength of Materials". The deflection curve derived is for the case of a concentrated load acting on an infinitely long bar, resting on an elastic foundation. The abo~e ' curve is modified to meet the conditions ~f the bar ending at a, hut is fundamentally the same curve. With this deflection curve high points of pressure woul d occur at points a and c, and zero ,,-< "z 0 0 ~ "' ..... + - - ~ - l ' - i.;: ~ ~ :~~ " Is.. ~~- ~'~ ~ ~ ~ ~ ~ ~~ ~'{~~ij (;}t I ~~ 1(!N ' ~ ~ ~~ Q ~ ~ ~N " ~ ~ I;: t~ Qi! §It-u ~~ ~ !I ~ :t) " ~~ ~~ "~"" () ~ ~~ I ::;i "' V)!"I 'li.i ~ C) t"!-i . , I I pressures at points band d. This analogy carried to a soil of partial continuous elasticity, would giv.e a pressure distribution similar to that obtained in this experiment for the longitudinal distribution. Any explanation of the relative intensities of pressure at the peaks arid low points is not attempted at this point, since sonsiderable verification of this pressure distribution is necessary before anything conclusive can be obtained. To compare the total load on the concrete block as meas ured by the pressure c·ells, with that applied by the jacks, the cell pressures in Run No. 10 (5000ff/ft. 2) were plotted on a plan of the block, and pressure contours drawn by interpolation between the plotted values. By measuring the contour areas ob tained, the total upward pressure recorded on the block was found to be 87% of the jack load. The north or left half of the footing gave a recorded upward pressure of 98% of the jack load. S ince this was the half that was unaffected by apQarent exdess mo isture, it was taken as t yp ica l of pressures obtained under ordi nary conditions wi th this t ype of soil. On this basis the idealized iso-pressure lines for a combined footing resting on a cohesive loam with active capillary water, were drawn, and are presented in the results . With this idealized pressure distribution flor an average load of 2.5 tons/ft. 2 , jack loads are accounted for. 96% of the F urther experimentation of course is needed to verify this distribution. By comparison of center line distribution with different magnitudes of loading, the total recorded pressures for the wal ls intact was calculated to by 67% of the total recorded pressures with the s ide wa lls removed. The remaining 33% of the total load was undoubtedly carried by friction on the sides of Z1 t 5e t TLe/'1ENT 1 .....,... - t t l V l---'l-+---4--✓-J.:t. -, I t + I+ V.L. 1 CI [ I . I-< i I? ~ 2-, ..._ t 'I":" I - ,. .. . f r ' I t • I . ,. I f • 28 the footing block. The settlements obtained during the course of these runs is shown in the settlement diagrams. These settlements sho w tha t the footing deflected slightly, but appear to have no bearing upon the pressure distribution obtained. A mechanical analysis of the soil on which the footing was placed was made according to the A.S.T.M. Standards (Serial D137-22T}. 0lay was taken as that part of the soil which did not settle out through 8 centimeters of water in 8 minutes. Of the soil which did settle out, that passing a 200 mesh 'fyler Standard sieve was taken as silt, and that retained on the sieve was taken as sand. The percentage of each size was based on the dry weight of the soil sample. Moi sture content was 14% of the dry weight at the time of the tests. The analysis showed the soil to b~ ~omp osed of 49% sand, 34% silt, and 17% clay. Accord- ing to the U.S. Bureau of Public Roads, this soil would be classified as loam. The extreme cohesion noted in the e x cavation is thought to be due to the large proportion of the sand particles ran g ing just above the ~00 mesh size. The most conclusive thing achieved from this research is the necessity for further verification of these results. Unfor- tunately, due to difficulties in financing the project, work was not started until the middle of the second term. Consequently with the necessary delays for concrete to harden etc., actual testing of the footing did not occur until the middle of May , and was limited to a very short period. In order to verify some of the theories and assumptions advanced here, test runs should be made on the footing with the soil dried out, and then with the re g ion soaked wi.th water. Also the footing should be lifted off the ground, and sand put in place, so that tests could be made on purely cohesionless material. Before starting any such tests, it would be advisable to lift the footing up and calibrate the cells in place in the footing. There was always a question as to whether the roug;h treatment and water infiltration during concrete pouring might not have affected the calibration curves of the cells considerably. The period of testing on this project should only be beginning at this point. The discussion in this the s is is based upon one set of ten runs, and because of the need of verification, is made extremely general in nature. On this basis, no recommendation as yet can be made toward any change in the customary procedure of combined footing design. CON::!LUSIONS 1. ~'hen a concrete combined footing is poured into an earthen form of cohesive soil, the side walls will take as much as one-third of the total load on the footing. 2. The pressure distribution under a rigid combined footing is not uniform. 3. The pressure distribution under a rigid combined footing seems to occur in waves, similar to what would theoretic-ally be expected for a rigid body on a continuous elastic foundation. 4. The pressure distribution under a rigid loaded footing is apparently quite sensitive to the relationship between moisture-content and cohesive qualities of the soil. 5. The necessity for further verification and ex per- imentation, precludes any recommendation at the present time, for any change in the present procedure of designing this type of footing. JI APPENDIX l. Field Data Sheets 2. Pressure Cell Calibration Curves .12 FIELD Jack SHEET No LOAD - SIDES INTACT Micrometer liead1r.tJ.s Corr. Cel(tF Loads ~ I :1r2 :11:J i.aAo Aver. Reat:I. , .L. I O . 65"30 .6S.JO ,6530 .65"30 0 2 0 .S</'15". 60/5 . 60/.f'". h0 /0 0 3 0 , 7060 . 7tJ~O . 705"S-. 7()60 0 4- 0 . 6090 . 609() . 6090. 609() 0 s 0 .4f2'-0 ,4270 ·1270 .4270 0 6 0 .5"665',S670 . .f""67S.S-670 0 7 0 .~380 .~380 .4J80,4J80 0 8 0 . .222.c -l22.o .12.20 .12'2.0 0 9 0 . 3tf70. 347f' ,3490, 3475"" 0 /0 . 642f". ,42$" -6425". ,425" 0 II 0 0 . 39q s-. '39'15' . 399.J .399S- 0 /2 0 . 45'5"S-. 45"5"0 . 45"60 . 45"5'.f 0 13 0 .5()65' -6'°~65 . .5"L>60 ,.f"06S" /4 0 .5'tJ66 ,5"tJ60 .s-tJ,o ,S-t>60 0 0 15' 0 .4660 .466S- .4h6$".«~,r 0 End Roel Reacltn9 Cent-er Readin:7 ,, #/ = 4..2.3 #2 -== ~- 2<> #3 = 1.'/45' :II4 ::. 4. 'JO #J = 4. 3 I '' *2 -= 4. 13 FIELD SHEET Jack Micrometer Readt~s Ce/litLot1ds -if I -1t- 2 :113 Aver: I 12000 ,{,S-l.O ,6S"7O ,bS-(,S" .tS6S'° 2. , S't'iO . S-'190 . SC/</0 . S-9(jO 3 fl . 7125" . 71/S- . 7/IS- , 7115"° 4 . 6D'i0 , 60'IS"' . 608$" . '()9,S- s- . 438~ . 4380 . 439,r. 4.!8S-, '7 ,, g If ,l3S0 .23>0 ,235"0 , 23>0 9 /I 10 II , 3'-S"S" . 3(.5'0 . 3(.S""O . 36)0 .6480 .,q1s .{,41s- .647S- II /2 ,, ,, .S7/o . s710 . 5712 . S7to -4':i"/5"° .4520 . 45'"1.5" .4s20 .-4180 ,4180 ,4180 ,4/80 -476S- .476S- .4771) .476.J 13 II .5'2/S" ,S-125" .5"22() ,$"220 14 IS° II . S-135"" ,5'l3S- . S'/40 , Sl3S- It . 4tH,S". 4875" .10 70 , 4970 End l?od /?ec?dtn~ # I = -1-. 26 h ~2. = 4.31 ~5 = -1-.93 # 4 == 1-. 'i I - 4.3/ ::. 4./7 ,, FIELD RuN # J JACK - 5HEET No LOAD - Mlcrornefer ~otl1n1s Corr: Cell# LoA/7.5 # I # Z # 3 S 10Es LO'IP Aver. Fqcfar {Chort) I 0 .6S$0 .6S30 .(,S-3O . 6S3O+-.0090 0 2. 0 .,020 . &010 ,60/0 .6010 r.0040 0 3 0 . 7030 . 7tJ30 . 7030 . 7030 +.0025" 0 4 0 ,&090 . {,095"" . 60'/0 . 60'iOtOOSS 0 s 0 . 4345""'. 434() . 1 3 40 . 4 3 40 -: 00/0 0 ' 0 .S-600 . 5"600 . 5"60o ,§"'too +,0/0S- 0 7 0 .437.r .+~90 .4-385". 4 3 80 1-,6090 0 8 0 , 22..,, . 220s. 22os- . 2210 "too40 0 q 0 . JS-45'" . 35"40 . 3S30 , "3S40 -.0070 0 IO 0 . ~1.qo . f>28S-. 628S". 628S- +.ooso 0 " 0 , "1070 . 4<>75'" . 407S- .4o7s-- ,oo<;s 0 /2. 0 . 4635"' , 463S"°, 46 3.> . 46 3.S- - ,(>OI~ 13 0 ,50'10 , S-095"" · SO</ 5"' . S"tJ9S- t-,C,005"" 0 0 I+ IS" 0 , soq-s- . S-095" . SO'IS-. S09S-·-r,0090 0 0 .~7)5" . 4715". 4730 , 472.S- End Roel Readtn<J 0 0 # I = 4.2S" :11 2 == 4-. 29 ~:; :::; 4. 94 :it 4 :If: I = -f-. 9 / ·= 4.2<; :lt2=- 4.17 . 11 INTACT F IE LD RvN # 4 SHEET - 3000 H/f't 2 - SIDES INTACT Micrornefer Jreadin9s Corr~. L.oAo fi.ess11K£ Cell# loAOS ;ft- I -ii:2 :I/.- 3 Aver: /feud {Chor-/) ro'11:t.i. JAcK I /8000 .6705 . 67tJO . G69S .6700 .6790 &9 ,69 2 ,, , 6030 . 6030 . 6030 . 6030 . 6070 23 , 2:S 3 11 , 7/40 , 7140 , 713$' , 7 / 40 . 7/6$"" /0$" /, 05" 4 II .6 /tJS- . {,!OS- . 6 110 . 6/0S- . 6 160 Jo . 10 , 45"00 , 449S- . 4495". 449.5' . 44/JS 18 , 19 . 5660 . .S-665" . 5bSS" , ~660 . 5765"" 57 , S'7 , 4£60 . 46$'"0 . 465""() . 46.S-5" , 4645"' 20 . 20 , 245"0 , 24 S-0 . 24:,-0 .24$""0. 2490 24 . 24 . 3770 . 3760 . 376S- . 37t.S- . 3&95" 33 . 33 s 6 7 8 ,, II ,, 9 /0 II . 6~0S- . t4tJS . t40S- . t4tJS- . 648S' q Z. . q2, II II ,32 /2 II , 4350 -435'0 . 434.F , 45'S-O . -?.ZY.5" 32 . 4960 . 49~~. 496() . 4960 ,494S- 42 13 II . 5}8S- . -5"29o , S285" , Y.28.S- . S".290 GO . 60 /4 II ,S.230 -5'23S- . 523S" ·.5'"2~S- . S3I S- 5'0 ,S-0 · St>4S. So4S- . S-~40 . S-04S" . S-04S S'S" . S'S IS- ,, End .Rbd /f'eC?cvny • .:# I *2 = 4- 3.7 4.22 11 . . 42 FIELD SHEET M1crornef-er Reotlin~ S Correc. LoAD rkESSVK/e Cell# LoAOS :# I *2 :1:1- 3 Aver &t1<I {Churf) To"f/,r:1.z. ..- tMCk I 2 3 24000 .684tJ . 6665 .6865" .6$60 .t59S"O 1.3'2. 11 11 76 . 76 7/SS . 716.S- . 7/60 . 7/6'0 . 7/$S 120 I .20 • 6105" • /.32 . 6100 . 609S . 6 / tJO . 6/40 4 '' . 6140 . 6/S-c; . b/S"O . 6/5"0. 6265" 2.~ . 2.5 ~ 1 ' .4620 .,1-62S . ~6.Zo .4620 .4b/O 3S .35 6 " .S"G90 .S69S. ~695' , S"G95,5&oo 76 . 76 7 " ,"1-78.f" . 4790 . 4790 ,4790 . 4860 52 8 11 • 2.S-30 q '' · 3<fCJS . 39oS" .39CJO · 39tJS ,J83S S-9 .52 . 31 . S9 IO II , 6220 -@220 . 6220 . ~2.20 II I/ , 4-#70 . 447CJ . 4470 . 4470 , 4375 -1S- .4D 12 II . S(JO.> ,S(JOO , S-tJ/0 .StJOJ. 4990 5"I .5'1 13 I/ ,J~,f() . S.f40. S-440 .,F,4,ftJ , .F41f" /IS- /.15 , 5300 . S-290 · 5"3t>o . 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