LABORATORY EXPERIMENTS A PRELIMINARY STUDY OF ADOBE SOILS AND CONCRETE SLAB TESTS FOR THE CALIFORNIA STATE AUTOMOBILE ASSOCIATION AND THE AUTOMOBILE CLUB OF SOUTHERN CALIFORNIA AS CONDUCTED AT THE UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA UNDER THE DIRECTION OF C. DERLETH, Jr. Dean, College of Civil Engineering C. T. WISKOCIL Associate Professor of Civil Engineering UNIVERSITY OF CALIFORNIA ADOBE SOIL TESTS A PRELIMINARY STUDY January 17, 1921 AUTOMOBILE CLUB OF SOUTHERN CALIFORNIA January 17, 1921. Mr. J. B. Lippincott, Consulting Engineer, Mr. H. J. Brunnier, Chairman Highway Committee, Gentlemen: I submit our preliminary report on adobe soil tests. The inquiries were limited to the period September-December, 1920. Had more time been available, or had we felt justified in making greater expenditures by using a larger number of assistants, perhaps our report would have been more inclusive. Our first studies and experiments were necessarily tentative and proved mostly negative. They showed us rather what not to do. As the tests proceeded, accumulating experience dictated changes in policy and program. In our program of October 25, 1920 (presented herewith in Appendix A) we proposed four groups of tests; thus far we have limited our inquiries to the first of these groups, "Contraction and Expansion." Tentative conclusions are outlined in the following preliminary study. These conclusions are spoken of as "tentative" because further studies, particularly on other samples of soils, may modify or enlarge our views. Air dried adobe soils made into a stiff mud gave an average increase of 37% in volume; the same soils made into a paste that could be poured, gave an average of 67%. Assuming the specimens to have had the same rate of expansion in all directions, these volume increases respectively correspond to linear changes of 11% and 20%. Experiments with fine sand and lime adulteration of adobe soil were limited in number. So far as our studies have proceeded, it would appear that adulteration with fine sand or lime does not greatly modify or reduce changes in volume due to changes in moisture con tent. More numerous and elaborate tests under different conditions of adulteration (particularly with gravel and crushed rock) and with greater variety of samples of adobe soils may give more prominent effects. We had expected to find a greater reduction in volume changes from the addition of fine sand and lime, but our results thus far have not justified that expectation. The addition of sand and lime helps in the tilling of adobe soils by breaking up the material mechanically into smaller lumps. Further adulteration experimentation is needed but will require effort and considerable expenditure. Tests were made to determine the amount of pressure required to increase the density of loose air dried adobe dust to that found in average air dried clods. Through the specially devised apparatus shown in Figure 5, a load of 6000 lbs. per sq. in. was applied to produce. this result. This test raises the question of the efficiency of the ordinary highway steam roller. We would suggest further tests on roller pressures needed to produce a particular density in a given adobe soil. Samples of soils after treatment with a steam roller might with profit be studied. In our laboratory experiments, after the confined adobe soil dust had been compacted to the density of natural clods under the high pressures just referred to, it was found that the specimen absorbed about 12% of water during a period of four days and that by absorbing the water it exerted an expansive pressure of 63 tons per sq. ft., or 880 lbs. per sq. in. Studies of this type are urged and may throw valuable light upon the heaving force of adobe soils. under pavement slabs, or when otherwise confined, for different conditions of soil density, moisture content and lateral confinement. |