Plate XIII

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T. Wiskoci

Volume Changes

DISCUSSION OF RESULTS

In order to compare the experimental data they were all reduced to expansions based on oven-dry volumes.

From the results of the linear-shrinkage tests it was possible to secure data with which to plot moisture-volume curves. The volume changes were computed from the decrease in length of test specimens on the assumption that the soil contracted equally in all directions. This assumption was apparently satisfactory because the changes in volumes determined from the tests in which the volumes were directly measured are on the volume curves computed from the linear-shrinkage tests. Shown thus on Plates III to IX.

The actual expansion obtained in the porous thimble test was very interesting in that it showed that expansions computed from shrinkage tests are comparable with actual expansions. See Plate VIII.

Additions of lime and sand did not seem to affect the volume changes, that is the total difference in volumes, but they did seem to decrease the rate of expansion for small amounts of moisture. See Plates X, XI, XII.

The lime and sand specimens were very brittle whereas those of the natural soil had considerable toughness.

The lime specimens showed much more cracking, breaking up into more pieces, than those of the natural soil.

Force in Confined Adobe Soil

The absorption of water by confined adobe soil caused a pressure of 880 lbs. per sq. in. or about 63 tons per sq. ft.

The specimen had absorbed about 12% water during the test which lasted over four days. During this time the specimen of soil was under a one-foot head of water.

Figure 7.

Sand content of adobe soils (material retained on a 200-mesh sieve). The two jars at the left show the size of the original sample of soil.

Moisture Content

The moisture content for the various soils in air-dry condition, whether lumps or average material (small lumps and dust) was taken, and was about 6%. This does not include soil No. 9 from Tulare County which had over 50% sand and would naturally have a lower moisture content.

Sieve Analyses

No attempt was made to determine the grain size of the soil passing the 200-mesh sieve. Only that retained on this sieve was rescreened. According to this division, soils 2, 5, 11 and 13 were the finest while 30% or more of soils 9, 10, and 12 was retained on the 200-mesh sieve. It could be said that these latter soils contained more than 30% sand.

Specific Weights

Average adobe soil weighed about 87 lbs. per cu. ft. This is for loose air-dry soil with no large lumps and only slightly jarred so as to completely fill the measuring box.

The soil in the lumps weighs about 121 lbs. per cu. ft. average with a maximum of 133 lbs. per cu. ft.

It is interesting to note that it required a pressure of 430 tons per square foot to compact the dry loose adobe soil to the density of the natural lumps.

Strength

The average tensile strength of four air-dry specimens taken at random was 180 lbs. per sq. in. The maximum was 270 lbs. per sq. in.

Only one compression test was made. It was an air-dry specimen made from a mixture of several soils. Its maximum strength was 2,500 lbs. per sq. in.

3.

4.

SUMMARY OF RESULTS

Volume changes were satisfactorily computed from experimental data on linearshrinkage tests.

The soils examined showed decided variation in grain size.

Air-dry adobe soils contained about 6% water.

High pressure was exerted by confined adcbe soil when it absorbed water.

5. Confined adobe soil absorbed a relatively small amount of water.

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7.

Sand added to adobe soil did not produce a marked decrease in its volume changes. An addition of 5% lime to adobe soil showed decided decrease in its volume changes when the moisture content was below 30%.

8. Lime added to adobe soil made it very brittle and caused it to break into smaller pieces than the natural soil.

9. Adobe soils increased about 1%% in volume from oven to air-dry conditions.

10.

From air-dry to a stiff mud, a condition not easily defined, the adobe soils examined increased about 37% in volume. The range was 16 to 51%. Table V.

11. With adobe mud thin enough to be poured the increase in volume from air-dry condition was about 67%. The range was from 28 to 93%. Table V.

12.

The soils examined required different amounts of water to bring them to the same degree of plasticity.

13. A pressure of about 864,000 lbs. per sq. ft. was required to compact loose, air-dry adobe soil to the density of the natural lumps.

14. The maximum tensile strength of a sample of adobe soil was 270 lbs. per sq. in. 15. The maximum compressive strength of a sample of air-dry adobe soil was 2,500 lbs. per sq. in.