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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.

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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 adobe soil when it absorbed water.
Confined adobe soil absorbed a relatively small amount of water.

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%.

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. 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.

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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.

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.

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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.

No. 1

October 22, 1920

APPENDIX A, PRELIMINARY REPORTS
ADOBE SOIL TESTS

Information of greatest immediate value would be obtained from tests similar to those now being made by the Bureau of Public Roads.

1.

Slabs of concrete 7 ft. by 7 ft. say 6 in. thick.

2. Placed directly on subgrade of adobe soil in various conditions of moisture content and adulteration.

3.

Similar slabs on good gravel or crushed rock subgrade.

4. Test under static load, determine deflection of slab and fiber stresses at various positions in the slab during the progress of the test.

Supplementing these tests the bearing power of adobe soil and other characteristic California subgrade materials could be determined as follows:

1. Pulverize sample, place in container about 14 inches in diameter and 8 inches deep. Let the soil absorb water from below, then dry the sample in air. Repeat this process until a constant volume is secured. (This method is far more satisfactory than mechanical tamping or other manipulation to secure uniform compactness of sample.) 2. Fill space between test sample and container with Plaster of Paris and place in testing machine.

3. Apply test load through flat-bottomed circular plunger (10 square inches in crosssectional area) to center of upper surface of sample.

4. Get data for load-deflection curve.

I think that it would be impossible to prepare samples of adobe soil in this manner. The most practical and satisfactory method could only be determined by actual experiment but the test itself could be made as outlined.

The adobe soil could be adulterated in any manner and the effect of such treatment on its bearing power readily determined.

This is essentially the method used by the Bureau of Public Roads. I got the information verbally from a remote source, and it may not be correct, but there is no doubt that this method would be only our starting point if we attempted such tests. Adobe soil is in a class by itself and demands individual methods.

If possible the grain size should be determined, see A. S. C. E. proceedings August, 1920, page 913. Change in volume under various moisture conditions should also be noted. (Signed) C. T. WISKOCIL.

No. 2

TESTS ON ADOBE SOIL

1. Contraction and Expansion
la Adobe in natural condition (as found).

October 25, 1920.

lal Prepare samples, made of adobe soil paste, as large as can be conveniently manipulated.

1a2 Dry sample at room temperature for seven days.

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1a4 Compute percentages of contraction (percentage reduction in volume on

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basis of 1a1).

Resoak sample.

1a6 When sample has absorbed all the water it will, determine its volume.

1a7 Compute expansion as percentage increase in volume on basis of volume in 1a3.

1b Adobe soil mixed with foreign materials, such as sand, gravel, rock, or lime. (Same procedure as in 1a.)

2. Bearing Power

2a Adobe soil in natural condition (as found).

2a1 Pulverize sample, place in container about 14 inches in diameter and 8 inches deep. Let the soil absorb water from below, then dry the sample in air. Repeat this process until a constant volume is secured. This method is far more satisfactory than mechanical tamping or other manipulation to secure uniform compactness of sample.

2a2 Fill space between test sample and container with Plaster of Paris and place in testing machine.

2a3 Apply test load through flat-bottomed circular plunger (10 square inches in cross sectional area) to center of upper surface of sample.

2a4 Get data for load-deformation curve.

3. Strength of Concrete Slabs on Adobe Soil Subgrade

3a1 Prepare slabs of cement concrete 7 feet by 7 feet by 6 inches.

3a2 Place cured slabs on subgrade of adobe soil.

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4. Grain Size

3a21 Natural adobe soil-vary moisture.

3a22 Adobe soil mixed with foreign materials vary moisture as in 3a21. Test slabs under static load, determine deflection and fiber stresses at various points in the slab during progress of test.

See A. S. C. E. Proceedings, August, 1920, page 913.

(Signed) C. T. WISKOCIL.

December 8, 1920.

No. 3

Dear Professor Derleth:

At our conference during which the letter of December 1 was written to Messrs. Lippincott and Brunnier you instructed me to prepare a statement of what we have been doing in the laboratory on adobe soil.

The first revised program of October 25, also contained in your letter of October 29 to Messrs. Lippincott and Brunnier, has been adhered to insofar as possible. The results however while in a sense negative will be of great value in showing us what not to do.

The laboratory work I am about to describe was done during the month of November. We attempted to follow the procedure recommended by Professor Shaw to the engineers of the Bureau of Public Roads working under Dr. Hewes. The actual work for this Bureau was done by Miss Edith Phillips, working in Professor Shaw's laboratory in Hilgard Hall. The work they were particularly interested in was the amount of shrinkage and expansion in adobe soil. This work was carried on as follows:

The sample of soil was crushed by hand and then pulverized. A small quantity of this pulverized soil was mixed with water until it attained the consistency of rich cream. The mud in this condition was poured into small cups 2 inches in diameter and 1-inch deep, weighed and set aside to dry. After air-drying to a constant weight, the volume of the specimen was determined by micrometer measurements. In most cases a solid single piece of soil was obtained. Where the soil in drying cracked or adhered to the container the specimen was rejected. This method of obtaining volumes was later abandoned because of its known inaccuracy. The results as finally recorded were obtained by immersing the dry specimens in mercury. While this method is no doubt more accurate than the one first used it nevertheless was quite cumbersome and could be improved upon.

Up to this time the results to my knowledge have not been computed and tabulated; but Miss B. F. Monroe, Professor Shaw's laboratory assistant, thinks that the average shrinkage was about 20% in volume, on the basis of the original mud paste.

inches long.

With this information in mind our first idea was to obtain a larger specimen than that used by the Bureau of Public Roads so as to reduce the experimental errors in determining the volumes. We therefore decided to use a cylindric shape. We adopted two sizes as follows: (a) 2.4 inches diameter by 41⁄2 inches long; (b) 1.3 inches diameter by 4 (These molds are in the background of Figure 8.) Soil No. 12, from Ventura County, was crushed and passed through a 100-mesh sieve. It was then mixed with a known amount of water to such a consistency that it could be poured from a container into a mold. Instead of air-drying these samples we attempted to accelerate this part of the test by more rapid drying, but without success. The specimens cracked and checked so that they were of no further use. Even approximate determinations of shrinkage could not be made.

Another source of inaccuracy was the condition of the mud paste; but to be sure that our molds were of the proper size we tried a smaller set 1.3 inches in diameter and 2.7 inches long. (One of these molds is shown in Figure 8.) These molds were paraffined and placed on greased plates. The mud paste was poured into the molds and they were set aside to dry. After a week's time only the upper portion had dried. The lower part still filled the mold and was in its original wet condition. One of these smaller molds is in the background of Figure 8. It became obvious, therefore, that the entire surface of the specimen must be subjected to air instead of just the top as was the case in any of the molds used. In doing this it is clear that the mud must be of thicker consistency. Just yesterday we tried mixing the pulverized adobe soil with small quantities of water, thoroughly kneading the mixture with additional water until it had a uniform texture. The consistency was about that of modeling clay and very easily handled.

We now propose to make up cylindric specimens of adobe soil of the consistency mentioned, remove the mold immediately and in that way we will be able to expose a large surface to dry and thus overcome one of the previous obstacles. The procedure will be as follows:

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Weigh out pulverized adobe soil, add water until the proper consistency is secured.
This will require very thorough kneading. Record the amount of water used and cal-
culate the percentage of moisture on the basis of dry material.
Compact the adobe paste into the oiled mold.

Remove specimen from mold. If it has any air bubbles or other holes visible, discard.
If it appears to be a solid homogeneous mass, weigh and immerse in kerosene to deter-
mine its volume.

Set aside to dry under room conditions.

5. To check the percentage moisture in the material molded into specimens make a moisture determination in the usual way by drying to a constant weight at 100 degrees Centigrade.

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After the specimen has dried one week in air place in a cold oven which will gradually be brought to a maximum temperature of 70 degrees C. After they have dried to a constant weight raise the temperature to 110 degrees C.

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Immerse the specimen in kerosene until it will absorb no more. Then determine the volume as was previously done with the wet specimen.

9. Compute percentage reduction in volume on the basis of dry volume.

We think, at this time, that this procedure will give us uniform, comparable results that will be of value. We realize that we are not duplicating field conditions in that the mud may be wetter than we use it and field drying is not 100 degrees C. The conditions to be used are, however, consistent with general laboratory practice and when obtained under known conditions, results can be modified according to the judgment of the person using them. We propose to mold specimens from each sample obtained. We will choose one sample which

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