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consistency as measured by the slump test, gravel-sand ratio, and cement content. The various properties of the individual mixes and concretes directly determined by tests include slump, workability, finishing qualities, 28-day compressive strength, and elastic properties. These properties are analyzed in a number of ways and the recommended mixes to be used in various parts of the work are derived. The information obtained from the mix tests also furnish data for design purposes.

The tentative concrete mixes selected for use in the field are finally tested under conditions closely approximating those that will obtain on the job. Specimens to be tested for strength, volume change, permeability, elasticity, plastic flow, temperature rise, and heat of hydration are cast in thin metal containers and stored in specially constructed rooms wherein the temperature may be caused to follow any predetermined or automatic cycle. Figure 3 shows a view of the control panel for these rooms which are termed "adiabatic calorimeter" rooms. Coincident with the above tests which are made under conditions called "mass curing" the thermal properties of the selected mixes are determined with the aid of specially designed and constructed apparatus as illustrated in figure 4. The determinations for thermal properties include specific heat, or the amount of heat required to raise the temperature of the concrete 1°, thermoconductivity and diffusivity, having to do with the rate at which heat is transferred through the concrete, and density or unit weight. The tests just outlined supply essential data for accurately predicting the behavior of the concrete in the completed structure, thereby permitting the use of

optimum design conditions and values, and the intelligent design and planning of artificial cooling and grouting systems and operations.

Sufficient data and knowledge concerning the materials. available for the construction of a given project are obtained to insure that they will be processed, combined, and utilized in a manner to obtain the greatest practicable, economic, and qualitative benefit. In addition to the investigational work described, the Denver laboratories are constantly being called upon experimentally to solve a myriad of problems encountered in design and construction and in the utilization of numerous materials of construction.

The data and information collected in the Denver laboratories are transmitted to the field for the use and guidance of the field laboratory and control organization during construction. The field laboratory performs routine tests on the concrete materials as they are processed and made ready for use, of the concrete as it is mixed and placed in the structures, and of the hardened concrete, to insure that the desired quality, workmanship, and economy are obtained. Figure 5 shows the plans for a typical field laboratory.

While the investigational and control operations, as above outlined, may sound elaborate, it should be realized that a large percentage of the cost of most of the Bureau's projects lies in the concrete work. The cost of the investigation and control work usually constitutes a very small proportion of the total cost of the project. It is insignificant in comparison with the insurance afforded by way of increased integrity, quality, performance, and economy obtained in the concrete construction.

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EARTH MATERIALS LABORATORY OPERATION

BY L. W. HAMILTON, ASSOCIATE ENGINEER, BUREAU OF RECLAMATION

IN VIEW of the larger number of earth dams proposed for construction, the Bureau of Reclamation established the Earth Materials Laboratory in the fall of 1933. Primarily the duties of the laboratory were to determine the characteristics of proposed embankment and foundation soils, to work with the design section in planning field control tests on the foundation and compacted embankment, and to train construction inspectors in the test procedure.

Earth testing along these lines is a comparatively new field. Consequently, the laboratory staff had to design the tests and test apparatus, a matter which required much time and thought. Shortly after the establishment of the laboratory it was decided to follow, in general, the technique of laboratory testing and control developed by the Los Angeles Bureau of Water Works and Supply. This technique was described by R. R. Proctor in a series of four articles published by Engineering News Record beginning August 31, 1933.

The routine tests now made in the laboratory are: (1) Mechanical analysis, (2) compaction and penetration resistance, (3) percolation and settlement, (4) consolidation, (5) shear, (6) specific gravity, and (7) soluble solids determinations. The laboratory has made numerous tests and studies using different amounts of compaction with various impact or hammer compactions. The laboratory has also conducted studies and experimentation on hydrostatic pressure travel through soils, how the internal pressure lags with decrease in external pressure, how hydrostatic pressure is created by consolidation, and how rapidly this pressure is dissipated.

BRIEF DESCRIPTION OF TESTS AND DATA OBTAINED

Mechanical analysis is the determination of the distribution of particle sizes. This analysis includes: (1) the determination of the percent by weight of particles retained on 6-, 3-, 11⁄2-, 4-, and 1⁄4-inch screens; (2) the sieve analysis. on the standard sand screens plus the 200-mesh screen; and (3) the hydrometer analysis of the fines passing the 200mesh screen. These analyses are combined into a total analysis. The material passing the 200-mesh screen is analyzed by the rate of sedimentation in water, using a Bouyoucos hydrometer as the means of measurement, and using Stokes' law as a basis for the computation of particle sizes. The mechanical analysis gives data for the classifica

tion of the soil, and may indicate many of the soil characteristics such as permeability and shearing strength.

The compaction test is made to determine the relationship between moisture content and dry density. From the information obtained by this test the maximum density, the optimum moisture content, and the range of moisture contents which will cause only small variations in resultant dry density, may be observed. This test also indicates the density that may be expected in the compacted embankment. The test is made by compacting a series of samples at different moisture contents, with these variations in moisture covering a much wider range than would logically be expected in construction practice. Using the information obtained from this test the moisture content of the embankment material is controlled to secure the greatest practical embankment density.

In conjunction with the compaction test the penetrationresistance test is made. This test is a measurement of the resistance of the soil to penetration. It is determined by forcing a cylindrical-ended needle into the compacted soil to a depth of approximately three inches, and reading the force required to cause an additional penetration of onehalf inch per second. The factors affecting this test are particle lubrication (moisture), degree of compaction, and density. By standardizing the amount of compaction it has been found that for any one soil this resistance to penetration becomes a function of the moisture content and may be successfully used for construction moisture control. The percolation test is a measure of the seepage capacity of a soil. It is made by measuring the flow of water through a loaded soil specimen. In conjunction with the percolation test the settlement of the soil specimen is observed before and after loading and with complete saturation. The amount of consolidating load used depends upon the probable load conditions in the structure; but for routine testing the load is usually made equivalent to 20 feet of fill.

The consolidation test is the observation of the volumetric change of a soil specimen under load with respect to time. From the test data information is obtained which will indicate the settlement of the actual structure.

The shear test is made in a two-piece container so constructed that the upper half may be held stationary while the lower half is pulled out, thus causing the soil to slide on soil. The shearing strength resulting from any one normal load is obtained by placing the material in the abovementioned container, applying a constant load to the soil

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normal to the plane of sliding, and pulling the lower half of the container from under the upper half. The maximum resistance to sliding of soil on soil is the ultimate shearing strength for that one normal load. This process is repeated for other normal loads until sufficient data are obtained for determining the cohesion and the angle of internal friction. The testing machine used has a specimen container that is one foot square, and the load capacity of the machine is approximately 30,000 pounds for both horizontal shearing load and for compression or normal loading. Strain meters are so connected to the specimen that both the consolidation and deflection under shearing load may be measured accurately to one-thousandth of an inch.

The specific gravity is needed for computing the amount. of voids in soil specimens. The method of determination needs no explanation except that it is important to remove all entrapped air from the specimen. This may be accomplished by reducing the pressure on the specimen, by boiling, or by a combination of the two, the last probably being better than either pressure reduction or boiling.

The soluble-solids test is the determination of the amount of material that will go into solution in water. The test as usually made is not quantitatively correct as the degree of solubility of the salts or mineral is not taken into account. The test merely indicates that there are large or small amounts of solids that are readily soluble in water. If a questionable condition is indicated a process of continued leaching is used to more accurately determine the percent of soluble solids.

EXPERIMENTAL WORK

In addition to the above-mentioned routine tests, experimentation has been started to determine the rate of hydrostatic pressure travel through soils. Soil specimens are compacted in a cylinder with a hydrostatic pressure cell at one end, this end being sealed watertight. At the other end water pressure is applied and the pressure measured until full hydrostatic pressure is obtained at the sealed end. Then the hydrostatic pressure is released at the end opposite the cell and the pressure on the cell is observed, thus de

termining the rate at which the specimen drains. It is believed tests of this nature will give data which may be used in the computation of hydrostatic pressure in embankments after reservoir draw-down.

Another phase of hydrostatic pressure for which experimentation has been started is the hydrostatic pressure created by the consolidation of soil specimens. It is known that all soil specimens artifically compacted contain soil, water, and air. If these specimens are consolidated, either the air or water must escape or the air will be compressed, thus creating pressure as indicated by Boyle's law on the compression of gases. The amount of pressure thus produced will be dependent upon the rate of escape of the air or water from the specimen as the consolidation occurs. The plan for measuring this pressure is to place a hydrostatic pressure cell in the middle of compacted specimens and observe the hydrostatic pressure created by consolidating the specimen and also observe how rapidly this pressure is dissipated. It is believed that tests of this nature will show clearly the advantages gained by artificial compaction and may indicate the artificial compaction necessary for different load conditions. This may also indicate the limiting moisture content for different load conditions or for different embankment heights.

The laboratory has also done considerable work with different amounts of compaction and with different types of compaction. For impact or hammer compaction the number of blows per layer has been varied over a wide range, the weight of the hammer has been varied, and the length of stroke has been varied. For pressure compaction the number of load applications, compacting load, size of compacting foot, and amount of rock in compacted specimens has been varied. The laboratory staff realizes that there is a large field for experimentation in soil mechanics; also that many improvements probably are yet to be made in the test apparatus and procedure now in use. As much time is given this phase of the work as routine tests will allow. The accompanying photographs show the principal equipment and machines now being used in the earth. materials laboratory investigations.

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