ments made by the Beach Erosion Board of the Corps of Engineers (fig. 6). Information relating to the development and performance of this instrument is presented in Technical Memorandum No. 121 by the Beach Erosion Board, September 1960.

UTILIZATION OF RADIOISOTOPES IN SEDIMENT TRANSPORT STUDIES

Considerable progress has been made recently in the development of techniques for tagging sediment samples with radioisotopes and tracing the movement of the sample particles in studies of sediment transportation associated with beach and estuary problems. Radioactive gold and scandium fused in glass particles have been used to determine the pattern of sand movement in harbors. The tracer material is placed on the harbor bottom, then detection instruments are moved over the bottom to determine both the direction and speed of movement of the radioactive particles. A study of this nature is described in a report by the Corps of Engineers, San Francisco district, dated April 1960, entitled, "Radioactive Tracer Techniques as an Aid in Resolving Sediment Problems." To date little use has been made of radioisotopes for investigation of sediment transportation problems associated with inland streams. Steps have been taken to initiate a joint study by the interagency sedimentation project and the Atomic Energy Commission to investigate the practicability of utilizing radioisotopes in development of a method for instantaneous measurement of suspended sediment concentrations in flowing water.

FIGURE 6.—Schematic layout of probe radioisotope gage for measuring in-place density of submerged sediment deposits.

SITE INVESTIGATIONS

Since the economics and design of engineering projects depend to a very considerable extent on subsurface geological conditions, the Corps of Engineers has utilized a variety of new exploration devices for investigation and sampling of subsurface soil and bedrock conditions. These methods and materials may be grouped conveniently into the following categories: (1) Those which combine geological interpretation with physical measurements, and which are therefore designated geophysical investigations; (2) those which permit interpretation and classification of subsurface conditions by means of soil and bedrock samples recovered by drilling and sampling devices, and frequently referred to as geomechanical methods. Some of the newcomers in these categories which have been used, or are considered for use, by the corps are described below.

GEOPHYSICAL EQUIPMENT

During the past 2 years several compact portable modifications of earlier bulky seismic and electrical resistivity instruments have been developed, which have greatly enhanced the application of geophysical methods to engineering investigations. Two instruments which have particular appeal at this time are discussed first.

Miniature direct-reading refraction seismograph.-This new instrument has been, and will continue to be, used extensively by the Corps of Engineers for preliminary determination of soil and bedrock foundation conditions, to depths generally not exceeding 50 feet. Due to considerable recent competition in its manufacture and very effective advertising, the instrument has found application in highway, railroad, dam, water supply, and military engineering projects.

Basically, seismographs are instruments for measuring the velocity of shock waves passing through the earth's crust. They have been modified as tools for determining both the character and depth of soil and rock materials through which artificially produced shock waves pass. In this field, the corps has conducted considerabie basic research. Until recently seismic instruments have been extremely bulky and for the shallowest depth exploration required the use of explosives. With the development of the new miniature models equipped with transistorized velocity timers and sufficiently sensitive to record shock waves produced by pounding a steel plate with a common sledge hammer, the problems associated with the storage and use of dynamite have been eliminated. Also eliminated is the time-consuming operation of developing and interpreting photographic film records before depths of materials can be determined (fig. 7).

Tests to date indicate a fairly high degree of accuracy in determining thicknesses of soil and bedrock materials to depths not exceeding 50 feet. Where only one more or less uniform soil material covers the bedrock, the position of the latter has been measured to depths ranging between 60 and 80 feet. Therefore, in the experience of the corps,

FIGURE 7.-View of typical light weight portable refraction seismograph showing control and millisecond signal timing panel with cable used to connect geophones or shock sensing devices with the control mechanism.

it may be stated that the usefulness of this new and lightweight instrument, portable by two men, is limited principally to determining the thickness and depth of two soil or rock materials within a depth range of 50 feet. The device is readily available from at least five manufacturers at costs ranging between $700 and $2,000. At least five firms manufacture this instrument and our field installations are using seismographs produced by three of these firms.

Portable electrical resistivity instrument.-Like the miniature seismograph this new instrument has opened the door to wider use of geophysical exploration methods to supplement the more costly_drilling investigation procedures. Its application is similarly confined to quick depth and thickness determinations of considerably lower magnitude than those within the capability of conventional heavier equipment. While not used to date by the corps, the instrument has been recommended for experimental testing to one or two field installations where its usefulness and accuracy in determining the thickness and depths of sand and gravel deposits are of interest.

The 20-pound instrument consists simply of a compact batterypotentiometer-galvanometer-range selection control cabinet, supplemented by four small ground electrodes and wire leads. The equipment is employed to measure various resistances of soils to electrical currents generated through them. Loss in potential of these currents between preselected input locations is translated to the resistivity of the soils through which the currents pass. Resistances and electrode spacings, in turn, are related to soil types and depths, respectively. The Corps of Engineers was active in modifying the original four electrode resistivity apparatus for use in investigating foundations and construction materials. On the basis of this pioneer work, the limitations of this exploratory device to delineating one or two layers of soils, particularly sands and gravels, to depths not exceeding 50 feet is recognized. The cost of the instrument is approximately $500.

GEOMECHANICAL EQUIPMENT

By far, the greatest amount of subsurface investigation for design of engineering works is accomplished by purely mechanical means. These include earth and rock core-drilling machines (designed for both intermittent or continuous care sampling), borehole pumping and hydraulic pressure testing equipment, and borehole photography and television. Through their interest in improving mechanical techniques for obtaining accurate subsurface information, the corps in cooperation with industry has pioneered improvements in these devices which have resulted in mutual benefits to the Government and industry alike. A few of the more important newcomers in this field, which have been tested and used in varying degrees, are described below.

Processed diamonds.-The most advanced mechanism for obtaining continuous undisturbed drill core samples of bedrock is the rotary core drill, equipped with a diamond-studded coring bit. Diamonds used in this device are the rejects or byproduct of the diamond jewelry industry. They range in size from 10 to 20 stones per carat and vary in price from $4 to $12 per carat, the latter being controlled by stone quality and world market prices. Due to gradually increasing prices of these commercial diamonds (bortz), the diamond drill

bit industry has conducted research which indicates that larger worn diamonds may be reclaimed from used bits and, by factory processing, may be upgraded for reuse in new bits. The processing involves an abrasion and polishing operation which removes microfractures and angular projections, thereby restoring the original durable cutting properties of the stones. Processed stones are then combined with new stones in new bits, when top quality bits are not required.

While this development has been too recent for performance testing and rating by the corps, early reports of performance by one or two large diamond bit manufacturers are encouraging. The corps will conduct tests at its diamond bit testing laboratory in Dallas to determine the drilling efficiency of various combinations of processed and unused diamonds in bits designed for cutting different types of rock. Since the corps procures several hundred thousand dollars worth of diamond bits annually, the savings which might be realized by an effective partial substitution of lower cost processed diamonds may be substantial.

Rubber sleeve core barrel.-For many years the Corps of Engineers and the core drill manufacturing industry have directed their research toward improving devices for recovery and preservation of undisturbed soil samples. One of the principal deficiencies in the present state of this art has been the lack of a device which not only will preserve the newly recovered soil sample's natural undisturbed structure and moisture as it penetrates the sampling tube, but which in addition will provide an instantaneous elastic sleeve cover that will assure against damage to granular soils or fractured shales during transportation to the laboratory. Such a device has now been developed by one of the leading core drill manufacturers. It consists basically of a soil sampling barrel which, as it cuts and recovers the sample, covers it immediately with a rubber sleeve of a diameter less than that of the sample core, thus protecting the sample from grinding or blocking as it is brought to the surface.

The corps plans to utilize one of these new sample barrels in connection with our study of improved techniques for recovery of weak shales for triaxial shear tests.

Cylindrical color photography of boreholes the borehole camera.-The Corps of Engineers has long recognized the need for supplementing conventional small-diameter core drilling investigations of bedrock foundations with economical photographic techniques, to assist in the interpretation of subsurface conditions which do not respond completely to the usual drilling and sampling procedures. Inaccurate appraisal of gaps in core recovery could result in overlooking serious foundation conditions. To remedy this problem, which until 1952 could be solved only by resorting to costly large-diameter shafts and borings, geologists in the Office, Chief of Engineers developed a camera capable of obtaining continuous 360-degree color photographs of the interior of 3-inch diameter boreholes.

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