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First, both diffractive and drag loads decresase with the slope of the cover; they are maximum for a structure with vertical sides and zero for a structure which does not stand above grade.

Or, if the earth fill over the structure is the part that is aboveground, they are maximum with an earth cover with a steep slope, minimum with an earth cover with a low slope.

Second, some of the force must be expended to move a mass of earth before the earth can apply force to the structure.

Third, the earth itself is capable of carrying load if it is thick enough and has been sufficiently compacted to arch over the structure and carry a part of the load.

One of the conclusions resulting from this project was that a shelter should be completely buried to obtain maximum blast protection from a minimum thickness of earth cover. In other words, if you want a combination of minimum thickness and maximum protection, then you should go completely below ground.

With that, I want to go into below-ground structures.

On Operation Plumbbob, project 34.3, that consisted of the test of 2 buried 7-foot-diameter 10-gage structural-plate pipes, each of these 20 feet long. They were buried so that the grade was 10 feet above the crown of the pipe and were placed to be subjected to overpressures of 195 and 265 pounds per square inch.

The purpose of the tests was not to evaluate multiplate pipes as personnel shelters, but rather to check their effectiveness as an inexpensive alternate to the expensive reinforced concrete tunnels required for certain test facilities.

For test purposes, the pipes could be permanently deformed, as long as deformations would not prevent restricted access through the pipe tunnel.

In other words, for the purpose for which these were designed, they could have been significantly deformed provided afterward people could still get through them.

Unfortunately, the desired deformations were not achieved because the overpressures were less than anticipated and because the arching action of the soil may have been greater than expected. Actual deformations-all less than 1 inch-probably would be acceptable for a personnel shelter, although not suitable for areas with a high water table. Neutron radiation was not measured inside the pipes, again because they were not intended as personnel shelters; gamma radiation inside the pipes was well within the acceptable limits.

If used as a shelter, these pipes, since they were only 20 feet long, could accommodate not more than 10 persons.

It should be emphasized that the blast loading transmitted by earth on belowground structures is very difficult to predict, even though the air blast pressure at the ground level is accurately described. The reasons for this difficulty in predicting soil pressures are: (1) The grossly different characteristics of various soils,

(2) The fact that there are few truly homogéneous soils even for a single soil type,

(3) Differences among soils in the extent to which cementation occurs that is cementation ordinarily increases with the time the structure has been buried-and

(4) Significant variation with time of moisture content, a major variable affecting transmission of blast loading.

Blast loading will be carried through saturated soils rather than those with low water content.

Even with these uncertainties, blast loadings can be prodicted with adequate accuracy. A procedure for estimating this type of loading, as well as those discussed earlier, is covered in an excellent FCDA publication I refer here to their TR-5-1 published in January of this year, Recommended FCDA Specifications for Blast Resistant Structural Design Method "A." This presents the information in a form which is suitable for use by the average engineering firm.

While the procedures described in the publication for estimating the loading are not as refined as they might be, I want to emphasize that they are as refined as they need be, at this time.

I will get into that point a little later.

I want to go back to Operation Teapot (1955) for one more belowground structure. This was an underground group shelter sponsored by FCDA on Operation Teapot (1955) and the planned view sections are shown in the following slide (fig. 12).

4' ABOVE FLOOR

FIGURE 12.-Underground group shelter sponsored by FCDA on Operation Teapot (1955), planned view sections.

SECTION A-A

Mr. HOLIFIELD. Will you give the year in each instance?

Mr. VORTMAN. This was in 1955.

The structure consisted of an underground room 25 feet long with utilities in one end, and entrance way, and a horizontal sliding door of reinforced concrete at the entrance.

Now, I want to point out that this structure did make use of the principle that I explained earlier of a ground level structure, such that there was no enhancement of drag or overpressure due to either diffractive or drag phenomena.

The interior of this shelter is shown in the following slide (fig. 13).

FIGURE 13.-Interior view of underground group shelter sponsored by FCDA on Operation Teapot (1955).

This view just gives you a general idea of the interior.

As tested, the shelter would accommodate 30 persons and it could be enlarged merely by increasing the length of the basic structure. The adequacy of the shelter to provide protection from neutron radiation was not firmly established.

The fast neutrons varied from 2×106 to 4X108 n/cm2. That is to say, the situation varied from adequate protection to one in which 45 percent of the occupants would suffer radiation sickness. We cannot be sure that adequate protection was furnished, because there is reason to suspect the measurements.

Mr. LIPSCOMB. Is this information from 1955 as applicable today as it was in 1955?

Mr. VORTMAN. Yes. So far as blast loading is concerned, that is true.

Mr. LIPSCOMB. In other words, the tests you conducted in 1957 and 1956 have not added anything in the way of new material?

Mr. VORTMAN. No, that is not true. They have added quite a considerable amount of material. As one goes along chronologically with tests, one increases the amount of knowledge as one goes along.

Mr. LIPSCOMB. Have you done any more on this particular test? Do you have new information?

Mr. VORTMAN. Yes. I may point out that the shelters for Operation Plumbbob were included in CETG program 30.

Mr. LIPSCOMB. In this FCDA pamphlet, are they using 1955 information in that pamphlet?

Mr. VORTMAN. Someone from FCDA will have to answer that question. I assume, based on the date, that they have used 1955 information and preliminary 1957 information.

Mr. LIPSCOMB. Did you reconduct this test that you are just describing in 1957 to correct for any radiation hazards, or to obtain other necessary technical data?

Mr. VORTMAN. At this point I want to complete the statement I began earlier, that program 30 of Operation Plumbbob in the 1957 series included the shelters, and someone-the program director of program 30, I guess-will describe those later.

Mr. CORSBIE. I believe it is intended, sir, to cover that when FCDA presents its testimony.

Mr. LIPSCOMB. I just get the impression that I have heard some of this before.

Mr. VORTMAN. I am sure you have.

The shelter described above was designed to withstand 100 pounds per square inch and was successfully tested at a blast pressure just a little less than 100. When a shelter is designed for test purposes to withstand 100 pounds per square inch, it is desirable that the shelter be unscathed when subjected to 100 pounds per square inch but show signs of failure at overpressures only slightly higher.

One is seeking an optimum design for controlled yield and point of burst. If such a shelter exhibited no signs of failure at a pressure significantly greater than 100 pounds per square inch, it would be considered overdesigned, it would not be the most economical design to resist 100 pounds per square inch.

I want to point out here that one should keep in mind that a structure which fails at 100 pounds per square inch from a small-yield weapon would be expected to fail at a somewhat lower pressure from a large-yield weapon, just on the basis of a longer loading from the higher yield device. În full-scale tests of shelters, loading and response information is sought to evaluate the design procedure. The procedure, if adequate, can then be applied to the design of other shelters without the necessity of testing them. In effect, we build up a fund of knowledge which will find a multiplicity of applications. The advantage of this approach is that it avoids the expense of continuous ad hoc testing of each individual shelter design, designed by the same procedure. But one does want to test and evaluate each one designed by a new procedure.

Now, the precision desired for testing need not be extended to shelters designed for actual use. A shelter designed to resist 100 pounds per square inch must be able to withstand that pressure and it is not only permissible, but even desirable that it withstand some additional overpressure. An optimum design cannot be attempted