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The CHAIRMAN. What you mean by the curtain is the wall made of concrete or stone, or whatever it may be, that comes next to the water, and then you fill it behind with earth? Is that it?

Mr. WALLACE. No; I mean a subterranean construction, a subsurface construction that will go clear to the bed rock.

Senator SIMMONS. What is that to be made of-masonry or timber?

Mr. WALLACE. That can be of timber if it is entirely submerged, or it can be of concrete, or it can be of any material, of whatever nature, that will be permanent and that will, without any question, shut off the percolation or flow of water underneath your dam.

Senator TALIAFERRO. That is, this curtain has no part whatever in the foundation except to prevent an underflow of water?

Mr. WALLACE. That is it, exactly. Now, there are two things to be guarded against

Senator SIMMONS. This curtain goes down to the rock and extends the full length of the dam?

Mr. WALLACE. Yes. To make it clear to you, perhaps I can explain it a little differently.

The foundation for a dam has to perform two functions. One is to support the structure upon which it rests; the other is to prevent the water from running through underneath it.

Senator TALIAFERRO. And undermining it?

Mr. WALLACE. Either undermining it or else exhausting your reservoir of its water supply. You may have a flow of water underneath a dam that may drain the dam area and still may not ruin your structure as a dăm. Have I made that clear to you?

Senator TALIAFERRO. Well, Mr. Wallace, if there was sufficient underflow to drain that dam, would it not in all probability impair the structure itself?

Mr. WALLACE. Possibly, but not necessarily so—that is, it might be possible to drain that water off absolutely and not be able to hold water in your dam and still not destroy the dam. But I would not take the chances on it if I were building the dam, though I can not say that it would not be possible to do it.

Senator SIMMONS. Is it proposed to construct any such curtain as you now speak of at Gamboa ?

Mr. WALLACE. At Gamboa? No, sir; because at Gamboa the foundations themselves go to the bed rock. At Gamboa the primary rock foundation comes up-not the indurated clay, where you do not know what is underneath it, but the actual basaltic rock, of which the backbone of the continent itself is made. The deepest part at Gamboa is only at sea level. You only have to go 45 or 50 feet below the bed of the Chagres River to put the foundations of your dam right down on the bed rock, which is the backbone of the continent, and there is not any question at all about its integrity.

Senator SIMMONS. But after you get to bed rock at Gamboa you propose to construct an earth dam there with a masonry core, do you not?

Mr. WALLACE. No; not necessarily so. That was a detail that was left for subsequent determination.

Senator SIMMONS. Has anybody suggested anything there except an earth dam with a masonry core?

Mr. WALLACE. Yes, sir; I understand that the majority of the advisory board suggested a masonry dam, and a masonry dam is the thing, it seems to me, to construct there. The only reason that anyone ever suggested an earthen dam with a masonry core at Gamboa was from the fact that there was so much material to dispose out of Culebra that the material could be wasted at the site of that dam, and it would afford a place to put that material. As far as an engineering proposition is concerned, the proper thing to build there is a masonry dam.

The strength of a masonry dam founded on the bed rock is a matter of absolute mathematical determination. There is no guesswork about it. You can logically reason from the integrity of one masonry dam to another masonry dam, because you have elements there that are what we call determinate. But you can not so figure on an earthen dam. An earthen dam is absolutely a question of judgment and opinion. You can build an earthen dam in Massachusetts or New York or Colorado that will stand a head of 85 feet of water, and it is no criterion at all that the same dam at Gatun or in South Africa or at any other place would stand it, because the conditions are never the same. I mean the conditions under the surface. That is a matter of what you may call engineering judgment if you are in favor of an earth dam, and you may call it engineering guesswork if you are not.

Senator TALIAFERRO. Mr. Wallace, is not this earthen dam which is proposed by the minority at Gatun a dam of unusual strength ?

Mr. WALLACE. Yes; it is; but as I said awhile ago, the unusual size that you make that dam may affect its integrity as a dam if there is no water flowing under it and if it is on a proper foundation. But there are two gorges that are underneath it. The deeper you go in those gorges the more water-bearing the material is. You find the same mass of loose gravel in the bottom of this gorge here that we found in the bottom of the gorge at Bohio, a really water-bearing stratification, practically a subterranean river. Now, when you add to that a pressure of some 38 pounds to the square inch, due to this 85 feet of head of water which is behind this dam bearing on this water to press it through that stratification, no engineer can tell you what is going to happen there.

Senator SIMMONS. Mr. Wallace, I want to see if I understand you. If I understand you, your position is that you can not guarantee the safety and integrity of an earth dam unless it is either built upon a rock foundation or unless there is a curtain going down from the surface, through the subsurface, to a rock foundation ?

Mr. WALLACE. Yes, sir.
Senator SIMMONS. That is your position?
Mr. WALLACE. That is my position.

Senator SIMMONS. And that curtain must extend the full length of the dam?

Mr. WALLACE. The full length of the dam; yes, sir. Senator SIMMONS. That is, not only across those gorges-down where those gorges are—but down where this indurated clay is?

Mr. WALLACE. It must go into that. If this indurated clay is all indurated clay, if you get a good foothold into this clay it is a proper foundation,

Senator TALIAFERRO. I want to ask you a question before you get away from that.

Mr. WALLACE. May I finish this matter-just a minute? Senator TALIAFERRO. Yes, sir. Mr. WALLACE. But no man can tell what is here. While it is possible and while it is probable that that indurated clay does extend to the bed rock, you have not any surety of it.

Senator SIMMONS. What I want to get from you is your opinion as to whether that curtain has to go down through that indurated clay until you get to bed rock, in order to guarantee the integrity of your dam.

Mr. WALLACE. No, sir; not if that indurated clay does go to bed rock; but I would find that out first, before I built a dam that this great work was dependent upon.

Senator SIMMONS. It must be demonstrated that the indurated clay goes to the bottom, or you must go through the indurated clay with your curtain down to the bed rock?

Mr. WALLACE. Yes, sir.

Senator TALIAFERRO. Now, Mr. Wallace, your answer to Senator Simmons seems to lay down the general proposition that an earth dam is not safe unless the foundations go to bed rock, or there is this curtain going to bed rock, for the purpose of cutting off an underflow of water.

Mr. WALLACE. Yes—that is, of course, unless the material may be of an impervious nature.

Senator TALIAFERRO. You did not put that qualification in your answer before. I was going to ask you if there were not a great many earth dams in this country which have been proved to be safe?

Mr. WALLACE. Yes; but if you had a thousand earth dams in this country that were proven to be safe, they would be no criterion that one at Gatun would, unless the conditions at Gatun coincided with the conditions of these that had been proved here.

Senator TALIAFERRO. Exactly so; but it would answer your general proposition that earth dams were unsafe?

Mr. WALLACE. Oh, no; I did not say that; I beg your pardon.
Senator TALIAFERRO. Well, that earth dams were not safe?

Mr. WALLACE. What I say is this, that I, of my own judgment, would not construct an earthen dam in an important work like this or with a great head of water behind it unless it was either founded on the rock or founded on some impervious material like indurated clay or unless it was protected by a curtain that was taken to the bed rock or to the indurated clay or to a surface through which the water would not flow.

Senator TALIAFERRO. I thought you meant that.
Mr. WALLACE. That is what I meant; yes.
Senator SIMMONS. What did you call these two formations here?
Mr. WALLACE. Down in the bottom here there is loose gravel.

Senator SIMMONS. But what did you call that [indicating] ? We have been calling it a gulch. Mr. WALLACE. I called it a geological gorge.

Senator SIMMONS. A gorge; yes. Now, Mr. Wallace, eliminating those two gorges altogether, supposing that they were not there, but that the formation where those gorges are was the same as this here (indicating]-in other words, that it was all indurated clay clear

across the line of this canal and that that indurated clay extended down 200 feet, the full length of the canal, do you give it as your opinion that you could not construct upon the surface of that indurated clay an earthen dam which would be absolutely safe?

Mr. WALLACE. No; I do not say that, because, as I said before, it would depend entirely on what was underneath it here.

Senator SIMMONS. I am asking you a hypothetical case.
Mr. WALLACE. Well, you have

to takeSenator SIMMONS. I can not get at your views unless you will allow me to ask you a hypothetical case.

Mr. WALLACE. I know, but engineers do not decide things on hypothetical cases.

We decide the form of our structure on what we find underneath.

Senator SIMMONS. But it might be of some benefit to us to have your opinion on a hypothetical case. Of course I do not want it unless you are willing to give it.

Mr. WALLACE. Well, engineers are a good deal like some judges; they do not give their

opinions on hypothetical cases. Senator SIMMONS. I will ask you the question, and you can answer it or not, as you see fit. Eliminating the two gorges there, supposing that the material in those gorges was the same as that on either side of it, and that you have, therefore, for the full length of the dam, a foundation of indurated clay extending 200 feet below the surface, I ask you the question if, with those conditions, an earth dam there would not be practically secure and safe?

Mr. WALLACE. I would not put an earth dam in there in an important work of this character under the conditions you name unless I knew what was under that 200 feet of indurated clay, and unless I knew how far above and below the dam the indurated clay extended.

Senator SIMMONS. That is an answer to the question.
Mr. WALLACE. Yes, sir.

Senator ANKENY. Mr. Wallace, that indurated clay is never of uniform thickness, is it?

Mr. WALLACE. That is the reason, or that is one of the reasons, why I would not trust an important structure of that kind on it unless I knew what was underneath it.

I have had some experience with this kind of clay. I put in a bridge across the Missouri River quite a number of years ago for the Santa Fe Railroad Company. When I first took charge of that structure they had a line of borings across the valley. That line of borings struck an indurated clay that was a great deal heavier and harder than this indurated clay is; and the engineer that was sent to take those borings had reported that he had found a foundation substantial enough to construct the piers of that bridge on; and the bridge was planned with its caissons and its piers and everything to go to that depth.

When I arrived on the ground I commenced to examine the character of some of the rock borings and this clay, and I was not satisfied that he had gotten into a permanent stratification. Se we went on through, and after going through some feet of this material we struck gravel and sand, and so forth, below. We found indurated clay in there that weighed 120 pounds to the cubic foot, and that was more of rock nature, really, than this indurated clay is here; and underneath it we found a mass of loose material, gravel and so on,

that we had to carry the piers of that bridge down through. Instead of going, as we expected, about 20 feet, we had to go 65 or 75 to 80 feet below the surface of the water, depending on stage of water.

Senator MORGAN. What place are you speaking of now?

Mr. WALLACE. I am speaking of a bridge foundation that I put at one time across the Missouri River at Sibley.

There is a section of country that Senator Kittredge is familiar with—in the Dakota country—I think up near the Jim River district, where the Missouri River comes down along the mountain slope of the continent at a much higher elevation than a large part of Dakota. In South Dakota you can put down almost anywhere a pipe of almost any size and get a flowing artesian well from it. I think the Senator will bear me out in that, at least to a degree.

Senator KITTREDGE. That is right.

Mr. WALLACE. That water evidently comes underneath from the Missouri River, and it flows through this sand and gravel and a fine silt—the silt is much finer than the material we found in this

gorge here—and under several hundred feet of indurated clay. Am I correct in that?

Senator KITTREDGE. That is absolutely right.

Mr. WALLACE. And so much water flows through that stratification that there are times when there is more water in the Missouri River at Yankton than there is at Sioux City. Is not that correct, Senator ?

Senator KITTREDGE. That is true.

Senator DRYDEN. Mr. Wallace, do the engineers who have examined the subject all concur with you in your opinion?

Mr. WALLACE. I do not know; I have not conferred with any of them.

Senator DRYDEN. You have not read their testimony?
Mr. WALLACE. I have read their testimony; yes, sir.

Senator DRYDEN. Have you observed in your readings whether they do or do not agree with you?

Mr. WALLACE. Some of them do and some of them do notor, rather, to put it the other way, I agree with some of them and with some of them I do not agree.

Senator DRYDEN. It comes to the same result?
Mr. WALLACE. It comes to the same result; yes, sir.

Senator DRYDEN. There are a number of very eminent engineer who hold that that dam can be safely built across those gorges.

Mr. WALLACE. I presume there are; but I am simply speaking fo: myself. Senator DRYDEN. Precisely. Mr. WALLACE. And for myself, I would not do it.

Senator DRYDEN. Will you point out to me, if you please, whil we are on this subject, on the map which shows the dam, just wher the dam will cross these gorges?

Mr. WALLACE. I am not familiar with these maps; but one of th gorges in the map is here, and the other one is here—that is, thi hill, in which they evidently have designed the sluice gates—th spillway—is in between these two gorges.

Senator Dryden. About what is the length of the mouth of thos gorges—the top of the gorges?

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