US2382763A - Submarine foundation - Google Patents

Description

SUBMARINE FOUNDATION Filed Aug. '7, 1944 INVENTOK L. yb n rTo/e/vfy.

Patented Aug. 14, 1945 UNITED STATES PATENT OFFICE Signal Oil and Gas Co mpany, Los Angeles,

Calif., a corporation of Delaware Application August 7, 1944, Serial No. 548,413

12 Claims.

This invention relates to the construction of submarine foundations. The problem of supporting structures in the bottom of lakes and in the ocean subjected to storm conditions is of ancient origin. A useful and widely used method is to employ piling. Many forms of piling have been suggested, among the most useful being structural shapes such as I and H beams and prefabricated concrete piling, as well as wooden beams.

The problem of supporting structures on such piling is associated ywith the limitation of load which may be supported by such piling. The slenderness ratio, i. e., the ratio of the unbraced length to the radius of gyration (L/R), is the limiting factor in design of such structures. Engineering experience as codified in various standards and building laws has imposed limitations on the permissible value of the L/R. For example, the following table gives the L/R, ratio permitted by various representative associations and representative building laws for steel main members for safe practice:

American Bridge Co L/R 120 It is safe to say that a, value of L/R of more than say 150 to 200 is excessive. This means that bracing must be placed to support the piling in order that the unbraced length of the piling be not greater than that required by the above limits.

This imposes severe limitations upon the use of piling in deep water, particularly when employing concrete or steel piling which is to be driven into the submarine bottom. Thus, for a, slenderness ratio of 120, using 141/2" O. D., 47#/foot pipe (radius of gyration 5.018), as a pile, the unsupported length of the pile cannot be more than 50 feet. This would limit the pile to a water depth of about 30 feet, assuming a free board of 20 feet to be sufficiently high to prevent waves breaking over the platform supported by the pile. In order to permit the structure to be supported in deeper Water, it is necessary to provide for bracing. Underwater bracing, particularly in steel or concrete piling, is limited to the use of clamps. Such clamps tend to corrode or to be vibrated loose. The piles themselves tend to be corroded at an accelerated rate at the clamps. For example, a structure such as is illustrated in the Roberts Patents No. 1,867,030 was built oil' the coast of California, and although it stood for a number of years and gave useful service, it finally collapsed. Welded bracing under water between driven piles cannot be employed.

I have devised a method of construction for deep water employing piling, where the total water depth, including the height of the structure above water, exceeds the slendemess ratio permissible for commercially practical piling. In my method I do not need to employ under-water cross bracing members between piles.

I have devised a submarine foundation particularly useful in constructing islands unconnected to the shore. All the principles are useful in the construction of piles or other structures connected to the shore. t

Pile-supported structures are superior, particularly where positioned in water subjected to vigorous Wave action resulting from winds or storms, since they expose a minimum resistance to the force of the waves. By my method of construction I make possible the pile unsupported by bracing in deeper Waters than have hitherto been possible, or, in the alternative employ piles of smaller radii of gyration. I do this by reducing the unsupported length of the pile by forming a mound upon the submarine bottom and driving the piles into the mound. The height of the mound is sumcient so that the unbraced length of the pile above the mound is such as to give the desired slenderness ratio.

It is, however, an important element of my invention that such a mound should be of such height and the piles, when driven into the mound, should have suicient penetration so that the length of the unbraced pile, when measured to a datum line in the mound below the limit of wave action, will be no greater than that requiredto give the desired slenderness ratio for the pile structure, and so that the pile is driven into the mound for a distance below the datum line to give by itself,l even though the mound be removed down to such datum line, a suicient penetration to result in adequate and safe bracing for the pile.

Preferably, in order to assure that the mound does not cause destructive wave action, the mound should not be so high as to cause storm waves to break as they pass over the mound and through the piling of the superposed structure.

The wave height, i. e., the distance from crest to the trough of the wave depends on the still water depth, the wind velocity and the reach of the wind, i. e., the distance of unobstructed travel of the wind and wave train (see Wave Pressure on Sea Wall and Breakwater, by David Molitor, Proceedings of the American Society of Civil Engineers, May, 1934, vol. 60, 5, Page 653). With few exceptions ocean waves rarely exceed 45 feet in height. However, no formula can be given for ocean waves due to the fact that the reach of the wind will range from 50 to 90 miles as extremes, and wind velocities may range up to 75 miles an hour. Local observations are therefore necessary. Experience on the Pacific coast indii cates that about 30 feet may be taken as a reasonable height for storm conditions.

In deep water, the level of the crest of the waves above still water is taken by experts in this art as about two-thirds of the height of the wave, and. a greater fraction of the height of the wave in shallow water. The trough of the wave is thus below the still water line in amount equal to about one-third or less of the height of the wave. For the maximum storm conditions given above the trough of the wave is from above to feet below still water line. The formulas for ascertaining the above values of the height of the crest and the depression of the trough from still water level are given at page 656 to 657 of the above article.

A wave approaching an obstruction will clear the obstruction if the trough passes over the obstruction. However, the drag caused by the passing of vthe wave across the ocean bottom also inuences the breaking of the wave. Thus, the depth of water in which a wave of given height will break will depend on the character of the bottom and on the wind velocity. The depth of water in which a wave will break is from 1.2 to

1.9 times the wave height for storm waves, with the wind in the directionof wave travel. Local observations are necessary to determine the exact figure. Waves will not break if the water is deeper than that indicated. Experience has shown that if an obstruction is submerged below still water line to a depth of from 30 to 60 feet, the waves will not break even with waves from 10 to 35 feet in height. This water depth is referred to in this specification as the limit of wave action. An obstruction positioned in the bottom which does not reach to a height within such dis' tance from the still water line is termed to be below the limit of wave action.

The pressure of a wave impinging or breaking against an obstruction rises to very large amounts.

The most satisfactory manner of determining suon methods of observation have shown that the obstruction of a wave results in gigantic forces being imposed on the obstruction. Thus, in the case of the conditions previously recited, a 30-foot wave, if obstructed, would rise to a distance of 40 feet above still water line, i. e., rise to feet above its crest. Such a wave would exert upon the impinging structure an impact of above 100,000 to 150,000 pounds per foot of width for the height of 50 feet above the trough of the wave. The impact forces and overturning moment thus imposed on such structure is extremely large and the structure must be suiliciently rugged to resist such forces. A mount against which such wave breaks must be specially designed and built. In fact, as has been shown by past experiences, only reinforced concrete structures are safe and earth fills will not resist such wave action, but will be carried away by the waves.

I therefore desire, in order that the mound shall not be subjected to destructive action, that the wave be not obstructed by the mound so as to cause the wave to impact with a force suillcient to disrupt or disturb any fill from which the mound is formed or any armor coat placed upon it. In order to assure this I prefer to keep the top elevation of the mound at a level below the trough of the largest waves to be encountered in the area. For example, in the case of the above 30-foot wave, I will keep the elevation of the mound 10 feet or more below the still water line. The submergence of the wave below still water line suilicient to be below the trough of the wave produced by severe storms occurring in the locality is herein termed as below the trough line.

There is, however, another consideration which imposes a. critical limitation on the height of the mound. It is desirable in erecting a pile-supported structure that the structure be not sublected to breaking waves but that the waves will pass through the structure without breaking. If waves break against the structure, the surface pressures which are generated are of such magnitude as to seriously threaten the stability of the structure. Two influences determine the breaking of the waves. One is the exposed surfaces of the structure. In an open structure such as a pilesupported deck, island, pier, or wharf the surface exposed is that of the pile, and from practical experience this is no more than from 5 to 10% obstruction. Such waves will pass through without breaking or creating any serious forces beyond that for which the structure may be designed. If, however, the wave breaks on passing through the structure, impact pressures mount seriously so as to endanger the structure,

The other condition which controls the breaking of the wave is the depth of the submarine obstruction below the still water line, as previously set out. It is preferred that the mound level does not rise to a height greater than about 1.2 to 2 times the extreme height of storm waves', and, as previously explained, this will, for a 30foot wave, be from about 35 to 60 feet below still water. Thus, the top of the mound is below the limit of wave action and the wave will not break against the structure or against the mound.

It is, of course, necessary that the superstructure supported by the piles be above the still water line so that the crest of the waves will pass underneath the structure, for otherwise they will break. For the 30foot wave previously assumed as the maximum wave to be encountered in the Pacific Ocean, this will require that the superstructure be 20 feet or more above the still water line. For a li5-foot wave where the extreme is encountered in ocean storms, this will be 30 feet.

By applying the above considerations to locally determined conditions, those skilled in the art may determine the maximum height of the mound and the level of the superstructure.

It will then be necessary to choose the piling so that the unbraced length of the pile driven into the mound gives the desired L/R or slenderness ratio. Thus. assuming a 30-foot wave, the mound top may be safely taken as 60 feet below the still water line and the deck be placed 25 feet above the still water line, i. e., the distance from the deck to the mound top is about feet or .more. The cross bracing above water may be placed a maximum of 18 feet below deck elevation, leaving an unsupported pile column of 67 feet. For a slendemess ratio of 130 we employ a standard H beam, to wit, a 14" x 14" section, 130 pounds per foot, the H beam having a radius of gyration of 6.31 inches.

The invention will be further understood by reference to the accompanying drawing, in which:

Fig. l is a side elevation of a structure built according to my invention; and

Fig. 2 is a section taken on the line 2-2 of Fig. 1.

Fig. 1 is a structure such as an island unconnected to the shore, or it may be a wharf, pier, or other structure connected to the shore. 2 is a mound.

Such mounds may be made by methods conventional in breakwater construction. The location is chosen to give adequate bearing for the mound. The ll material may be dirt, quarry waste rock, or other suitable lill. Such structures have adequate stability if they are properly designed and constructed. In this regard there is little distinction between such a mound and a breakwater. An important feature of my invention resides in the limitations on the position of its top 2 which has been described above. The side 3 of the mound may be sloped and if desired, although this is not necessary, may be armored with rock. Line 4-4 is the still water level. The letter h indicates the height of the waves passing the structure, i. e., the distance between the crest and the trough of the waves. The areas of the wave surface above and below the still water line are equal. For ocean waves the height of the wave above the still water line may be taken as 2%; h, and the distance between the still water line and the trough of the wave as 1/3 h. The distance between the top of the mound and the still Water line d is designated as d.

The invention nds its most advantageous application to areas subjected to wave actions which are of sufiicient magnitude as to cause disruption or destruction of breakwaters. It is the common experience of civil engineers that no fill-type breakwater which is subjected to storm conditions is stable, 'but that the top of the breakwater is carried away usually to a level of minus 10 feet, i. e., 10 feet below mean low water level. All such breakwaters must be repaired after severe storms. Observations on the Humboldt breakwater at Humboldt Bay, California, have shown that 50-ton boulders used to armor the breakwater are tossed from one side of the breakwater to the other side by the force of the waves during storm conditions.

My invention is therefore peculiarly applicable to ocean or lake sites which are subjected to storms suliicient to cause disturbance of lls placed on the submarine floor, and I avoid such disturbances by the limitation of the height of the fill in accordance with the principles previously set forth.

The limitation of the height of the mound to the distance d below still water, previously referred to, is of little or'no significance in still water or in water which is subjected to waves of small height, say 5 feet or less, or one in which the reach of the wind causing the waves is so small as to impart a relatively minor impact pressure when the wave breaks. Thus, if the reach is 5 miles or less, with winds up to 75 miles an hour, the wave will be 5 feet or less in height, and the maximum pressure developed by such a wave is not suflicient to be a problem.

The peculiar advantage of my invention is realized when operating in depths of water ranging above about 30 feet and in other areas where the ocean depth is such that the unbraced length of the piling will give an undesirable slenderness ratio in excess, for example, of 200, and particularly in areas subjected to storms developing waves of about 10 feet or more in height. In such areas I attain the objective of my invention by raising the mound to a sufficient height to permit use of piles of conventional or commercially available design and employ a safe slenderness ratio, as stated above. I, however, preferably also limit the height of the mound so that the maximum elevation is such as to prevent breaking of the waves as they pass through the pile-supported structure placed on the mound. I, therefore, build up the mound above the marine foundation to a. height such that its top is a. distance from the still water line for 1.25 to 2 times the height of the extreme wave height experienced for the worst storms in the locality. The distance is safely 60 feet or more for the worst storms experienced on the Pacific Ocean, California coast. A distance of feet will meet the most extreme ocean waves of 45 feet, such as are rarely experienced, since such waves require wind velocities of 75 miles an hour and a wind reach Vof 900 miles. Ocean storms are more or less local and do not cover more than 50 to 100 miles. An outside figure for ocean storm waves of 30 feet is, according to experience, therefore a safe gure. My structure thus finds particular utility when it is desired to construct the foundation in more than 60 feet of water. I thus build the mound so that its top is 60 feet or more below the still water line.

Into this mound I drive the desired piling to give the necessary support. The piling is driven preferably, but not necessarily to refusal, in the mound to give the necessary bearing support for the pile. The support for the pile is thus, as is well known, merely the frictional resistance of the pile in the earth into which it is driven. The importance of my invention in producing a stable mound which is not destroyed by the wave action is thus further made critical by this requirement. As will be clear to those skilled in the art, if a pile is driven to an adequate depth in the mound and :then the top of the mound is cut away by wave action, the structure is unstabilized by two instability factors thus introduced. The depth of penetration is reduced by the depth of the mound which is washed away and the unsupported length of the pile is thus increased by such depth of earth removed, resulting in an increased slenderness ratio for the structure.

My invention thus eliminates such hazards from the design of submarine foundations.

The superstructure 4 illustrated as a deck is mounted on Ithe pile.` 'I'he distance d, which is the height above the wave crest, should be such as to pass any wave under the extreme wave and storm conditions encountered in the locality. As has been indicated, this will give, for example, a deck at a height greater than about 20 to 30 feet above the still water line 4 4. For waves of 30 feet, 25 feet will be ample. Usually bracing 5 extends to a depth of about 18 feet below the deck. Employing standard H or I beams, or .beams of equivalent sections, the wave obstruction produced by this structure is no more than about 5 to 10%. Such a structure will permit of the use of standard shapes and give slenderness ratios about to 150, and less than 200, and will be safe.

Bracing below water has been found to be the cause of early failure due to accelerated corrosion at bracing points. This, it is believed, is caused by slight movement of members at bracing point which removes products of corrosion.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim as my invention:

l. A submarine supported structure, comprising a submerged mound artiflciallyerected on the submarine bottom beneath said structure, the top surface of said mound being below the limit of wave action, piles driven in said mound, a structure supported on said piles, the unsupported piling having an L/R ratio of less than 200.

2. A submarine supported structure, comprising a submerged mound artiilcially erected on the submarine bottom beneath said structure, the top surface of said mound being more than 35 feet below still waterline, piles driven into said mound, a structure supported on said piling at a level more than 20 feet or more above still water line, the L/R. ratio ofthe unsupported length of the pile being less than 200.

3. A submarine supported structure, comprising a submerged mound artificially erected on the submarine bottom beneath said structure, the top surface of said mound being more than 60 feet below still water line, piles driven into said mound, a structure supported on said piling at a level more than 20 feet or more above still water line, the L/R ratio of the unsupported length of the pile being less than 200.

4. A submarine supported structure, comprising a submerged mound articially erected on the submarine bottom beneath said structure, positioned in a submarine bottom where the reach of the wind is such as to give waves of feet or more with winds ringing up to 75 miles per hour, said top being positioned below the limit of wave action. piles driven in said mound, a structure supported on said piles, :the unsupported piling having an L/R ratio of less than 200.

5. A submarine supported structure positioned in a submarine bottom subjected to action of waves up to 30 feet in height, which comprises a mound articially erected on the submarine bottom beneath said structure, the top surface of said mound being more than 35 feet below still water line, piles driven into said mound, a structure supported on said piling at a level more than feet or more above still water line, the L/R ratio of the unsupported length-of the pile being less than 200.

6. A submarine supported structure positioned in a submarine bottom subjected to action of waves up to feet in height, which comprises a mound articially erected on the submarine Y bottom beneath said structure, the top surface of placed, raising said mound to a height such that the top surface of the mound beneath said structure is below the limit of expected wave action, erecting piles on said mound, and erecting a structure on said piles.

8. A method for constructing submarine supported structures, which comprises forming a submerged articial mound on the bottom beneath the water-in which said structure is to be placed, raising said mound to a height such that the top surface of the mound beneath said structure is below the limit of expected wave action, erecting piles on said mound, and erecting a structure on said piles, said piles on said structure being such that the piles have an L/R ratio for their unsupported length less than 200.

9. A method for constructing submarine supported structures positioned in a submarine bottom subject to wave action of waves up to 30 feet in height, which comprises forming a submerged articial mound on the bottom beneath the water in which said structure is to be placed, raising said mound to a height such that the top surface of the mound beneath said structure is more than 35 feet below the still water line,-

erecting piles on said mound, and erecting a structure on said piles, said piles in said structure being such that the piles have an L/R ratio for their unsupported length less than 200.

10. A method for constructing submarine supported structures positioned in a submarine bottom subject to wave action of waves up to 30 feet in height, which comprises forming a submerged artificial mound on the bottom beneath the water in which said structure is to be placed, raising said mound to a height such that the top surface of the mound beneathsaid structure is more than 60 feet below the still water line, erecting piles on said mound, and erecting a structure on said piles, said piles-in said structure being such that the piles have an L/R ratio for their unsupported length less than 200.

1l. A method of erecting submarine structures in water of d epth of about 30 feet or more at 1ocations where the wind velocity and reach of the wind give waves of 10 feet or more in height, which comprises erecting a mound beneath the location where the structure is to be erected, raising the top of the mound to a level which is below the still Water line for a distance equal to 1.25 feet or more times the wave height expected during storms at said location, erecting piling in said mound and erecting a structure, on said piles, above the crest of such expected waves, bracing said piles above the water level such as to give an unbraced length of said piles having an L/Rratio of less than 200.

12. A method of erecting submarine structures in water of depth of 30 feet or more at locations where the wind velocity and reach of the Wind give waves of 10 feet or more in height, which comprises erecting a mound beneathv the location where the structure is to be erected, raising the top of the mound to a level which is below the still water line for a distance equal to 1.25 feet or more times the wave height expected during storms at said location, erecting piling in said mound and erecting a structure on said piles, and bracing said piles above water level such as to give an unbraced length of said piles having an L/R. ratio of less than 200.

GARTH L. YOUNG.

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