US3508406A - Composite arch structure - Google Patents

Description

April 28, 1970 c.v| FISHER 3,508,406

COMPOSITE ARCH STRUCTURE med June 1o, 1969 `s sheets-sheet 1 l NvENToR/s CHR/sToPHE/a L. FISHER,

ATTORNEYS April 28, 1970 c. L. FISHER 3,508,406

1 I COMPOSITE ARCH STRUCTURE Filed June lO, 1969 l Y 3 Sheets-Sheet 3 |NvENToR/s CHRISTOPHER L. F/SHER,

c.' 1 .'-Flsr-1ER 3,508,406

COMPOSITEARCH STRUCTURE April 28,1970

3 Sheets-Sheet 5 Filed June 10, 1969 I I7 i f3 LL 6I INVENToR/s United States Patent O 3,508,406 COMPOSITE ARCH STRUCTURE Christopher L. Fisher, Winnipeg, Manitoba, Canada, as-

signor to Armco Steel Corporation, Middletown, Ohlo, a corporation of Ohio Continuation-in-part of application Ser. No. 677,278,

Oct. 23, 1967. This application June 10, 1969, Ser.

No. 834,219 Claims priority, application Canada, Oct. 15, 1968,

Int. Cl. F161 9/22; 01g 5/06; E21d 9/00 U.S. Cl. 61-16 23 Claims ABSTRACT F THE DISCLOSURE A composite arch structure and method of making it. The composite arch structure comprises a pair of flexible retaining wall portions and a top portion extending therebetween. Longitudinally extending load spreading means are provided on either side of the vertical axis of the structure at positions where a radial force acting on the structure forms an angle of about 45 or more to the horizontal.

CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part application of the c0- pending application in the name of the same inventor, Ser. No. 677,278, filed Oct. 23, 1967, and entitled Composite Arch Structure and now abandoned.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to new and useful improvements in composite arch structures, particularly relatively large dimensioned composite arch structures.

Description of the prior art The term composite arch structure is intended to include arch structures having a planar base thus forming, when viewed in end elevation, a truncated ellipsoid; arch structures having a planar top portion; and arch structures which might be considered to consist of a pair of flexible retaining wall sections together with top and bottom sections so that they present, when viewed in cross section, a pure ellipsoid, circle or modification of these shapes.

They include all such structures manufactured from relatively thin arcuately curved, corrugated sheets used for culverts or tunnels or bridge liners depending upon the size thereof.

The conventional rigid arch design is at present being superseded by a relatively flexible design utilizing flexible retaining wall structure similar to that described and claimed in U.S. Letters Patent No. 3,282,056, issued Nov. 1, 1966 in the name of the same inventor.

So long as the dimensions of the arch remain relatively small, no difliculty is encountered in the backiilling procedure.

However, with this flexible design, support is achieved with a minimum of moment strength in the tunnel or arch lining, which in fact is just enough to properly install the lining. However, this strength is not to any extent capable of supporting the superimposed load after installation inasmuch as the inherent strength of the device is obtained from the back filling operation and ilexible retaining wall structure as defined in the above mentioned patent.

The difference in moment strength may only be a small fraction of what would be designed for conventional rigid structures.

The design features of such structures utilizing the composite arch principle are, of course, dependent upon the shear and thrust values of the soil, the proper related curvatures of the flexible lining and the type of soil enveloping the underground stucture when finished.

However, under normal conditions the lining which is normally manufactured from corrugated steel sheet material, can be relatively thin even though the internal dimensions may -be relatively large.

Dii'liculties are encountered with relatively large structures inasmuch as the initial stability of the tunnel liner is very small until supported by the backfilling material. Of course, it will be appreciated that the thickness of the material could be increased in order to stiffen the structure prior to backlling but this is relatively expensive and is not a satisfactory solution to the problem.

The principal difficulty appears to be encountered when attempting to backll and compact the soil along or around the junction lines between the flexible retaining walls which have a relatively sharp curvature and are situated substantially vertically, and the flattened flexible arch which extends therebetween. As the compaction proceeds, the horizontal component of the load becomes greater than the vertical component thus causing distortion of the structure which can only be avoided by extremely careful backlling from both sides.

The present invention overcomes this particular fault of the structure by providing load-spreading buttresses formed of concrete and extending along or adjacent to the junctions between the flexible retaining walls and the flattened arch structure.

This provides a length of consolidated material at the location where compaction and backilling equipment cannot effectively work thus enabling the compaction and backlling to continue without distortion occurring to the arch structure.

Once these areas ha-ve been backiilled and compacted, soil is compacted upon the upper vside of the flattened flexible arch and the ring compression of this portion of the structure is run out into the consolidated and known soil material where the concrete buttresses have been attached. This results, of course, in less load on the footings.

In addition to the above, it has been found that when the width of the arch structure is relatively small, in the neighborhood of 20 or 30 feet, there is suicient inherent rigidity in the structure when it is assembled and buttresses applied, to maintain the shape of the structure during the backiilling and compacting process so that when the backlling and compacting process is complete, the liner or arch structure maintains the predetermined curvature which, of course, is controlled by the design parameters of the structure.

However, I have found that the principles inherent in this structure may be extended to cover relatively large liners having a width in the neighborhood of 60 feet or more under which circumstances, the static load of the material would normally cause deection until the backfilling and compaction is complete and, furthermore, the action of backiilling and compacting would (and often does) distort the design curvature of the structure.

It has therefore been found necessary to provide some means to stabilize the structure during the backlling and compacting process.

Thus, for wide width arches, the present invention contemplates the use of one or more stiffening members extending between the buttress means and over the top portion of the arch structure.

3 SUMMARY OF THE INVENTION The present invention relates to improvements in relatively large com-posite arch structures made of relatively thin arcuately curved metallic sheets of the type generally used for small culverts and the like.

In accordance with the present invention longitudinally extending concrete buttresses are affixed to the exterior of the arch structure along areas where the flexibility of the arch structure is such that, in the absence of such buttresses, normal backfllling and compacting procedures could not be used. The buttresses enable conventional backfilling equipment and compacting equipment to be utilized on relatively large dimensioned flexible arch structures thus eliminating the necessity of increasing the thickness of the material forming these arch structures. They can be precast and secured to the surface of the arch structure or, alternatively, they can be cast in place when required. The buttresses are simple in construction and economical to manufacture.

When a composite arch structure of the present invention is relatively wide, the present invention contemplates the use of additional structure in the form of one or more stiffening members extending over the top portion of the arch structure and affixed to the buttresses. The one o-r more stiflening members control the flexibility of the arch structure and prevent side sway during the erection and backfllling operations.

It will of course be appreciated that once the backfilling and compaction process is complete, in many circumstances the stifleners are not required. They can, of course, be removed if desired but it has been found preferable and easier to leave them in place thus adding to the strength of the structure although primarily they are included to prevent distortion of the structure during erection and backfllling.

- It should be pointed out that in relatively wide structures of thin walled plate, the static deflection load can mount to 6 to 12 inches in the center. By the provision of the present construction and the stiffening members used, the top section of the arch liner is, in effect, hung from the members thus preventing side sway and also preventing static load deflection. It should be stressed that this deflection would, of course, disappear when backfllled completely but, with relatively wide spans, distortion would be diflicult to control and in fact, excessive deflection may cause collapse if these stiflening members are not used. Use of the present invention enables the operators to maintain a more accurate curvature to the thin plate during assembly and backlling and also when loaded with live loads.

The stiffening members can be cast in place with the buttresses or, alternatively, can be bolted to bolts extending from the buttresses, if it is desired to make the members detachable.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE l is a cross sectional view of a composite arch structure showing the invention in situ.

FIGURE 2 is an alternative embodiment of the invention.

FIGURE 3 is a cross sectional view showing the invention utilized with flexible retaining walls and a precast arch or spreader beam extending therebetween.

FIGURE 4 is a fragmentary enlarged sectional view showing one method of providing the buttress.

FIGURE 5 is an isometric fragmentary view showing the buttress in place.

FIGURE 6 is an isometric view of a composite arch structure showing the use of stiiening members.

FIGURE 7 is an end view showing an alternative construction of the composite arch structure utilizing stiflening members.

FIGURE 8 is an enlarged fragmentary end elevation showing a method of attaching the stiffening members to the arch structure.

FIGURE 9 is a fragmentary end elevation showing an alternative form of one of the stiffening members.

FIGURE 10 is a fragmentary side elevation of a composite arch structure illustrating the use of stiflening members.

FIGURE 11 is a fragmentary end elevation showing an alternative method of attachment of the stiffening members to the buttress.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings like characters of reference indicate corresponding parts in the different figures.

Safe composite support for loads upon underground flexible structures have been obtained by using a proper relationship between the size of structure, to thickness of material, to type of soil adjacent to the structure, and the degree of consolidation imparted to that soil.

Installation problems increase as structures become larger and therefore have too much flexibility. Economical compaction and earth moving equipment cannot safely be used near such a structure with too much flexibility so that the size has been limited up to the present time for this $35.0.-

The present application is therefore related to relatively large structures manufactured from relatively thin gauge sheet corrugated material.

QInFIGURE 1, the composite arch structure collectivelydesignated 10 comprises a pair of flexible retaining wall structures 11 and a flexible flattened arch structure 12. The/.bases of the flexible wall structures 11 are secured within footings 13 and soil 14 is compacted around footings and around the outside of the flexible wall structures 11 as described in the above-mentioned patent.

VIn the embodiment of FIGURE l, the radius arrows 15 indicate the change in radius between the flexible wall structures 11 and the flexible flattened arch structure 12. Theareas in which this radius change occurs may be considered as the junctions between the flexible wall structures and the flexible flattened arch structure.

In all of the embodiments illustrated herein there is a noticeable radius change between the wall portions and top'portion of the arch structure. Such a radius change would not occur if the arch were circular in cross section. However, the junctures between the wall portions and the top portion, even if not identifiable by radius change, will occur on either side of the structure where a radial force acting on the structure forms an angle of about 45 or more to the horizontal.

When the compacting of the soil 14 has reached adjacent these areas identified by the reference characters 16, conventional earth moving and compacting equipment cannot operate because of the danger of distortion and perhaps collapse of the arch structure. This is due to the fact that the horizontal component is considerably greater at this point due to the rapid change in direction of the curvature of the liner.

Therefore buttresses are provided designated 17 along these, junctions or areas which spread the compacting load at these points thus enabling conventional earth moving and compacting machinery to be utilized.

It will be noted that these buttresses are substantially triangular when viewed in cross section and are provided with a substantially horizontal wall 19 and a substantially vertical wall 18 extending therefrom.

The arcuately curved wall 20 extending between the distal edges of walls 18 and 19, follows the curvature of the tunnel liner at this point and is secured thereto by means of anchor bolts or similar means 21 extending through the tunnel liner.

These buttresses extend clear along the junction area 16' and provide support for the structure so that soil can be compacted against the vertical wall as indicated by the arrows 22 thus spreading the load over a greater area at this vital point.

Once these buttresses have been covered and the soil therearound compacted, the remainder of the compaction up to the upper surface 23 may be undertaken in the normal manner as the composite arch structure is now well supported by the compacted soil.

FIGURE 2 shows a slightly dilferent embodiment in which the composite arch structure extends across the base 25 in the form of an arcuately curved structure.

Here again the buttresses 17 are provided adjacent the areas 16 where there is a change of radius between the flexible retaining wall structures 11 and the flexible llattened arch structure 12.

In this embodiment the Vertical and horizontal surfaces 18 and 19 are concave thus assisting in locating the structure during'the compaction process as well as spreading the loads over a greater area of the compacted soil or lill 14.

The provision of the buttresses 17 enables the arch structure to be modified as shown in FIGURE 3. Here the llexible flattened arch structure is eliminated and a spreader beam or plate 26 substituted.

This spreader beam which may be of precast concrete, is formed with rabbetted edges 27 which engage over the horizontal surfaces 19 of the buttresses 17 as clearly shown. The back-filling material is compacted up to the areas 16 whereupon the buttresses are secured and then the spreader beams may be placed in position to form a solid roof and the over burden 23 compacted into position.

If the composite arch structure is relatively spherical as shown in phantom in FIGURE 3, it is necessary to provide some means of preventing rotation of the structure during the backlilling and compacting process. An elongated keel 29 is provided depending downwardly from the arcuately curved base 30 and when soil is compacted around this keel, rotational displacement of the structure cannot take place.

It will, of course, be appreciated that the buttresses 17 may either be precast and then secured to the areas 16 by means of hook bolts 31 extending from the buttresses, and through the Walls of the liner or, alternatively, they may be poured in place. If they are poured in place then conventional cribbing 32 may be positioned and supported by braces 33 and concrete poured into place under which circumstances the hook anchors 31 may be inserted prior to the pouring of the concrete.

summarizing, when the dimensions of composite arch structures become relatively large, it is diflicult to compact the soil in the general area identified 16 and specilically between points 16a and 16b, due to the flexibility of the economical lining used.

Concrete buttresses 19 are poured in place or precast and fastened to the outer flexible surface thus providing consolidated material at this location where compaction equipment cannot work effectively.

By spreading the load at this point, good compaction can then be obtained against the vertical faces 18 of these concrete buttresses.

Furthermore, ring compression of the llexible flattened arch between points 16b on both sides is run out into the consolidated and known soil material 14 adjacent the concrete buttresses thus resulting in less load on the footings 13.

Although the foregoing description and the drawings describe and illustrate structures designed primarily for substantially horizontally extending structures, it should be noted that the inventive concept can readily be applied to tunnelling which is in any direction and any degree of inclination from the horizontal up to and including vertical where the direction of the load towards the structure is not uniform.

FIGURES 6 and 7 illustrate composite arch structures of the type having relatively large horizontal or width dimensions.

Proceeding therefore to describe the invention in detail, reference should lrst be made to FIGURES 6 and 7 which generally illustrates composite arch structures of the type to which the present invention may be applied.

The composite arch structure in FIGURE 6 (generally indicated at 34) includes a pair of flexible retaining wall sections 35 and 36 and an arcuately curved top section 37 joined to the retaining wall sections 35 and 36- to form, in cross section, a truncated ellipsoid shell.

The retaining wall sections 35 and 36 and the top section 37 are both formed of arcuately curved sheets of relatively thin corrugated steel bolted together and anchored to base abutments 38 and 39.

The junction'areas 40 and 41 between the retaining wall sections 35 and 36 and the top section 37 are each provided with a longitudinally extending concrete buttress 42 and 43 respectively. The buttresses 42 and 43 are of the type described above with respect to FIGURES l through 5 FIGURE 7 shows a modified arch construction which is ellipsoidal when viewed in cross section and which consists of llexible retaining wall side sections 44 and 45 and arcuately curved top section 46. Also provided in this embodiment, is a lower arcuately curved section 47 thus forming the ellipsoid.

This embodiment `also includes similar elongated buttresses 48 and 49 situated at the junction areas 50 and 51 respectively.

When the horizontal dimension of the structure becomes relatively large, both the arch structure shown in FIGURE 6 and the arch structure shown in FIGURE 7 become very flexible, particularly during the backfilling operation. In fact, the upper sections 37 in FIGURE 6 and 46 in FIGURE 7 may actually sag due to the static load inherent in the construction.

The present invention contemplates the use of a plurality of arcuately curved reinforcing and stabilizing members overspanning the top sections and being secured by the ends thereof to the buttresses. These stitfening members are curved to follow the curvature of the upper sections of the arch structures and are in contact with these sections when installed.

In FIGURE 6, stifening members 52 are illustrated as spanning top section 37, With their ends affixed to buttress 42 and 43. In FIGURE 7, stiffening member S3 is shown spanning top section 46, with its ends afiixed to buttresses 48 and 49.

The stiifening members 52 and 53 of FIGURES 6 and 7 may take various forms. A number of embodiments of stilfening members are illustrated in FIGURES 8 through 11.

FIGURE 8 illustrates a stitfening member (generally indicated at 54) fabricated of channel iron with an upper flange 55, a lower llange 56 and an intermediate web 57.

FIGURE 10 illustrates another embodiment of the stiilening members. In this figure the arch structure (generally indicated at 58) has side portionslone of which is shown at 59), a top portion 60 and buttresses (one of which is shown at 61). The top portion 60is overspanned by a plurality of identical stiffening members 62, each having an angle i-ron cross section with a vertical llange 63 and a horizontal flange 64.

Not shown, but equally effective, would be tubular members, square cross sectional members, I-beam cross sectional members and the like, depending upon the requirements and design parameters.

Under normal circumstances, the buttresses would be cast in place as described above. Before these buttresses a-re cast in place, the stiifening members should be placed in position ywith their end portions entering into the forms for the buttresses 15.

Once the stiifening members are in place, in spaced and parallel relationship as clearly shown, the buttresses could be poured thus incorporating the members into the buttresses and anchoring the members by the ends thereof.

FIGURES 6, 7, 8 and 1() illustrate stiffening members with their end portions embedded and supported by buttresses. In FIGURE 8, the buttress 65 is illustrated in dashed line.

Alternatively, if the buttresses are precast or if it is desired to make the stiffening members detachable, the ends thereof can be removably aixed to the buttresses. In FIGURE 1l an arch structure 66 is illustrated together with one of its buttresses 67. A stiffening member 68 is shown with one of its ends 68a angled to conform to the upper surface 67a of the buttress 67. The end 68a of the stiifening member is perforated to permit passage therethrough of bolts 69 and 70. The bolts 69 and 70 are embedded in the buttress 67 as at 69a and 70a respectively. Nuts 71 and 72 hold the stiifening member removably affixed to the buttress.

It is preferable although not necessary under all circumstances, to secure the upper or top section of the arch structure to the stiffening members and FIGURE 8 shows details of one method of attachment. Pairs of bolts 73 pass through the top portion of the arch structure 74 and the anges 56 and 55 of the stiffening member 54 and nuts 75 clamp the top portion to the underside of flange 56, it being understood that washer plates 76 may be utilized in the well known manner.

If the top portion of the arch structure is secured to the stiffening members, as shown in FIGURE 8, then, in effect, the top portion is suspended from the stiffening members which, due to the rigidity of construction thereof, maintain the arch structure in the desired configuration during backtilling and compacting.

In similar manner, the stiffening members 62 of FIG- URE 10 may have the top portion 60' of the arch structure 58 aflixed thereto. In this instance bolts will pass through perforations in the top portion 60 and the horizontal anges 64 of the stiifening members 62. Again washer plates may be used. This is diagrammatically indicate at 73 in FIGURE 10.

FIGURE 9 shows a side elevation of an alternative method of construction of the stiifening members and in this instance, the stiffening member is identied by the reference character 77. In this construction, the stiffening member 77 is cast in concrete integrally with the adjacent buttresses, conventional forming being provided for this purpose. It is normally desirable that reinforcing steel 78 be incorporated in this construction and concrete bolts 79 or similar fasteners may hold the top portion 80 and the arch structure to the stiffening member 77.

Normally speaking, a plurality of stiffening members would be situated in spaced and parallel relationship along the entire length of the top portion of the arch structure and these would remain in place after compacting and backilling.

However, underA certain circumstances, one pair of stiffening members may be secured to the arch structure and a narrow roadway built and compacted over the top of the arch structure between the two adjacent stiffening members. One of these may then be detached and moved outwardly as the road is widened until the entire length of the arch structure has been backilled and compacted in conjunction with the formation of the roadway overhead.

Various modiications can be made within the scope of the inventive concept disclosed. Accordingly, it is intended that what is described herein should be regarded as Iillustrative of such concept and not for the purpose of limiting protection to any particular embodiment thereof but that only such limitations should be placed upon the Ascope of protection to which the inventor hereof is entitled, as justice dictates. I

The embodiments of the invention in which an exclusive property or privilege is claimed are delined as follows:

1. A composite arch structure of the type having compacted backll material thereabout, comprising an elongated flexible liner, side portions at least of said liner being formed of relatively thin gauge corrugated sheet material, a pair of load-spreading buttress means, said buttress means comprising elongated bodies extending longitudinally of said liner, said buttress means being affixed to the exterior surface of said liner on either side of the vertical axis thereof at positions where a radial force acting on said liner forms an angle of about 45 or more to the horizontal, each of said buttress means having a longitudinal surface abutting said exterior surface of said liner at said position where said buttress means is affixed to said liner and at least one additional load-spreading surface in Contact with said compacted backlill material.

2. The device according to claim 1 in which said liner has a top portion comprising a tiexible arch formed from relatively thin gauge corrugated sheet material.

3. The device according to claim 1 in which each of said buttress means when viewed in cross section, includes a substantially vertical wall surface comprising said load spreading surface, a substantially horizontal wall surface extending from the upper edge of said vertical wall surface and an arcuately curved surface extending between the distal ends of said wall surfaces thus defining a substantially triangular configuration, said arcuate surface comprising said surface abutting the exterior surface of said liner.

4. The device according to claim 1 in which each of said buttress means is precast and secured to said exterior surface of said liner.

5. The device according to claim 1 in which each of said buttress means is poured in place against the exterior surface of said liner.

6. The device according to claim 1, in which said liner includes an arcuately curved base extending between the lower edges of the said side portions thus forming a substantially oblate spheroid when viewed in cross section, and means to prevent rotational displacement of said liner during the compacting of backiilling material thereabout, said means comprising an antirotation keel secured to and extending outwardly from said base.

7. The structure claimed in claim 1 wherein said liner has an arcuate top portion, reinforcing and stabilizing means extend between said buttress means and overspan said top portion of said liner, said reinforcing and stabilizing means comprise at least one substantially rigid stiffening member being arcuately curved to conform substantially to the arcuate curvature of said top portion of said liner and means to secure said member by the ends thereof, to said buttress means.

8. The device according to claim 4, in which said liner includes an arcuately curved base extending between the lower edges of the said side portions thus forming a substantially oblate spheroid when viewed in cross section, and means to prevent rotational displacement of said liner during the compacting of backlling material thereabout, said means comprising an antirotational keel secured to and extending outwardly from said base.

9. The device according to claim 5, in which said liner includes an arcuately curved base extending between the lower edges of the said side portions thus forming a substantially oblate spheroid when viewed in cross section, and means to prevent rotational displacement of said liner during the compacting of backflling material thereabout, said means comprising an antirotation keel secured to and extending outwardly from said base.

10. The structure claimed in claim 7, including fastening means cooperating between said stiffening member and said top portion of said liner to secure and stabilize said top portion to said stiffening member.

11. The structure claimed in claim 7 in which said reinforcing and stabilizing means comprises a plurality of said stiffening members in parallel spaced relationship.

12. The structure claimed in claim 7 wherein said stiffening member is manufactured from steel section arcuately curved to a curvature similar to the curvature of said top portion of said liner, said buttress means being concrete poured in place around the ends of said stiening member to secure same.

13. The structure claimed in claim 7 wherein said at least one stiffening member and said buttress means are concrete poured in place to form an integral structure.

14. A composite arch structure of the type having compacted backll material thereabout, comprising an elongated ilexible liner having side portions and a top portion, the lines of juncture of said top portion and said side portions of said liner each being defined by a radius change in said liner, said side portions at least of said liner being formed of relatively thin gauge corrugated sheet material, a pair of load-spreading buttress means, said buttress means comprising elongated bodies extending longitudinally of said liner, each of said buttress means being aixed to the exterior Surface of said liner adjacent one of said lines of juncture, each of said buttress means having a longitudinal surface abutting said exterior surface of said liner and at least one additional loadspreading surface in contact with said compacted backll material.

15. The structure claimed in claim 14 wherein said top portion of said liner is arcuate, reinforcing and stabilizing means extend between said buttress means and over-span said top portion f said liner, said reinforcing and stabilizing means comprise at least one substantially rigid stilfening member being arcuately curved to conform substantially to the arcuate curvature of said top portion of said liner and means to secure said stifening member by the ends thereof, to said buttress means.

16. The structure claimed in claim 15 wherein said reinforcing and stabilizing means comprises a plurality of said stifening members in parallel spaced relationship.

17. A method of constructing a composite arch structure having compacted backfill material about a liner, side portions at least of which are formed of relatively thin gauge corrugated sheet material, comprising the steps of setting said liner in place, anchoring the base thereof against movement, backlling and compacting backfill material against the exterior surface of both sides of said liner to positions thereon where a radial force on said liner forms an angle of about 45 or more to the horizontal, securing to said liner elongated load-spreading buttress means on both sides of the vertical axis of said liner and extending longitudinally of said liner, each of said buttress means having a longitudinal surface abutting theexterior surface of said liner at one of said positions and a longitudinal load-spreading surface, backlling and compacting backill material against said load-spreading surfaces of said buttress means and then continuing backilling and compacting backll material to cover said structure.

18. The method according to claim 17, which includes the step of casting in place said buttress means.

19. The method according to claim 17, which includes the step of precasting said buttress means and securing same to said liner.

20. The method according to claim 17 including the steps of placing at least one arcuately curved substantially rigid stilfening member overspanning the top portion of said liner and anchoring said stiffening member by the ends thereof to said buttress means.

21. The method according to claim 17 including the steps of placing a plurality of arcuately curved substantially rigid stiffening members in parallel spaced relationship overspanning the top portion of said liner and anchoring said stiffening members by the ends thereof to said buttress means.

.22. The method according to claim 20 which includes the additional step of securing said stiffening member to said top portion of said liner at spaced intervals along the length of said stiffening member.

23. The method according to claim 21 which includes the additional step of securing said stiffening members to said top portion of said liner at spaced intervals along the length of said stilfening members.

References Cited UNITED STATES PATENTS 8/1938 Claybaugh 61--16 2/1967 Kaiser a 61-45 X JACOB SHAPIRO, Primary Examiner

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