POHANGINA BRIDGE SITE.

Manawatu Standard, Volume XLI, Issue 8508, 10 February 1908, Page 2

POHANGINA BRIDGE SITE.

A'i DIFFERENCE OF -OPINION.

Although a loan has now been car-f ried for the re-erection of the broken span of the Pohangiria; Bridge on its old site the question still '"arises as to •whether it is wise to do so or whether it would not be better to shift thewhole structure further up the river where there would be no possibility of its being damaged by slips from the adjoining cliff. At the meeting of the Pohangina County Council on Saturday the reports of three engineers. on the question were read, and while two, Mr Robinson, Governmentengineer, and Mr Routhwaite, C.E., are in favour of the old site, Mr Harland, the County engineer, is strongly against it, and maintains that .sooner or later a large slip must come down again. Appended are the reports:—

MR. ROBINSON'S REPORT. Since the accident to the bridge I have visited the site three times and oh each occasion have carefully considered the position; on the last occasion I inspected the subsidence of the earth about and above the upper road. The excavation of the lower road approach to the bridge clearly shows that a bed of hard papa extends parallel with the river and is fairly horizontal in longitudinal sections and so far as I can judge it is also nearly level in cross sections (at right angles to the road). Thil being so it forms a base for the superimposed earth, and the latter cannot slide off it but may crumble over its edge when the overlying loose gravel or earth is steeper than the angle of repose of its constituent parts. Apparently the first subsidence of the portion above the upper road occurred.over 20 years ago andas up to the present it has dropped but a few-feet it proves that its motion is very slow; it may therefore affect the upper read only to a small degree for some years to- come. I would recommend that before re-erecting the broken span the loose or overhanging earth and scrub resting above the papa ledge be removed by sloping it evenly between the edge of the papa cutting and the upper road, thus minimising the danger of further slides, and that the surface of the slope be sown with a deep rooting grass or with lupin or broom or some such vegetation. That the bridge span should then be erected, either on a pier of driven piles, if such can be done, or should the papa be too hard to permit of pile driving then the ground should be excavated until a solid foundation is obtained and a timber (or concrete) pier be constructed on a concrete base. Whether the bridge span should be constructed I as before ( in a straight line with the whole bridge) or on an angle up stream i is immaterial so far as safety is conj cerned as I anticipate no danger from a further large slide of earth. To build at aii angle would lengthen the bridge and thus increase the cost, givI ing no commensurate benefit. No doubt for some time—until the surface j is protected, by vegetation from the action of the sun and weather—portions of the loose mass above the papa will fall upon the lower road, but I do not anticipate it will\do so in such quantity as to endanger the bridge in any way. Ido not think the risk of further damage by slides of earth from I above is sufficient to warrant your ' Council incurring the heavy additional, [ cost of removing the bridge to a new site. While the new span is being erected I would recommend that action Ibe taken to deflect the river f fom erod- | ing the papa face - near the bridge and so cutting away its base. This can be | done by wire netting crate work near I the bridge or by a groyne vm stream. [ MR. ROUNTHWAITE'S REPORT.

In accordance with a resolution passed at the Council's last meeting Mr Robinson, Mr Harland and myself visited the broken span of the Pohangina Bridge. Mr Harland pointed out certain small fissures on the high road and showed where this road had considerably subsided since the 13th December. He also pointed out a much larger fissure at the top of the old slip and near to the fence bounding the flat land above. Three suggestions were made at the time as follows:— (1) The abandonment of the present structure and approaches entirely and the construction of a new bridge (of four or five spans) and approaches on or about the line of the ford. (2) The re-erection of the eas.tmbst span at an obtuse angle with the.existing stcuc* ture .and on a rising grade so as to work in with the level of the approach road. (3} The re-constructibn of the eastmost 'span on precisely the same site as that upon which it originally existed. As to the first of these suggestions it would be a serious matter for the Council and ratepayers of the district to abandon a work which has cost them so much money and to begin de novo and expend possibly a still larger sum in the construction of a new and a longer bridge with the necessary Nevertheless if I thought there was likely to bo in the future any real danger to the structure re-erected on the original site, or danger to life or property, I would not hesitate to recommend the adoption of such a course expensive though it might be. With regard to the sec- i ond suggestion the cost would be | greater and no possible advantage I gained. The east end of the bridge j i would be removed only a very short distance from the original site. An additional short span woufd be required ahd' H l> am; exceedingly doubtful if the necessary and the abutment could be constructed in the papa by means of piles at this point in a satisfactory manner. Furthermore I am inclined to the opinion that if such a thing as a heavy slip from above the high road were possible it would be more likely to land at this point than at the end of a bridge built upon the original site. Under these circumstances I am compelled to adhere to the opinion expressed in my report to you of the 13th ultimo, that is to reconstruct the new span on the original site with a pile foundation as shown Upon the drawings submitted at your meeting on the 11th inst., but I would again emphasise the necessity for dressing down the bank on top of the papa formation, removing some of the irregularities and sowing down the whole of the surface with a deep-root-ed grass, or with broom or some other suitable shrub so as to protect the surface from the direct action of weather and sun. . x MR. HARLAND'S REPORT. I beg to report that I again visited the site on the 20th January accompanied by Messrs Robinson and Rounthwaite' and members of the Council and took them over the whole of the locality and poii\ted tout the large cracks on the top road and also the place where the road had dropped 3 feet. I am of the opinion that a large slip is imminent and must take place within a short period, and in my opinion would result in damaging the bridge if re-erected on the present site. The only way in my opinion to prevent the whole of the top slip from coming down on the bottom road would ha> to face the papa bluff with a cement wall. The base of this cement wall to be 6ffc and to be built to the top of the papa ledge, which is about 34ft in height. This wall would prevent the action of the weather on the bluff and bo pro-

tect the present foundation. Itf%as decided that in view of,the. reconfmendations of Mr Robinson, verninent^Engineer, and Mr Rounthwaite,' CiE: 1 ; tor re-:erect v of 'the Pofiangiha Bridge dn-Jther'pre-sent to be; called accordingly.—Crs. Jackson—Handley. | T,hat the plans for the reconstruction of the damaged span of the Rau-mai-Pohangina Bridge be approved.— Crs. Handley—Jackson. That Mr Rounthwaite call.for tenders for the erection of the new span of the Pohangina Bridge, the same to close at the County Office 'on March 13th and that Mr be asked to attend the Council's meeting on the 14th proximo.—Crs. Jackson— Howie. !