POHANGINA BRIDGE.

Feilding Star, Volume II, Issue 492, 10 February 1908, Page 4

POHANGINA BRIDGE.

DIFFERENCE OF OPINION ON THE SITE.

Although a loan has now been carried for the re-erection of the broken span of the Pohangina Bridge on its old site, the question still arises as to whether it is wise to do so or whether it would he better to shift the whole 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. Government Engineer, 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 HEPOItT. Since the accident to the bridge I have visited the site three times, and on each occasion have carefully considered the position; on the last occasion I inspected the subsidence and the earth about and above the upper road. The excavation of the lower road approach to the bridge t showed that a bed of hard papa exI tends parallel with the river and is fairly horizontal with longitudinal | sections, and, so far as I can judge, it is also nearly level in cross sections (at right angles to the road). This 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 over lying loose gravel or earth is steeper than the angle of repose pi its constituent parts. Apparently the first subsidence of the portion above the upper road occured over 20 years ago, and as 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 road only to a small degree for some years to come: I would recommend that before reerecting the broken span, the loose or overhanging earth and scrub,, resting above the papa ledge, be removed by sloping it evenly betiveen the edge of the papa cutting and the upper road, thus minimising the clanger of further slides, and that the surface of the slope be sown with a deep footing 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 thepapa 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 as before (in a , straight line with the whole bridge) ; or on an angle up stream is iinma- ! terial so far as safety is concerned, ' as I anticipate no danger from a further large slide of earth. To build at an angle would lengthen the bridge and thus increase the cost, giving no commensurate benefit. No doubt for some time— until the surface is protected by vegetation from the action of the s\m 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. I do not think the risk of further damage by slides, of earth from 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 be taken to deflect the river from eroding the papa face near the bridge and so cutting' away its base. This can be done by wire netting crab work near the bridge or by a groyne up stream. In accordance with a resolution passed at the Council's last meeting, Mr Robinson and Mr Harland and myself visited the broken span of the Pohangina bridge. Mr Harland pointed out certain small fissures on the high grade, and showed where this road iiad 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 (or four or five spans), and approaches on or about the line of the ford. (2) The re-ereo-tion of tho eastmost span at an obtuse angle with the existing structure, and on a rising grade, so as to work in with the level of the approach road. (3) The re-construction 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 approaches. Nevertheless, if I thought -there was likely to be, in the future^ any real danger to the structure on the original site or danger to life or property, I would not hesitate to recommend the . adoption of such a course, expensive j thought it might be, With regard" to th& sdcbtfd' s"uffges«

tton. tlio cost Mould bo greater, and no possible advantage gained. The cast cud oi the bridge would be removed only a very short distanco from the original site. An additional short span would be required, and I am exceedingly doubtful if tlio necessary pier and the abutmonfc could be constructed in the papa by means of piles at this point in a satisfactory manner. Furthermore, lam 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 I you on the 13th ultimo., that is, to re-censtruct the new span on the original site with a pile foundation., as shown upon the drawings submitted at yoxir meeting on the 11th inst., but 1 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-rooted grass, or with broom or some other suitrfble shrub, so as to protect the surface from the direct action of weather and sun. MR HARLAND'S REPORT. I beg to report that I again visited the site on the 20th January, accompanied by Messrs Robinson and Bonn- , thwaite and members of the Council, and took them over the whole of the locality, and pointed out tho largo cracks on the top road, and also the places where the road had" dropped 3ft. lam of the opinion that a large slip is imminent and must take j place within a short period, a"nd in : my opinion would result in damaging tho bridge if re-erected on tho present-: j site. The only way, in my opinion, j to prevent tho whole of the top slip ! from coming down on the bottom road j would be to face the papa bluff with '• a cement wall. The base of this j cement wall to be 6ft, and to be built j to the top of the papa ledge, which : is about 34ft in heiglit. This wall would prevent the action of the weather on the bluff, and so protect the present foundation. It was decided that, in view of the recommendations of Mr Robinson, Government Engineer and Mr Rounthwaite. 0.E.. to re-erect the new span of the Pohangina bridge on the present site, tenders to be called accordingly. — Crs. Jackson — Handley. That the plans for the re-construc-tion 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 13, and that Mr Rounthwaite be asked to attend the Council's meeting on the 14th prox. — Crs. Jackson— Howio.