THE POHANGINA BRIDGE.

Rangitikei Advocate and Manawatu Argus, Volume XXXIII, Issue 9070, 10 February 1908, Page 5

THE POHANGINA BRIDGE.

Mr G. F. Robinson, Government Engineer, reporting to the Pobangina County Council on his examination of the Pohangina bridge site, said the excavation for the road approach to the road clearly shows that a bed of hard papa extends parallel with the river and is fairly horizontal in longitudinal section and as far gs I can judge is nearly leval in cross section (at right angles to the line of road). This being so it forms a base for the super imposed earth and the latter cannot slide off it, but may crumble over its edge when the overlying loose earth or gravel 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 and as up to the present it "has dropped but a few feet it proves that its motion is very slow; it may, therefore, effect- the upper road only to a small degree for some years to come. I would recommend before re-eretcing the ■ broken spars, the loose ovei hanging earth and scrub resting above the papa ledge, be sloping it evenly be

tween 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 vegatation: that the bridge span should then be erected, either on a pier of driven piles, if such can be done, or should the papa he too hard to permit of pile driving, the gravel should be excavated until a solid foundation has been 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 lino with the whole bridge, or an angle up stream, is immaterial so far as safety is concerned; as I anticipate no danger from a further large slidge of earth to build it on an angle would lengthen the bridge and thus increase the cost, giving no commensurate benefit. Ko doubt for seme time until the surface is protected by vegetation from tho action of sun and weather portions of the loose mass above the papa will fall upon the lower road, but Ido not anticipate it will do so in such quantity as to endanger tho bridge in any way. I do not think the risk of further damage by slidges of earth from above is sufficient to warrapFyour Council incurring the heavy additional cost of removing the bridge to a now 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 bass. This can be done by wire netting crate work near tho bridge or by a groyne up stream. Mr R. S. Rcunthwaito reported :—lt would bo a serious matter for the Council and for tho ratepayers of thejlistrict to abandon a work which lias cost them so much money andfto begin anew and expend possibly a still larger sum in the construction of a new and of a longer bridge, with the necessary approaches and the acquisition of land on the eastern side for' one of such approaches. Nevertheless, if id thought there was likely to be 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 second suggestion, namely that of placing the new span at an angle and upon a grade, I am of opinion that whilst the cost would be substantially greater no pqssiblo advantage would be gained. The east end of the bridge would be removed only a very short distance from the original site, an additional short span would he required and T am exceedingly doubtful if the necessary piles and abutment 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 the bridge built upon the present site. Under these circumstances I am compelled to adhere to the opinion expressed in my report to you of the 18th ult., 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 llth instant, 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 the whole surface with a deep "rooted grass, or broom, or some other suitable shrub, so as to protect the surface from the direct action of the weather and the sun.

The County Engineer reported.— ; I again visited the;,Pohangina bridge site on 20th January, accompanied hy Messrs Rouuthwaite and Robinson, and members of the Council and took the 'whole of these gentlemen over the locality and pointed out the large cracks on the top road, and also the place where the road Lad dropped three feet. lam of opinion that a large slip is imminent and must take place in 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 to the bottom road would be to face the papa cliff with a cement wall. The base of this cement wall to he 6 feet wide and to be built to the top of the papa ledge, a height of 84 feet. This would prevent the action of the weather on the bluff ’and so protect the present foundation. Cr. Jackson said Mr Rounthwaifce and Mr Robinson had taken upon themselves the responsibility of the re-erection of the bridge on the present site and he moved that in view of the recommendations of Mr Robinson, D R.B. and Mr Ronnthwaite, C.E.,, Wellington, to erect the new span on the Pohangina bridge on the present site, tenders to be called for the work accordingly. Cr. Handley seconded the motion which, after some discussion, was carried.