Road Over Peat Swamp

Manawatu Times, Volume 63, Issue 80, 5 April 1938, Page 5

Road Over Peat Swamp

DIFFICULTIES ENCOUNTERED BUIL.DIN G THE EMBANKMENT. Au interesting survey of the raising of the level of the road at Whirokino which leads away from the main highway and straightens out the route from Fusion to the Manawatu river was gi\eu in a paper delivered by Mr J. W. Grindley to the North Wellington branch of the New Zealand Institute of Engineers at Palmerston North on the occasional of a recent annual meeting. When preference had bcea given to the route the problem had been to decide upon grades, the length of the trestle, and that of the embankment, and at the time when the permanent pegging was commenced the proposal was to have a length of trestle of 1450 feet, and then a further length of bank and a bridge of 200 feet at the main drain, said the speaker. However, when test bores had been put down it had been found that up to 10 feet of soft clay and peat would have to be excavated to reach a secure foundation for me tilling. It was then decided to carry the trestle right through and to adopt the present length of 3600 feet. Across Peat Swamp. “The major part of the deviation is being constructed across a peat swamp, 77 said Mr Grindley. “Test bores and pits were put down to ascertain what would be encountered and these showed, in general, an overlying stratum of peat over a layer of blue clay, ranging in thickness from 5 to 15 feet. Beiow this again was fine sand a. 3 far as could be reached with the bores with occasionally a narrow layer of clay, Sand was about 10 feet down on the average. “Pe.U was deepest south of the Whirokino bridge where a depth of ei"ht feet was encountered/ 7 the speaker continued. “The profile of the upper surface of sand sloped away from the river on both sides, and ■ is an interesting point that the ground level across the swamp north of the river is only about one foot above normal water level. Gravel of about three-quarters of an inch diameter was met with at the Main Drain at a depth of 491 feet. When test piles were driven toward the latter end of last winter the driving at the Main Drain and near the river was fairly hard, while the two piles in the swamp section went down comparatively easily. 1902 Flood Level. “The length of main trestle and bridge was decided on after the volume of high flood flow had been estimated, and their respective deck levels were fixed to allow a flood of the 1902 kind to pass under the beams. A short length of trestle at one point was added to provide an exit for flood waters, which would be pocketed between the new road and a ridge which is being used as a stop-bank. The flood grade of the 1902 flood was taken generally as a basis for the formation level on the new* road. The length of level road in Patrick street (in Foxton) is hardly consistent with this grade, but it was impracticable to follow here the desired grade closely on account of difficulty of access to property*. Further, one length of bitumen is also well belvw the required level and the intention is to build this up when the pre sent surfacing needs renewal. “Work was started on the northern end in May, 1936, and by the end of July formation work on the northern deviation was complete. The building up of the existing section of road in Patrick street presented some difficulties, and the water cart had to be kept going continuously as the filling was being done with fine saud from ridges near Foxton. Broken axles on trucks employed on the job were regrettably frequent at times, and passing traffic was not immune. However, care was taken to minimise the inconvenience to traffic as much as possible and few* •: mi plaints were heard. Peat Excavated. “The first stage was the excavation of peat wad soil‘down to a good sand foundation along the whole length of the filling/ 7 Mr Grindley said. “This was done at the side of the existing road along Patrick street, and a considerable quantity of material was removed along the length of the main deviation and across the river. The total of excavated peat and soil mm ounted to 18,940 cubic yards. By November, 1936, this part of the work was done along the whole length north the Main Drain. One of the troubles experienced was loss of sand by wind action. When brought up to formation level fillings had to be covered imme fiiatelv with flax waste until soil could be replaced on the batters. A further u*o for this flax waste was found when 1« *ri<\s were used on the fillings, the v. :id« being employed to prevent their sinking in.

“On the length south of the Whirokino bridge, excavation was commenced with hand labour, using side tip-trucks and horses, but this method became too expensive when the depth of peat began t© increase and an excavator of three-eighths of a yard capacity, acting as a drag-line, was used very successfully and economically in removing the remainder of the peat. The machine shifted spoil at a cost of 1U.37d a cubic yard as against 2s lOd a yard by* hand labour. When the fillings were complete the previously excavated peat and soil w*ere replaced on the batters with the excavator to Lipid the sand and prevent scour by wind and flood. The machine was ideal for this work. Willows have been planted round the toe of the main deviation in order to assist further in preventing scour. Progress to Schedule. “The progress to date on the earthwork section has been pretty well up to schedule, 55 men being employed at the peak period from March to June of this year/ 7 said the speaker. “The filling is completed except for the approaches to the trestles and the bridge, and a further five feet of filling will be necessary just north of the bridge. The new road from the northern end to the point where the deviation leaves the old road has been metalled and 55 chains of it primed. The fencing on the section north of the Main Drain has been practically completed and the four-chain curve leading into Union street has also been formed and metalled and awaits priming. The building of the concrete trestle was not commenced until some three months ago and progress has been slow. Pile making has been going on and several of the piles art driven, some of the driving having been fairly hard. 77