Suspect Road Edge,Gladstone Road,Levin.
Thursday 20 August 2009
Overdependence on cables, to hold a bank below Gladstone Road's edge, above the Makahika River, Levin topographical area, Horowhenua District, New Zealand.
Observation and photos taken on Saturday 25 July and updated with clearer views of rusty rope on Thursday 20 August 2009.
The structural units, of timber and a railway iron, held by buried galvanised steel wire rope, or cables, back to concrete deadmen in the virgin rock of the road, are at risk of failure. These cables are holding the bank from falling into the river about ten metres below.
The wall shown is held at the top with an obviously badly rusted wire rope into the bank. They pass through secure railway posts on the north end and directly into the bank on the south end. However, there are also the buried cables back to the concrete deadmen blocks in the road from the inner wall. These could be even more rusted than the visible one shown. It seems like bushman engineering, that's more applicable to a tempory logging road lasting a few months, than a permanent, busy road.
All stormwater and springs should be routed away from the bank below the road, via metal shutes, onto non-errodable material, before meeting the river.
The ideal concrete deadman system has 25mm diameter, hot dipped, galvanised rods, wrapped in Denso tape or Petro tape, to inhibit rusting, for connection to the wall, holding the bank above the river. The galvanised nuts and plates are painted with cold galvanising paint as well.
Cables, or wire ropes, have a much shorter life than rods, because each strand is subject to rusting towards their centre. This results in their being reduced to cotton thread thickness at various spots in the cable. The combination of a cable's strands surface area, is obviously much more than a rod's. A galvanised cable's failure from corrosion, is far earlier than a galvanised rod's of the same diameter. The diagram below attempts to explain that the strands of the cable are each subject to rusting and that cable failure can be unexpected because it's not always realised how cables, unlike rods, rust from the inside out. A combination of strands, gradually reduced to cotton thread thickness, at various points, brings sudden cable failure.
The disused pipe bridge at Poads Road, collapsed from rusted cable strands in the 1980's. They didn't all rust in one cross section of the cable, but were staggered, in spots, over a metre or more. The writer collected some rusted strands that were reduced to a hair's breadth in 10mm spots, to prove why the pipe bridge collapsed. He was amazed at the reaction of officials who didn't want to openly admit that it was a gradual process over decades, as the cable samples suggested. The official cause of the collapse was speculated as a choice between a recent car accident at the bridge and/or a recently tar sealed road nearby, causing corrosive run-off that attacked the cable. The truth to the writer is simply that the cable gradually rusted away to failure stage, just as the cables holding the section of the bank, in the photos, below will be.
CABLES MUST ALWAYS BE THOROUGHLY GREASED, AS IS DONE REGULARLY WITH THE WELLINGTON CABLE CAR'S CABLE. HOWEVER, THIS IS NOT POSSIBLE FOR CABLES USED TO SECURE BANKS, AS SHOWN IN THE PHOTOS BELOW. HOT DIPPED GALVANISED RODS, THAT ARE ALSO PAINTED WITH ALUMINIUM PAINT AND WRAPPED IN PETROTAPE, ARE REQUIRED THERE FOR PERMANENCE.
Many roadsides and especially their footpaths, in Wellington's hill suburbs, are held with rods to concrete dead man anchors, set into the virgin rock section of the road. The rods were wrapped in Denso tape against damage and corrosion.
The writer, Richard Tingey, of Levin, was alarmed, on 25 July 2009, at the site, to see that the very rusty top cable was tight and carrying the weight of the road. Buried cables from the inner, older wall system, back to the concrete dead man anchors are the primary source of support for this section of road's bank above the river. The front ends are hidden by the newer wall. They could be badly rusted too. He saw these hidden cables being installed about twenty years ago. They appeared to be galvanised, which is not permanent underground.
The top exposed cable, which the writer also saw being installed, wasn't galvanised. It is indicative of the other hidden ones. It's rusted beyond an obviously safe state. Such a cable reliant system of bank support is a temporary measure. The galvanised steel crash barriers might suggest a permanent solution at the site, but the rusted cable is the weakest link in the system. It will eventually break from gradual rusting. Without the top wire rope, it's possible, in very wet conditions, that the front steel railway posts and planking could fall into the river.
The clock is ticking towards a time of possible road edge collapse at the suspect site, unless the probably rusting cables, to the concrete deadman anchors in the road, are added to with, preferably, 25mm hot dipped galvanised rods, with plates and nuts.
Trenches in the road, back to its inner bank, should then be dug and the 25mm rods, with threaded ends, bolted to new concrete dead man anchors at the back and a horizontal, hot dipped, drilled, 100mm box section steel beam, spanning the railway posts at the front. These rods would be at the ends of the wall, as well as centre sections. With four of these rods fitted, the wall unit would then, in my layman's opinion, be safe.
# ↑ ↑
Suggested locations, outside the steel crash barrier, for newly installed rail line posts as anchoring points, for a temporary, 20mm diameter galvanised steel wire rope. It would pass along the front of the newer wall, under welded cleats on the rail line posts, to prevent the rope riding up and over the tops of these posts. Since the top wire rope is in such a rusty condition and the effect of its sudden parting is unknown, a medium sized excavator's bucket, located on the front wall posts would offer worker safety when welding the cleats and positioning the temporary wire rope. This rope would allow four, 25mm diameter hot dipped galvanised rods, to details elswhere on this page, to be fitted to new concrete dead man anchors.
Black location triangle on steel crash barrier ↓↑
↓ Black location triangle on steel crash barrier ↑
Southern post of newer wall ↓ ↑
↓ Mid post of newer wall ↑ Southern end of top wire rope enters bank
Southern post of newer wall ↓ ↑
Southern post of newer wall ↓ ↑
Mid post of newer wall ↓
Mid post of new wall ↑↓
Mid post of new wall ↑↓
Below Photo O/N in reference to enlarged sketch of (10)
Older and newer walls visible in this photo. The older wall is to the right.
The north end steel post of the newer wall is to the left, in the centre photo of three.
It relates to O/N in large diagram (10). The newer wall timbers' ends are in the last of these three photos.
This rusty ungalvanised steel wire rope threads the older wall to the newer.
The newer wall is mostly held by dead man anchors back in the road that hold the older wall via galvanised wire ropes.
The ends of these wire ropes to the older wall are hidden by the new wall. This means their condition is always in doubt.
Please see detail of sketch (10).
↓D Post of crash barrier
↑B Ends of boards on newer wall ↑ Post C of older wall
Northern end of top wire rope ↑ ↓D Post of crash barrier
Northern end of top wire rope enters bank ↑ ↓
Northern post of newer wall↑ Post C↑ ↑ Post A of older wall
Post A ↑ ↓
Post A ↑ ↓
Post A ↑ ↓
↑ ↓ Northern end of newer wall with wire rope still mainly covered with debris
Older wall's Post A ↑
Ends of boards on newer wall↓ B ↑
Below New Wall in three photos with another of the base of the wall
↑D Post of crash barrier↓
Please see article on wire rope corrosion on page 10 of this PDF
Please note comments on wire rope having a surface area 16 times greater than a rod's of the same diameter.
This means that a wire rope rusts 16 times faster than a steel rod equivalent.
A comparable wire rope needs to be 64mm in diameter, to equal the rust resistance life of 4 mm thick, number eight fencing wire. That's strictly in the context of rust resistance. The 64mm diameter steel wire rope strands could actually be 4mm thick number eight fencing wire and the rope obviously dozens of times stronger than a strand. Both the rope and a single strand, however, would theoretically individually rust away together.
In other words, the originally 12mm thick wire rope below, that's now expanded with rust to 14 mm, would need to be 192mm thick to match the rust resistance life of a 12mm equivalent steel rod.
Is it alarmist to suggest that, the possibly fifteen year old, 12mm thick cable pictured below, is only as strong now as an equivalent rod would be in 240 years time?
Click→ http://www.pacifichousingsystems.com/images/Corrosion_of_Helical_Anchors_in_Soil_Whitepaper.pdf rate of corrosion article
If the estimated rate of rust penetration, on bare steel, in the environment pictured below, is 0.01 mm per year, or one hundredth of a millimetre, then a rod would be reduced in good steel diameter by double that, or by a fiftieth of a millimetre per year. Now given that a steel wire rope rusts sixteen times faster, that's a reduction in good steel diameter of 0.32mm per year in the wire rope. For the 15 years that it's been there, that's 12mm minus 4.8mm = 7.2mm of good steel wire rope left. That's the eqivalent of just over three strands of number eight fencing wire.
Should we feel comfortable about the outer, newer wall being held by that? However, these calculations are on the conservative side since they don't take into account that, as the wire rope's good steel is reduced by rust, the loss speeds up. It's because above calculations don't consider that when rust halves the rope thickness, the area left to rust, is far less than for the halving in original thickness.
The crucial question is, when will this wire rope below fail and the wall go it's own way? More importantly, when will the wire ropes in the road, from the older wall to dead man anchors, likewise fail? Unlike other sections of wall nearby, this wall's wire ropes' ends around railway steel posts, from the dead man anchors, are obscured by the newer wall, built in front.
In theoretical terms, to match the rust resistance life of a 25mm thick steel rod, holding a Wellington road's retaining wall, the equivalent wire rope would need to be 400mm thick. (25mm x 16 = 400mm) That's a four lane suspension bridge's, main support cable size. However, in practical terms, we only need to dwell on the needs of the structure concerned and of thorough rust resistance.
The two sides of the retaining wall issue.
Twelve points for and against it being adequately strong, safe and in sound condition, with added comment.
|For current Condition||Against Current Condition||What the photos Tell and further Comment|
Gladstone Road river valley section is not as inherently structurally safe as Tararua Road and vehicles use it at their own risk, with extra care and attention, as well as the local Council maintaining it adequately.
|The road must be classed as either safe of unsafe, irrespective of the terrain and reduced to one way traffic where necessary, plus given heavy netting fences to hold rock falls from cascading out onto the road.||There is still room on this bend, to close half the width, for one lane of traffic, as a sensible strategy where the retaining wall is is so obviously compromised, with the very rusty top wire rope and unknown condition of the hidden wire ropes, from the front of the earlier wall, to the dead man anchors.|
|2||The double wall has been there for over fifteen years without any complaints.||Like office buildings, elevators, bridges, ships, aircraft and trucks, crucial road support structures need to be inspected, reported and certified as fit for use, with a renewal date.||The obviously badly rusted crucial top wire rope should have been uncovered for inspection ten years ago and reported by the periodically scheduled, road inspection contractor, for immediate replacement, owing to it not being galvanised.|
|3||The steel rail lines are massively strong and are not rusty.||All components need to be trustworthy such as the exposed, crucial, top wire rope and hidden, inner wall's galvanised wire ropes, to dead man anchors, of unknown condition.||The railway line posts and the 300 x 75 wooden planking are certainly sound but it's their wire rope connecton to the bank, which should be with steel rods, that's substandard,|
|4||The galvanised steel wire ropes to the dead man anchors from the older, hidden, inside wall, are most probably sound.||Buried, galvanised steel wire ropes are prone to acid producing anaerobic bacteria attack, such as helped to spot-reduce many strands in both of the collapsed Poads Road pipe bridge cables, in about 1986.||The top wire rope is obviously beyond its replacement date and not worthy of the local Council's strongly promoted slogan of, The best rural lifesyle district in New Zealand.|
|5||The 300 x 75 tanalised pine boards are sound, strong and wired to the rail line posts.||The side posts of the newer wall would not have been enough to hold the planking on their own and the centre posts were too long to rely soley on their ground penetration, so the top wire rope was added and must be well maintained.||Without the top wire rope's attachment, it's uncertain whether the centre rail line posts, that are wired to planking, could stand up under their own weight and not take the wall with them in a moderate earthquake or upon the breaking of this wire rope, when it would inevitably rust to it's resistance limit, if not replaced.|
|6||The Council sensibly installed the second wall soon after notification of the undermined first one, plus gave additional confidence with the excellent galvanised steel crash barrier at the site.||The combined new and old wall unit allows the falacy to thrive, with some people, of a doubly strong, road supporting structure, when each wall has faults that are individually like two thoroughly rusted links in a chain.||The newer wall, with a wire rope only along the top could, one day, be undermined too, with the bottoms of its posts losing their ground hold, to allow all this outer wall's posts and planking to slide into the river together, while the crash barrier wouldn't stop a bus going over the side, if the road subsided.|
|7||Steel wire ropes are made from specialist steel that is much stronger then ordinary, mild reinforcing steel.||Corrosion prevention is absent on the outer wall's solitary wire rope attachment and a serious design fault. This wire rope should have been at least galvanised and not allowed to be covered with debris that damages a galvanised rope's zinc coating||Even stainless steel corrodes in the ground, in time and lubrication of highest quality steel wire rope is still essential, as is seen on Wellington's Cable Car grip rope. The rusty steel wire, top rope is only together because it's not active, like on a crane. As a most basic display of engineering competance, a temporary 20mm diameter galvanised wire rope needs installing as described above in #.|
|8||The deeply embedded side posts of the newer wall, firmly anchor the thick, 75 x 300 planking of this wall.||The ground here is prone to subsidence and slips, such as the slip that undermined the first wall, which makes it essential for each post to be attached to a dead man anchor, but they're not.||Specialist road bank stabilization companies offer a guaranteed solution option to subsidence threats, with earth nails and rock anchors that involve boring an anchor hole rather than digging a trench in the road which, nevertheless, still might be the best option for the site.|
|9||While the wall may not look as achitecturally excellent as the Manawatu Gorge road's concrete supporting structures, it's adequate for the task.||The assurance of engineering excellence is in the certified drawings and job inspection certificates, which probably don't exist for the un-code-numbered retaining walls along Gladstone Road, above the river, unlike the Manawatu Gorge highway walls that do.||While every storm water culvert, that passes under Gladstone Road, has a necessary maintenance location code number, the numerous slip prone retaining walls do not, but should have, to allow immediate public reporting of slips or washouts for emergency crews, as well as regular inspection, upgrade and certification.|
|10||The wall is as sound as it was on the day it was built and more so with tree-root bank stabilization.||The crucial issue is the tensioned, top rusty wire rope that could break, after further rusting, with a sudden release of the outer wall under stress and unknown effects on the inner wall.||The wall is 4060 from the base to the road level, which is the height of an upstairs window of a two storied house, to mean only a slight overhang at the top, putting a high burden on the holding strength of the posts' foundation soil.|
|11||If the road was at risk of falling away, there would be tell tale small cracks in the seal near the edge, but there are none.||The outer wall is held partly by the rusted top wire rope, who's sudden breaking would possibly cause some road cracks, but they may only last half a day before letting in water during rain, which could cause the older wall to fall, if this inner wall's dead man anchor wire ropes had secretly severely weakened from rust.||A subsidence sequence, in wet weather, could start with the snapping of the rusted top wire rope and trigger a chain reaction, over ten minutes, ending in a deep hole, some metres to the inside of the steel crash barrier, as the saturated ground slowly slumps away, without supporting structures.|
|12||This is not an urgent job because it's a theoretical potential problem, on a little used road that's obviously prone to rock falls and slips for motorists to avoid, plus minor road edge subsidence to be promptly cell phone reported by users.||There is no other road edge retaining wall with a secondary one in front of it on Gladstone Road, nor one that's nearly as high, at 4060, whose failure could take away much of the road width, suddenly, without warning cracks in the road edge tar seal.||The Accident Compensation Commission would expect that a high standard of road maintenance is one of the Horowhenua District Council's undisputed prime responsibilities, with drinking water purtiy control and waste water removal and sanitary treatment, irrespective of a road's usage or topography.|
|For current Condition||Against Current Condition||What the photos Tell and further Comment|
|1||A busy lifestyle block road that's the route to Makahika Outdoor Pursuits Centre for bus loads of children on their class trips.|
|2||The writer, Richard Tingey, has biked on the road for over 25 years. He saw the older and newer walls' installation details.|
|3||The writer informed the 1980's Council of the undermined older wall that's now hidden by the four metre high newer one.|
|4||The newer wall is held by a top wire rope to the bank, in front of the older wall. It's a unique, Gladstone Road, double wall.|
|5||The older wall is held by hidden, possibly badly weakened, galvanised wire ropes, to dead man anchors back in the road.|
|6||No wire ropes hold the outer wall's rail line posts, midway down, as on other walls. It's only held by a rusted top wire rope.|
|7||The outer wall's side rail line posts may support most of the wall, if the top wire rope broke but their holding soil may slip.|
|8||There's no way to prove that the rusty, top wire rope is crucial to holding the wall without high risk. Ropes' insides rust first.|
|9||The rusty steel wire rope, clearly shown since 27 July 2009, hints at gross neglegence of Horowhenua District Council staff.|
|10||Emails, with website, to Horowhenua District Council Staff, CEO & Mayor. Handwritten letter to CEO has no reply at 2/9/09.|
|11||The writer's motivation is safe bus loads of children going from the Makahika Outdoor Pursuits Centre. Busses aren't rusty.|
Murray Richard Tingey
May Peace Prevail On Earth