Roadside Work

Sunday 5 December 2010

Ohau River Bridge Maintenance, On State Highway One, South Of Levin, New Zealand.

♦The Ohau River, at the curved highway bridge, needs to be maintained exclusively in a channel at the present northern bank. This is well away from the southern bank.

♦The Ohau River bridge's southern bank is in need of urgent remedial action.  It's of boulders in clay, which gradually erodes. It needs protection to, in turn, protect the supporting wall, up the bank, by stepped rock baskets or gabions.

 

♦The southern channel then needs filling with large rocks between the river bank and the adjacent well anchored supporting pile wall in the river bed. That is the second pile wall from the south. The first pile wall from the south is up on the river bank. 

 

♦Another wall of stepped gabions needs to be installed, on an angle, to stop the river going

through the southern channel.

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♦Photos

(1) The Location

(2) The Problem

(3) The Remedy

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(1) The Location

 

 Map on Google Earth (Imagery date 1/2/2005)  Please Click:-  

http://maps.google.com/maps/mm?t=h&hl=en&ie=UTF8&ll=-40.671134,175.241117&spn=0.005102,0.008647&z=17

 Street view is available. Click and drag the yellow human figure above the zoom in / out bar.




Above Please note how the river veers to the left under the road bridge, while further downstream it veers back to the right. This indicates that the flow could be confined to below the truck and trailer and the present south channel totally blocked off.



Photo in a 2009 earthquake engineering study:- 
click >  http://db.nzsee.org.nz/2009/Pres45.pdf

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 (2) The Problem

Undermining risk to south wall, up the bank, in a flood.











Vulnerability List

(1) Flood

(2) Earthquake

(3) Flow allowed to concentrate in southern channels.

Main Point
Good maintenance is not leaving the stability of the boulder and clay bank to good luck. It's essentially about foreseeing the worst case scenario.

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(3) The Remedy

Bulldose normal river channel to below the truck   Regularly maintain this exclusive flow.



                    Better river channel to bulldose  

  Remove tree debris to limit scouring



Protect  bankwith stepped rock basketsor gabions.

 Fill this channel with large boulders Protect with another stepped wall of gabions, angled to deflect flow

 Gabion photo   Click   http://image.made-in-china.com/2f0j00OvNtwmcJeMkY/Gabion-Engineering-SW006-.jpg

 

Two photos below:- newer concrete under the closer pile. It was shown to be needed in the pdf file photo, in the text link below. 

Please note newer concrete, lower on the supporting wall. Thin under-piles were exposed from gravel extraction. The south supporting wall, up the bank, would have such under-piles too, one would hope. The casual observer might ask how deep these under-piles go? Also, what's the solidarity, or ability of the bank material to hold together, in flood conditions? It's not solid rock but a clay, sand and boulder mix.

It has a variable degree of abliity to hold together. Concrete-like thin iron pan layers, can be in the Horowhenua District's silt, and boulder mixes. It's an uncertain situation where some layers errode quicker. Logs in a flood can cause a bank to errode faster. They may dislodge embedded boulders.

Please see the photo in Fiqure 10:3 of the following pdf file. It shows five of the, probably, eight piles under each supporting wall or pile cap. The piles would be very long. However, while letting the central piles get exposed was unavoidable, as metal was extacted from the river, the southern wall's piles have no need to be exposed. It's necessary for earthquake protection that the piles are not exposed from a washed away bank.  

The bridge's structural integrity should be respected and maintained to at least the designers' intentions. They couldn't anticipate gravel extraction here though. Gravel extraction doesn't directly affect the southern pile wall.  The soil around this wall, now needs to be protected with gabions and rock fill, for proper maintenance after this gravel extraction has lowered the river bed by so much. It simply isn't reasonable to leave the future of this crucial highway bridge to chance, whatsoever. There's no alternative route. 

pdf file, pile photo (Figure 10:3)  Click  http://www.nzta.govt.nz/resources/bridge-inspection-maintenance-manual/docs/10-waterway.pdf

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Possible bad situation in diagrams below.

Fast erosion is likely, if the thin strip of hard bank erodes away, below the south pile wall.

The remaining thin strip of hard bank could be washed right away, after binding tree roots fall out in a flood. The bank has boulders which are otherwise tightly held in the clay. Tree roots break up the bank but hold the boulders, unless rammed with logs in a flood.  It's elswhere a hard surface that erodes slowly. However, the back-fill around the pile wall, up on the bank, would be much softer and would erode easily, should the present hard bank strip be completely eroded away.

Maybe 2 metres of softer, back-filled ground would surround the pile wall. Steel concrete formwork for the pile wall,  needed that much excavated room for men to work around it. The south abutment of the bridge is closer to the south pile wall than is the second pile wall to the south pile wall. All of the softer back-fill, up to the abutment could be washed away in a single flooded river event, if the thin strip of hard bank is first lost.  The crucial abument stability would then be seriously threatened and urgent work would be required.

Blocking off the south river channel entrance with gabion rock baskets, in front of large quarry rock fill, is vital. The sooner the better. It would be much cheaper than waiting for the pile wall's softer back fill to be quickly eroded. The eight exposed pile posts would then need a concrete base added around them, as have the other pile walls. Public confidence in the bridge could also be lost.  However, bridges  should never have their abutments exposed to river flow, otherwise the road deck is jeopodised. Letting the southern pile wall's present hard bank erode away, is not an option. 

This is a case of logical, preventative maintenance, versus a hazardous, wait and see attitude. The 22 February 2011 Christchurch earthquake was a turning point for all structural engineering maintenance. Colapse of supposedly safe, modern, certified as earthquake resistant buildings, has changed the engineering scene in New Zealand, for ever. This bridge must now be properly maintained, to best standards, without delay.  

 

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SUNDAY 21 NOVEMBER 2010

 

URGENT REMEDIAL ACTION NEEDED



PLEASE HELP PREVENT MORE OHAU RIVER, SOUTH RIVER BANK LOSS,

FROM BENEATH THE STATE HIGHWAY NUMBER ONE CURVED BRIDGE.

 

THE SOUTH BRIDGE SUPPORT WALL, HIGHER UP THE BANK, IS NOW

DANGEROUSLY CLOSE TO THE RIVER EDGE AND DEEP POOL.


A NEW CHANNEL MUST BE CUT MUCH FURTHER NORTH, THE PRESENT

SOUTH BANK PROTECTED WITH THICK WIRE ROCK BASKETS AND RIVER

BED MATERIAL PILED HIGH IN THE PRESENT DEEP POOL.

 

THE PICNIC SPOT THERE WILL SIMPLY HAVE TO BE REDUCED IN SIZE.

 

BRIDGE SAFETY MUST COME FIRST

 

PLUS

 


IMMEDIATE REMOVAL OF MASSES OF TREE DEBRIS IS FIRST

REQUIRED FROM TWO BRIDGE SUPPORTS THERE.

 


 
RIVER BED SUPPORTS ARE A BRIDGE'S

 

HEART

 

 

 

 


THEY MUST BE RESPECTED AND PROTECTED.

 

 

THE THREATENED SOUTH BRIDGE SUPPORT WALL, NEAR THE BANK BESIDE A DEEP POOL, AND THE ADDED STRAIN IN FLOOD, CAUSED BY PILED TREE DEBRIS, IS NOT GOOD BRIDGE CARE.

 

 

1. THE TREE DEBRIS MUST INITIALLY BE URGENTLY TRUCKED AWAY.

 

 2. A TEMPORARY NEW RIVER CHANNEL NEEDS TO BE THEN CUT BESIDE 

         THE PRESENT NORTH  BANK BESIDE THE TREES .

 

3. THICK WIRE ROCK BASKETS NEED TO BE IMMEDIATELY  INSTALLED,

        STEPPED HIGH AT THE PRESENT SOUTH BANK.

 

4. A NEW PERMANENT CHANNEL, MUCH FURTHER NORTH, UNDER THE

         BRIDGE, MUST BE THEN  CUT, WITH THE DUG OUT STONES

        TEMPORALILY PILED TO THE SOUTH OF IT, TO CONTAIN THE FLOW.

        THIS WILL REDUCE THE SIZE OF THE PICNIC AREA, BUT IS NECESSARY

        FOR THE SAKE OF THE BRIDGE.

5. THE DEEP POOL NEEDS TO BE  THEN FILLED HIGH AGAINST THE ROCK

         BASKETS WITH RIVER ROCK.


 6. THE ROCK BASKETS NEED TO BE MAINTAINED WITH PLENTY OF RIVER

         BED MATERIAL PILED AGAINST THEM AND THE CHANNEL KEPT TO ITS

         NEW COURSE. SOME NORTH SIDE ROCK BASKETS WILL POSSIBLY

         BE NEEDED.

 


 RICHARD TINGEY 

 

MURRAY RICHARD TINGEY

56 KINGS DRIVE
LEVIN


PHONE (06) 368 8587


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