Wednesday, 30 September 2015

Waikokopu Slip, 1957

On August 12, 1957, a huge slip carried away a 250-metre section of the Napier-Gisborne railway line just short of Waikokopu. It took heavy earthmoving machinery four weeks to restore the railway line and the Nuhaka to Opoutama road which runs closely alongside it.

Google Earth map of the area, the slip area is roughly in the centre of this picture. The well known S bend in the railway line at Waikokopu can be seen to the right.

NZRM map of the same area. Waikokopu station site can be seen at right.

Gisborne Photo News spread from 22 August 1957. See the GPN website for larger photos.

More photos appeared in the Gisborne Photo News edition of 19 September 1957.

Sunday, 27 September 2015

Analysis of Maintenance Costs of the Napier-Gisborne Line (Updated 2015)

This report was originally based on Kiwirail's line closure proposition dated 2012. Since that time more information has come to hand. 

Text in italics is quoted from the Kiwirail report from 2012.

6.4. Current and Future Costs associated with the PNGL North of Napier
KiwiRail is currently spending approximately $2.1m annually to maintain the infrastructure to its current condition north of Napier. This is very minimal amount and with the increased frequency of trains over the recent summer season, local staff had observed that increased maintenance would be required in order to hold the existing level of service.

Increasing Drain and Culvert Resilience: Because of the susceptibility and climate conditions in this part of the country, there may be a case to increase the culvert inspections from once every six years to annual inspection. This would add a further $6,000 to the inspection cost per year. The maintenance work found from the extra inspections is estimated to cost between $40,000 and $70,000 per year.

Geotechnical Risks and improve resilience: To decrease line outages due to slips and drop outs, prudent engineering works could be undertaken. A detailed study has not been undertaken so reasonable judgement has been used to generate the estimates used in the table below. The Wairoa to Gisborne section is more vulnerable than the Napier to Wairoa section and this is reflected in the estimates. In this section alone there are at least 13 embankments of similar height to those that have failed recently and may pose a similar risk of embankment collapse due to upstream ponding. Added to this, between Wairoa and Gisborne there are an estimated 30 to 40 cuttings and slopes above track that may pose varying levels of risk to the operation of the line.

River Works: One high risk area that will need a significant amount of work is between 347km and 350km where the rail runs alongside the Kopuawhara River. A combination of river protection, slope stability and drainage work is required along this stretch of the corridor. To remove the risks associated with this area could cost between $0.8m to $1.5m. A nominal amount has been included for the Napier to Wairoa section to acknowledge there are numerous rivers that the railway runs close to.

Bridge 218 PNGL
Ahuriri Estuary Bridge: To maintain bridge 218 PNGL to its existing condition is estimated to cost between $400k and $800k over a 15 year period. This estimate is based on the recent pile repair work completed in 2010. To renew this bridge would cost between $8m to $10m. Other bridges on the line between Wairoa and Gisborne that will require significant work within the next 15 years are bridges 251, 253, 257 and 260 PNGL. These bridges are located within 500m of the coast and the steel plate girders are severely corroded and therefore have reduced section loss in the steel. These spans will need to be renewed with refurbished spare spans and will have a combined cost of approximately $800k and $1.2M. 

Other Incidents: Taking into account past history, we could expect a major infrastructure outage due to weather to occur every 2 to 4 years. Given the geography of the line, it makes it difficult to eliminate damage against such events. However, we can certainly limit the damage by improving at risk areas and by maintaining drains, culverts and slopes after severe weather events.

Regarding track related derailments, renewals expenditure north of Napier is very low. With large amounts of relatively old track infrastructure, it would not be unexpected to have an occasional derailment on this line.

Maps of the Whareratas section: These maps were drawn months ago and show accurately a number of areas of damage to this section of the line. They are not part of the KRL report quoted above. The aerial footage is from Bing Maps and is more up to date than current GE coverage. The report by KRL was done in 2012 and it is possible further damage has occurred since that time.

This is close to the area 347 to 350 km mentioned above where the line runs alongside the Kopuawhara Stream. A couple of small slips are visible in this section on the aerial imagery that was obtained of the line. These are shown below.
image image
The left hand aerial is the one at Kopuawhara Viaduct while the right hand one is between No.15 and 16 Tunnels. The slipping is extremely obvious in both those areas along with the debris that has ended up in the river. Clearly major works will be needed here to address the slipping, as Kiwirail has mentioned above.

This is the next map north of the one above and I have marked in three slips which are all clearly visible, as shown below.
image image image
The one on the left happened about three years ago and was a bluff above the line which collapsed onto the track. It took about a month to fix. The other two are due to the river undercutting the slope. Again this appears to be the same area mentioned. The one on the right shows the Wharerata Walkway station shelter.

This map shows the first of the actual washouts at the top. This one is between No. 22 and 23 Tunnels. The Kiwirail report calls this Site 1 and locates it at 353.7 km.
The size of this washout should not be underestimated. It is expected the volume of fill required is around 54,000 cubic metres. The estimated period of repair is around 5 months and the cost around $2 million.

The second major washout is between Tunnel 23 and the site of the daylighted Tunnel 24. KRL calls this “Site 2” at “Big Hut”, 355.6 km.
The estimate time of repair is around 3 months, requiring 22,000 cubic metres of fill at a cost of around $1 million.
The last map covers the two remaining washouts which are close together.
Firstly you have a slip north of former Tunnel 24, then you have the Beach Loop washout, and then the slip at Wharekakaho. KRL’s report only mentions the last two.

Slip north of Tunnel 24 is obviously developing or is the site of a previous slip that has been cleared along the line. There is obviously a lot of land movement happening here.
image image
This is the current Beach Loop slip. There have been several in recent times but this one has washed out the track at the south end (No.1 Points). It is at “Site 3”, 357.15 km.
This is about a 2 month repair job, will require around 10,000 metres of fill, and is estimated to cost up to half a million dollars.
The last of the four sites is at the Wharekakaho Stream. Historical photos show this embankment was constructed by the classic PWD method of filling in a temporary trestle, as was probably the case all along this section. Kiwirail describes it as Site 4, 358.400 km. Although the track was not washed out at the time, the estimated repair would require a substantial volume of 46,000 cubic metres of fill and would take up to 3 months and cost close to $1 million.

Other Work

Several bridges that would require maintenance are mentioned above. Many of the bridges have wooden piles that will deteriorate and need renewing. Bridge 256 at Nuhaka was found to be susceptible to teredo infestation following its collapse in 2005. Although there are still two sets of wooden piles under this bridge, the piers in the main channel of the river were replaced in concrete when the bridge was rebuilt, lessening the problem. Bridge 297 crossing Gisborne Harbour is susceptible to teredo and has had piles replaced at various times during its life, although the bridge is beyond Gisborne station and has been disused for a number of years.

The major maintenance issue for bridges is Bridge 290, 10 km south of Gisborne, crossing the Waipaoa River. The original 220 metre bridge was twice extended, firstly in 1956-58 and again in 1988 due to flooding problems in the river catchment. The first extension of 110 metres allowed for the development of a new river channel as part of flood control engineering works and was carried out at the same time as the whole bridge was raised by 0.6 - 0.7 metres. The second extension of 145 metres covered a section of track that was washed out by a flood of more than 5000 cumecs (5 million litres per second) due to the Cyclone Bola storm of March 1988. Although the bridge has been safe for use by trains up until 2012 when the line was closed, and is to be brought into use by Gisborne City Vintage Railway later this year when they resume passenger services between Gisborne and Muriwai, since 1988 it has been subject to various operational restrictions. There is an axle loading limit of 14 tons and a low speed restriction, and the bridge is subject to temporary closure whenever certain flood flow or wind speed limits are reached in the riverbed. These are due to the fact that some of the original 1930s and extension 1950s piers are at minimal depth due to flood scour. Although the newest extension has deep founded piers the older foundations are at risk of collapse in future floods and so the bridge remains vulnerable to severe weather events.

Thursday, 24 September 2015

Napier Gisborne Line : Bridge 290, Waipaoa River Bridge [3]

A Gisborne Photo News photo of the Waipaoa Railway Bridge in 1957. The change of course of the riverbed can be seen which was part of the flood control works being carried out at the time.

This overview map shows the general area in which the bridge is located, including the former sawmill siding just to the north. The river has changed course a number of times, including as part of flood control works in the 1950s, and since the Cyclone Bola storm in 1988.

A closer view of the bridge itself, showing the three sections which were built at different times, and the deviation that was built in the mid 1950s to allow for the second section to be constructed.

Wednesday, 23 September 2015

Napier Gisborne Line : Bridge 290, Waipaoa River Bridge [2]

Continuing from the first post in this series here are a series of photos of the bridge extension works in 1958 nearing completion. Published in Gisborne Photo News in August 1958.

The bridge was finally completed two months later with a 56 hour block of line imposed as some of the girders had to be lifted off the temporary deviation and put onto the permanent alignment where the extension joined on at the south end of the bridge. Then the track had to be pushed across at the south end as well.

Gisborne Photo News photos from October 1958.

From the time that this extension was completed for the next 30 years the now 330 metre bridge seems to have had a relatively uneventful life. In March 1988 that all changed. The storm called Cyclone Bola passed up the east coast of New Zealand and caused a flood that peaked at 5360 cumecs (more than 5 million litres per second) in the river. This cut into the riverbank at the south end of the bridge and left more than 100 metres of track suspended in midair. The southern abutment was pushed out of line and the three other piers supporting the 1958 extension spans were all scoured significantly. Along with other damage to the railway, the re opening costs were estimated at around $3.5 million, which is equivalent to about $7 million of today's money. Because the railway at the time was operating uneconomically, NZR stated they wished to close it. However the Labour government had other ideas as Gisborne was a marginal seat and they wished to win it at the next election. Hence, the government provided extra funding to enable the line to be re-opened.

The work carried out in 1988 entailed a further extension of the bridge at the southern end to 475 metres, its present length. The undermining of the four piers supporting the ends of the 1958 extension spans all had to be remedied in some way as part of the works. The former southern abutment pier which had the most serious damage was underpinned with new piling while the scouring of the three others was addressed not by fixing their foundations, but by imposing new load limits on the bridge. Specifically, traffic was given a speed limit over this section and restrictions were also imposed on train running whenever the flood level in the river or wind speeds reached certain limits. The bridge also has a load limit overall of 12 tons in axle loading and in general double headed trains are not allowed to run on the bridge. The reasons the bridge was not fully repaired are due to the economic situation in relation to train operations in the area. Apart from these works another 12 spans were added at the south end to cover the washout of track. The total cost of repairing the bridge was $2.2 million, which is about $4 million in today's money.

These repairs were only a patch up short term solution because all of the pre-1988 bridge foundations have been severely affected by flood-induced scouring in which the pier footings are in a number of places only in the riverbed to a very shallow depth (as little as 1 metre in places) and this could be undermined in a future flood and will need to be addressed in the future. In addition most of these footings are in soft soil, a mixture of sand, silt and clay, rather than gravel or other hard rock material which would normally be the desirable type of foundation material for a large structure such as a bridge. Since the cost of replacing these pier footings would be very high, this represents a significant cost risk for the future operation of the line. The section containing Bridge 290 is still in use being leased to Gisborne City Vintage Railway, but at some future point the accumulated backlog of bridge maintenance will catch up and that section of line will most likely be permanently closed when that happens.

It should be noted that the Waipaoa River is very flood prone and this is exacerbated by the nature of the geography of the East Cape area where a lot of land is of a very soft character, being comprised primarily of weak mudstones and sandstones. The unstable land has been responsible for a great many slips and washouts on the railway in the time since its construction was completed in the early 1940s; and in the case of the river, in heavy rainfall, a lot of back country terrain is washed into the river and becomes part of its flood load, so that the river is liable to rise very quickly in a relatively short space of time. Morever this has also raised the bed level so that the current flood control stopbanks in Gisborne have become less effective over time. Hence the railway bridge is quite vulnerable to large floods in the river and it is only a matter of time before the bridge is too badly damaged to remain open.

References: [1] "Bridge Scour"; Bruce W Melville & Stephen E Coleman; Water Resources Publications (Colorado, USA), 2000. ISBN 1-887201-18-1.

Napier Gisborne Line : Bridge 290, Waipaoa River Bridge [1]

The largest bridge on the Napier-Gisborne Line is Bridge 290, which crosses the Waipaoa River between Matawhero and Muriwai. It encloses the 380 km peg which is about 1/3 of the way across the bridge when heading northwards, and is therefore 380 km from Palmerston North, 200 km from Napier and 10 km from Gisborne. The Waipaoa River bridge was built in the late 1930s and had an original length of 220 metres in 15 through-plate girder spans. The three photos below are from Auckland City Libraries. (Sir George Grey Special Collections).




This photo is also from Auckland Weekly News and was taken in 1942 when the line opened.

This photo is from Gisborne Photo News and was published in 1956 showing a large flood passing beneath the bridge.

A few years after the line opened, in 1948, there was a very large flood in the Gisborne district, which had an estimated peak flow volume in the Waipaoa River of 3960 cumecs (which is about 4 million litres of water per second) and as a consequence, flood protection works were constructed, with the result that the bridge was lengthened by 110 metres at the south end. This entailed rearranging some of the existing spans and adding three through girder spans; the entire bridge was also raised by about 0.6 - 0.7 metres. These works were carried out between 1956 and 1958. The 380 km peg is immediately to the south of these new spans. Part of the construction works required a deviation to be made at the south end of the bridge to keep the line open.

The next two photos showing the work being started on the bridge were published in Gisborne Photo News in November 1956.

The next three photos were published in Gisborne Photo News in May 1957.

It is hard to believe today that the bridge was originally as short as seen here: less than half its present length.

The next photo is from Gisborne Photo News published March 1958.

The second part of this article will have more photos of the 1958 deviation being completed and also describe the 1988 extension of the bridge.