Mohaka Viaduct facts for kids

Quick facts for kids Mohaka viaduct
The Mohaka viaduct crossing the Mohaka River in Northern Hawke's Bay, New Zealand
Coordinates	39°04′07″S 177°07′33″E / 39.0686°S 177.1257°E / -39.0686; 177.1257
Carries	Single track of the Palmerston North–Gisborne Line (mothballed 2012)
Crosses	Mohaka River
Locale	Raupunga, Northern Hawke’s Bay, North Island, New Zealand
Owner	ONTRACK (New Zealand Railways Corporation)
Characteristics
Design	Plate girder
Material	Mild steel
Total length	276.8 metres (908 ft)
Height	95 metres (312 ft)
Number of spans	12
History
Designer	John L. Cull and W. L. Newnham
Engineering design by	Components prefabricated by PWD workshops, Mt Maunganui, Bay of Plenty
Constructed by	The Public Works Department (PWD) for the New Zealand Railways Department (NZR).
Construction begin	1930
Construction end	June 1937
Opened	1 July 1937

The Mohaka Viaduct is a very tall railway bridge in New Zealand. It crosses the Mohaka River in the northern part of Hawke’s Bay. This amazing bridge was built between 1930 and 1937. It was a big project by the Public Works Department for the New Zealand Railways Department.

The viaduct is about 276.8 metres (908 ft) long. It stands at an incredible 95 metres (312 ft) high. This makes it the tallest railway bridge in Australasia.

Building a Railway Line

Building the railway line that uses the Mohaka Viaduct was a huge job.

Connecting Cities

• The railway line from Palmerston North to Gisborne started being built in 1872.
• The first part, connecting Palmerston North to Napier, was finished in 1891.
• The northern part, from Napier to Wairoa and then to Gisborne, took much longer. It was built between 1912 and 1942.

Tough Terrain

The section of the railway line between Napier and Wairoa was very difficult to build.

• Workers had to move a lot of earth.
• They also built five tunnels.
• Five tall steel bridges, called viaducts, were needed to cross deep valleys.
• Many other smaller bridges were also constructed.

Because of these challenges, building the line was slow. Parts of the line were opened as soon as they were ready.

Starting the Mohaka Viaduct

• By late 1930, the railway line reached the viaduct site from Wairoa.
• The steel parts for the bridge were made somewhere else. This is called pre-fabrication.
• These parts were then brought by train from a small port called Waikokopu.
• Workers started by digging deep foundations in the river bed. These foundations were about 18.3 metres (60 ft) to 21.3 metres (70 ft) deep.
• They used special sealed structures called caissons to work under pressure in the water.

Challenges and Delays

Building the viaduct faced many problems.

Financial Troubles

• In 1929, there was a big stock market crash in the US. This led to a very difficult time called the Great Depression.
• The New Zealand Government faced major money problems.
• To save money, work on most of the railway line was stopped.
• However, the foundation work at the Mohaka Viaduct continued for a while.

Earthquake Damage

• On February 3, 1931, a huge earthquake hit Hawke’s Bay.
• This earthquake caused a lot of damage to the railway line and the construction sites.
• Because of the earthquake and the ongoing money problems, all work on the line stopped.
• Luckily, the Mohaka Viaduct's foundations were finished before everything stopped.

Work Resumes

• The construction sites remained empty until 1936.
• A new government came into power, and the country started to recover from the Depression.
• Work on the viaduct began again in June 1936.
• The completed Mohaka Viaduct was officially opened on July 1, 1937. The Minister of Public Works, Bob Semple, opened it.
• In April 1938, big floods washed away part of a nearby road bridge. The viaduct was used temporarily for road traffic before trains started using it regularly.

How the Viaduct Was Built

The Mohaka Viaduct is a great example of clever engineering.

Design and Materials

• The viaduct was designed by two engineers, John Lelliot Cull and William Langston Newnham.
• It is made from a type of strong metal called mild steel.
• The steel parts were made at the Public Works Department Workshops in Mount Maunganui.
• They were then shipped to Waikokopu and taken by train to the building site.

Putting it Together

• Workers used a special cable-way system to move the heavy steel parts across the gorge.
• It took seven months to put all the steel together.
• The viaduct uses about 1,824.7 tonnes (1,795.9 long tons; 2,011.4 short tons) of steel.
• Around 450,000 rivets hold all the steel pieces together. Rivets are like strong metal pins that join parts.

Size and Structure

• When it was finished, the Mohaka Viaduct was the fourth highest in the world.
• It is still the highest railway bridge in Australasia.
• The bridge has twelve main sections, called plate girder spans.
• These spans are supported by six tall trestle piers. Piers are the strong columns that hold up the bridge.

The Viaduct Today

The Mohaka Viaduct is an important part of New Zealand's history.

Recognized History

• This bridge is recognized as a significant part of New Zealand’s engineering history.
• A special plaque was put up to show how important this railway viaduct was for the country's development.
• The New Zealand Historic Places Trust has listed the viaduct as a Category 1 historic place. This means it is very important historically.

Current Use

• In 2012, a big storm damaged the railway line between Wairoa and Gisborne.
• Because of this, the northern part of the line, including the Mohaka Viaduct, was mostly closed down. This is called being "mothballed."
• However, in October 2016, it was announced that logging trains would start using the line again.
• These trains began running on weekends in October 2017, carrying logs from Wairoa to the Port of Napier.