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The New Zealand Railways Magazine, Volume 6, Issue 1 (May 1, 1931)

Recent Earthquake does Extensive Damage to Structure

Recent Earthquake does Extensive Damage to Structure.

An indication of the magnitude of the earth movement associated with the earthquake in Hawke's Bay on 3rd February is afforded by its effect on the Westshore Bridge (at Napier), and approaches. The bridge, which has a total length of 1,232 feet, is of reinforced concrete construction on reinforced concrete piles, and consists of twenty-one spans of 50 ft. each, five of 25 ft. each, and one of 57 ft. (the latter designed to allow the passage of boats and launches).

The old road bridge which crossed the inner harbour from Port Ahuriri to the south end of the shingle spit at Westshore, was in need of renewal in 1914. At the same time the Napier-Eskdale railway was about to be constructed, while the Napier Harbour Board was also contemplating the construction of an extensive inner harbour in the Ahuriri Lagoon. An agreement was, therefore, entered into between the Public Works Department, the Napier Harbour Board and the Hawkes Bay County Council (an agreement subsequently confirmed by legislation), by which a combined road and railway bridge was to be constructed in a position that would not interfere with the proposed inner harbour.

The bridge was constructed by the Public Works Department at a cost of £44,708 18s. the County Council contributing approximately half the cost. (The embankment leading to the bridge was constructed by the Harbour Board, the Public Works Department contributing £20,000 out of a total cost of about £47,000).

Provision was made for the Railway Department to maintain the bridge and embankment and collect, from the County Council, the cost of maintaining the portion carrying the road, the County Council being assigned the work of maintaining the road surface.

To allow for expansion and contraction, the bridge was constructed in units, each consisting of two 50 ft. spans, one 25 ft. span (in centre) and two 50 ft. spans, making an expansion joint at every 225 feet. The 25 ft. spans were strongly braced against longitudinal forces, and also provided with raker piles acting in the same direction. The waterway was designed to allow for ample provision for the discharge of the Tutaekuri River and also for the movement of sufficient tidal water to and from the lagoon, this provision being for the purpose of preventing the shoaling of the entrance to the inner harbour.

The earthquake of 3rd February, 1931, was accompanied by an earth movement from the north of a magnitude sufficient to push the south end span entirely off the concrete pier at the expansion joint, allowing the span to fall into the estuary. The pier (with the expansion joint placed centrally) is three feet wide. The photograph shews the second span well over the edge of the pier. All piles in the piers to the northward were cracked on the north side, but the reinforcing remained in place. The southern-most pier was broken by the falling of the span.

The whole structure was raised 5 ft. 10 in. by an upward movement of the bed, and this movement extended for many miles in both directions. The maximum uplift noted on the railway was 6 ft. at Bay View (about 5 miles north of the bridge), steadily decreasing to 3 ft. 9 in. at Eskdale (9 miles northwards). The uplift was less where swamps were crossed, due to the semi-fluid earth flowing page 24 towards the lower levels. For some distance south of the bridge the uplift was about 5 ft. 6 in. The long embankment was spread and flattened almost throughout its entire length, and the road on both sides of the bridge was badly broken up. A number of motor vehicles which were on the embankment when the earthquake occurred became embedded in the fissures, the fine bitumen surface being shattered.

The bridge was thrown only slightly out of line, but the embankment at the south end was forced to the westward so that the roadway on the embankment lined up approximately with the railway track on the bridge.

The uplift considerably reduced the portion of the lagoon covered by tidal waters, and in consequence yachts and launches which formerly sailed the inner lagoon were left high and dry. The railway line between Napier and Eskdale will require to be regraded, and in many places relocated to suit the altered conditions.

The Second Span of the Westshore Bridge. (Railway Publicity Photo.) View of the bridge after the disastrous earthquake on 3rd February, 1931.

The Second Span of the Westshore Bridge.
(Railway Publicity Photo.)
View of the bridge after the disastrous earthquake on 3rd February, 1931.