Ishioka No. 2 Power Station Penstock Retaining Wall

The wall that holds the pipe in place

The Ishioka No. 2 Power Station Penstock Retaining Wall is a granite retaining wall totalling 49 meters in the Ishioka district of Nakagocho, Kitaibaraki City, Ibaraki Prefecture, built in 1913 (Taisho 2) and listed as a Registered Tangible Cultural Property on 3 August 2006 (Heisei 18).

It follows the single steel penstock that drops from the head tank to the generating hall. Two kinds of work are covered by the one registration: the masonry wall at the foot of the head tank where the pipe is anchored, and further lengths built parallel to the pipe wherever the ground needed holding, including the sections next to the spillway channel and the road.

A penstock is not a passive object. Water falling through it presses outward at every bend and pushes hard against the anchors, and the pipe expands and contracts with temperature all day. If the ground beneath shifts, the joints work loose. The Ishioka No. 2 Power Station Penstock Retaining Wall exists so that the slope under the pipe stays where the engineers put it.

Granite, set point down

The stone is granite, laid in tanizumi coursing: squared blocks turned on the diagonal so each sits point down against its neighbours, with the joints running at an angle across the face. The method distributes a load sideways through the wall rather than straight down a vertical joint, and it forgives the slight settlement that any cut slope produces in its first decades.

Granite was chosen for hardness rather than appearance. Close up the faces are rough, the blocks vary in size, and the coursing adjusts to the ground as it climbs. That variation is the mark of a wall built by masons reading the site rather than following a repeated detail, and it is the clearest evidence at the Ishioka No. 2 Power Station Penstock Retaining Wall of how the work was actually done.

An unusual thing to register

The Ishioka No. 2 Power Station Penstock Retaining Wall is registration number 08-0217 from the 51st round, listed under the criterion of contributing to the historic landscape of the country. Retaining walls are ancillary works, and they are seldom recorded as heritage on their own account. Registering this one with the five other structures of the plant reflects a view of the site as a complete system in which the supporting works count as much as the buildings.

There is a practical side to that decision. The pipe itself has been replaced and repaired over the years, as pipes are. The masonry beneath it has not needed to be. Under the Registered Tangible Cultural Property system the wall keeps working while it is protected, which is the only sensible arrangement for a structure whose failure would take the penstock with it.

Seeing the Ishioka No. 2 Power Station Penstock Retaining Wall

The Ishioka No. 2 Power Station Penstock Retaining Wall stands on land owned by Tokyo Dengen Co., Ltd. within an operating, unstaffed plant. Entry is not possible and there is no marked viewpoint. From the public road below, the line of the penstock climbing the slope is the thing that catches the eye first; the masonry underneath is easiest to pick out where the pipe passes close to the road.

Public transport does not reach the site. Minaminakago Station on the JR Joban Line is around 5 kilometers away, about 15 minutes by car, and taxis or a rental car are the way in. Photographs from the roadside are unrestricted. Scrambling up the slope to follow the wall is both trespassing and a genuine risk, since the ground beside a penstock is steep and loose.

Each autumn Kitaibaraki City opens selected cultural properties, and the upstream Ishioka No. 1 Power Station has been among them by advance application in past years. The Ishioka No. 2 Power Station Penstock Retaining Wall has not been included, so anyone planning a visit around the announcement should confirm the current details with the city's Lifelong Learning Division.