Senbon Dam Reinforcement Work


Senbon Dam, located in the eastern part of Shimane Prefecture, is a gravity concrete dam completed in 1918 and dedicated to water supply.
One hundred years after its construction, it still supplies a quarter of Matsue City's water supply.
The surface of Senbon Dam is masonry. And it was built of rubble concrete with concrete filling between large stones.
It is a structure of high historical value, having been designated as a nationally registered tangible cultural property and a civil engineering heritage recommended by the Japan Society of Civil Engineers.

Senbon Dam Outline

Purpose: Water supply
Type: Gravity concrete dam
Height of embankment: 15.8m
Length of embankment crest: 109.1m
Volume of embankment: 7,000m3
Total storage capacity: 387,000m3
Location: Aza-Hosoda, Nishi-Inbe-cho, Matsue City, Shimane Prefecture
Client: Matsue City
Completion: 1918
Reinforcement: 2020


PS anchor for the dam body of Senbon Dam

A dam integrity survey confirmed that there was a problem with the stability of the dam body during earthquakes, so seismic reinforcement work was undertaken.
In order to maintain the functionality of the dam and preserve its appearance as a cultural heritage, Obayashi Corporation reinforced the dam body with a seismic reinforcement method using PS anchors for the first time in Japan.


In the PS anchor method, anchors made of bundles of PC steel wires are inserted from the top edge of the dam body and anchored to the foundation bedrock to prevent the dam body from overturning in the event of a large-scale earthquake.
Thirty-eight PS anchors, the longest of which is approximately 30 meters long, were installed.

Specifications of the PS Anchor for dam body
・Anchor length: 30.5m (longest)
・Design tension force: 1702 kN (maximum)
・Anchor installation interval: 2.5m
・Number of anchors: 38


Construction status of the PS Anchor for dam body


Anchor drilling

Drilling the dam body by a hole drilling machine to insert anchors.


Tendon insertion

Insert tendons (e.g., high-strength steel wire) that will serve as the anchor body


Tendon insertion

To preserve the landscape and historic appearance

Devices for construction of the top of the overflow section (Etsuryu-bu) where water flows out of the dam lake.

The top of the existing masonry was removed and PS anchors were installed; after reinforcing with PS anchors, new masonry was applied to recreate the appearance before the reinforcement work.

Installation Procedure

Confirmation of texture with homogeneous stone

Restoration in consideration of the connection with the existing part

Cleaning of the existing section

Reduce the color difference between the restored and existing areas (restore the appearance of 100 years ago)

Consideration when constructing scaffolding

To protect the masonry surface, the scaffold was assembled using base fittings without anchors.

Water quality preservation

Leakage prevention

After PS anchors are inserted into the dam body, cement milk or other grout is injected to fix the anchors in place, and measures are taken to prevent leakage of the injected cement milk.

◆Pre-check and repair of masonry joints
◆Confirmation of permeability by full hole hydrostatic test


Construction drainage measures



The 3rd Civil Engineering Award, Award of the Japan Federation of Construction Contractors (Japan Construction Contractors Association)
Special Prize, 27th Shimane Landscape Award (Shimane Prefecture)
2020 JSCE Technical Award Group II (Japan Society of Civil Engineers)
2020 Japan Society of Dam Engineering Technology Award (Japan Society of Dam Engineering)
The 1st JSCE Infrastructure Maintenance Project Award (Japan Society of Civil Engineers)