We discovered that the top plate had been dropped in the central parking area at 6 PM.
The largest plate is almost 30 cm. The time of the fall is between 4 and 6 PM.
Fortunately, nobody was injured and nothing was damaged.
We will conduct a safety check of the tunnel by the Kamioka mine company.
Kamioka mine company, Hayakawa, Yamaguchi, Ohmae, Uchiyama
We performed the necessary safety treatments.
We also discussed this with the Kamioka mine company about the safety check on August 29.
Kamioka Mine Company, Hayakwa, Uchiyama
We conducted a safety check by the Kamioka mine company. We inspected the central parking and the tunnel between the bypass tunnel and the central parking. We used a high-altitude work vehicle and a forklift with a workbench. While looking at the wall, we knocked the tunnel or ceiling at the central parking with a long stick-shaped tool to check the sound and dropped stones that could be removed.
There are now 3 things to pay attention to in the ceiling of the central parking. Each is marked.
Marking point 1 (where there was the falling stone on August 22): They dropped a stone that was 4 to 5 times the size of the stone on August 22.
Marking point 2, 3: From the hitting sound, it was marked that there was likely to be an empty space between the bedrock and the mortar layer. There is no crack on the surface, so it does not collapse immediately.
We are waiting for the report from the Kamioka mine company to judge how we will do.
Fundamental concept of mortar treatment from Furuta-san,
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Sprayed mortar is fundamentally used as a temporary method until the permanent concrete overlay is applied. Generally, in civil engineering, permanent structures made of mortar are limited to slope protection works. While it is used in slope works with poor conditions, measures are taken to address cracking and delamination.
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I understand that with managed quality and standard construction, it can last about 30 to 50 years. However, I believe the quality control and construction management of the mortar used at the problematic locations were not under conditions that would generally guarantee quality.
So, this is the reason we never believe the general lifetime of mortar in the KAGRA site. The only method to keep its condition is simply to check the mortar conditions every week.
Actually, even in the XY-arm tunnel where the regular checks have been done, the drop of small-sized mortars was sometimes observed, maybe because of sudden pressure by spring water.
The inspection results of Kamioka Mining have been submitted.
In addition, as a comment, “We didn't see any floating stones or cracks that needed to be countered right now." I received.
So we discussed with the relevant parties and decided to return the operation of the parking lot to normal.
We will continue to implement measures and regular inspections to ensure safety.
Hayakawa and his team
They cleaned up the triangular cones and returned the parking lot to normal.
Kamioka mining company, Hayakawa, Takahashi, Sawada
We are carrying out repair work on the ceiling in the central parking area by Kamioka Mining Company. (Photo. 1)
They reinforced the surface of three places that were found to be careful in the previous inspection (https://klog.icrr.u-tokyo.ac.jp/osl/?r=34974) with wire mesh. (Photo 2)
The ceiling repair work has completed on December 3rd.