Excavating next to a pipe you can replace is a scheduling problem. Excavating next to a pipe that is carrying cold water into a live petrochemical plant is a different job, and the difference is not the digging.
At Yansab in Yanbu Industrial City we are exposing two live 3.0 metre diameter GRP cold water pipelines along approximately 400 metres of the Zone 3 pipe rack area, to a depth of about 7.0 metres. The mains stay in service throughout. This note is about how that scope is made safe on paper before anybody breaks ground.
The face, and why it is benched
A 7.0 metre face in this ground will not stand vertically, and a trench box is not an option across a 400 metre run beside two large diameter mains. The face is self supporting and benched, with a reinforced shotcrete facing.
Benching costs excavation volume and it costs programme. What it buys is a face whose stability you can calculate and demonstrate, rather than one you have to trust.
Soil nails as a contingency, not a default
A built in soil nail contingency covers any zone that does not perform to the geotechnical baseline. That distinction matters. Nailing the whole run would have been simpler to specify and considerably more expensive. Nailing only where the ground disagrees with the baseline keeps the cost proportional to the risk actually found.
The design basis
| Item | Basis |
|---|---|
| Excavation standard | OSHA 29 CFR 1926 Subpart P |
| Client requirements | SABIC and Yansab SHEM |
| Site authority | Royal Commission |
| Face stability | Factor of safety 1.5 against a Yansab specific geotechnical baseline |
| Monitoring | Daily, by a Competent Person |
The factor of safety is stated against a named baseline rather than a general one. That is deliberate. A factor of safety quoted without saying what it is a factor of safety against is not a number, it is a reassurance.
Why the monitoring protocol is the real control
Shoring design gets the attention because it is the drawing. On a live industrial site the daily Competent Person inspection is what actually keeps the face standing, because ground conditions, weather and adjacent activity all change after the drawing is signed.
The protocol is daily for the duration, not at milestones. On a 400 metre run that is the only interval that means anything: a face that was sound on Sunday and is being loaded by plant on Tuesday needs looking at on Tuesday.
What we hand over
Shoring design and structural calculation, the excavation itself, dewatering coordination, and the geotechnical monitoring record. The last of those is the one clients ask for months later, and it is the one that is hardest to reconstruct if nobody was writing it down.