Unsupervised lateral, unlocated service and a flat grade
Your company is installing drainage on a residential subdivision. A labourer with no drainlaying qualification is laying 100 mm laterals while the site supervisor is off site. The trench is 1.5 m deep, unshored, with a 1.0 m spoil heap at the edge. The labourer proposes to "cut through" a marked service because the plan shows a different alignment. The approved plan shows a 1:60 grade, but the labourer has laid 25 m at 250 mm fall (1:100).
1. What immediate health and safety actions are required?
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Stop the work and take control of the excavation:
- keep workers out of the 1.5 m deep unshored trench; provide shoring or a trench box, or batter to a safe angle in the actual ground conditions;
- move the spoil heap back from the trench edge to the distance required by the site's excavation plan (commonly at least 1 m) and prevent spoil and materials falling in;
- barricade and sign the open trench and provide a safe access route;
- do not cut or probe the marked service. Treat it as live until located and proven by the service owner/utility locate; obtain approval before any crossing or diversion;
- check for other services and underground hazards before further excavation;
- record the stop-work instruction and notify the site supervisor/manager.
2. Assess the laid grade and state what must happen to the 25 m of lateral.
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Grade = 250 mm ÷ 25 000 mm = 0.010 = 1.0%, i.e. 1:100.
The approved plan requires 1:60 (1.67%). 1:100 is flatter and does not comply. The 25 m section cannot be certified as laid.
Options are to relay the lateral at 1:60, or obtain an approved variation if the designer can demonstrate compliance by another route (for example a different pipe size with a permitted flatter grade, subject to capacity and connection checks). The as-built levels must be re-surveyed after relaying. The fact that the labourer "thought it looked fine" is not evidence of compliance - the level must be checked with a laser or level and recorded.
3. What supervision, competence and contractual issues does this raise?
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Key issues:
- Competence and supervision - drainlaying is licensed/restricted work; a labourer must not be left to set out and lay laterals without adequate supervision by a suitably qualified person. The supervisor's absence and the lack of a competent person on site are the root cause.
- Site instruction and variation - the service conflict and any alignment change must be documented and approved by the designer/territorial authority before work proceeds. "Cutting through" a marked service without locating it is not a variation, it is a risk.
- Rework and cost - the non-compliant 25 m is the contractor's risk; the builder should be notified promptly in writing, with a remediation plan and programme impact.
- Records - diary notes, stop-work notices, toolbox records, supervision records, amended plans and re-surveyed levels should all be retained.
- Certification - the certifying drainlayer must not certify the non-compliant grade or undocumented service crossing; certification should follow inspection, testing and approved documentation.