Groundwater, Bedrock, and a Low Tank Outlet During Installation
A crew is installing a 1,500-gal concrete tank and a six-trench system. The approved plan shows the tank outlet invert at 36.0 in above the distribution-box invert and trench bottoms at 30 in below grade with 24-in separation to bedrock. During excavation, groundwater seeps from the 30-in depth and bedrock is encountered at 42 in. The tank is set on undisturbed soil, but the outlet invert measures 34.5 in above the distribution-box invert-1.5 in lower than plan. The first two trenches are excavated to 36 in to reach what the operator calls better soil, reducing separation to bedrock to 6 in. The homeowner asks the crew to backfill before the inspector arrives because rain is forecast.
1. Identify the conditions that require stopping work and explain why they cannot be treated as routine field adjustments.
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Stop work. Groundwater at 30 in and bedrock at 42 in mean the planned 30-in trench bottom with 24-in separation is not achievable. The first two trenches at 36 in leave only 6 in to bedrock, far short of 24 in. That is a design change, not a field adjustment. The 1.5-in-low tank outlet also changes the hydraulic grade and can cause backup or unequal dosing. Backfilling would conceal noncompliant work and create a false as-built. The rain forecast is a site-protection issue, not a reason to cover noncompliant construction.
2. What corrective actions and approvals are needed before work resumes?
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Notify the designer, the permitting authority, and the homeowner in writing. Re-evaluate the site with additional borings and consider a raised or mounded system, a different area, or an approved alternative with reduced separation if the code allows it. Reset or shim the tank so the outlet invert matches the approved grade, or revise the distribution-box and lateral elevations with design approval. Remove or re-excavate the over-deep trenches and restore the required separation. The operator's claim of better soil does not substitute for a revised design and permit.
3. How should the installer document the episode and handle the homeowner's request?
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Document with photographs, elevations, boring logs, and a written stop-work notice. Record the homeowner's request to backfill and the installer's refusal. Keep the inspector informed and do not backfill before verification. If rain threatens, cover or stabilize exposed work in a way that can be removed for inspection. The installer should explain that concealment could void the permit, create liability, and violate the registration's ethical duty to install according to the approved plan.