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Excavation Geometry
Working level is normally kept 0.5–1.0 m below the excavation base (IS 9759 practice).
Hydrogeological Setting GRAVITY
From pumping test / slug test / GEC-2015 formation range. Cite source in report.
Typically kh/10 in layered alluvium unless measured.
GEC-2015 convention: percent. Converted to fraction internally.
Bottom of the unconfined aquifer from the borehole log. Must be below the target water level.
1.0 if aquifer base is at/near excavation base. 1.3–2.0 where the aquifer extends well below the base (Mansur & Kaufman 1962; CIRIA C750).
Used to check that drawdown does not de-pressurise below aquifer top (else mixed flow).
Aquitard thickness between excavation base and top of the underlying pervious layer, from the borehole log — NOT an assumed value.
From a piezometer tapping the pervious layer below the plug.
Through-wall/joint leakage from the wall specification (panel joints, toe gaps). Enter 0 if watertight wall assumed; justify in report.
Method basis: IS 9759:1981 Guidelines for dewatering during construction, Cl. 6.2 (discharge formulae, Table 6 circular source to well), Cl. 4.4.2.3 (radius of influence), Appendix B (flow nets).
Equivalent-well substitution and partial-penetration guidance per Mansur & Kaufman (1962), Powers Construction Dewatering, CIRIA C750.
The excavation is represented as one equivalent well of radius re = √(A/π).
Steady Seepage Inflow
–
m³/day
Equivalent Well Radius re
–
m
Radius of Influence R
–
m (from pit centre)
Required Drawdown s
–
m
Step-by-step Computation
Initial Storage Release (one-off dewatering of aquifer storage)
Period over which the water table is lowered to target before excavation below WT proceeds. Storage-release pumping is spread over this period.
Monsoon / Rainfall Inflow
External area draining into the excavation (beyond the pit plan area). 0 if bunds divert all runoff.
CPWD: paved 0.80–0.90, gravel 0.35–0.70, open ground 0.20–0.35.