Wellpoint Drawdown Calculator

Inputs

Choose an analysis method, then define the wellpoint layout and soil parameters. The calculator uses superposition for multiple wellpoints.

Example data table

Sample scenarios to illustrate typical field inputs. Replace these with soil test values and site geometry.

Formula used

Confined aquifer (Thiem): drawdown at distance r from one wellpoint:

• s = drawdown (m), Q = discharge (m³/s), T = transmissivity (m²/s).
• R = radius of influence (m), r = distance to point (m).

Unconfined aquifer (Dupuit–Thiem): water thickness at distance r from one wellpoint:

• K = hydraulic conductivity (m/s), H0 = initial saturated thickness (m).
• If h(r)² becomes negative, it is clamped to zero.

Multiple wellpoints: the tool uses superposition, summing drawdown from each wellpoint using per-well discharge Q/N.

How to use this calculator

1. Select analysis type based on whether the aquifer is confined or unconfined.
2. Choose a configuration: single, ring around excavation, or a line of wellpoints.
3. Enter the total pumping rate and your best estimate of R.
4. Provide T for confined cases, or K and H0 for unconfined cases.
5. Complete geometry inputs (r, a, spacing, offset), then click Calculate.
6. Review design drawdown. Use CSV/PDF exports for reports.

Hydrogeologic inputs that control drawdown

Drawdown is governed by aquifer capacity and the assumed radius of influence. For confined conditions, transmissivity combines thickness and permeability, so higher transmissivity produces less drawdown at the same discharge. For unconfined conditions, hydraulic conductivity and initial saturated thickness control how the water table lowers. Radius of influence should reflect boundaries, recharge, stratigraphy changes, and nearby pumping.

Selecting a suitable wellpoint layout

Layout defines how multiple wellpoints interact through superposition. A ring around the excavation targets lowering across the footprint while keeping access for installation and maintenance. A line layout suits trenches, walls, and corridor work where dewatering follows an alignment. Spacing governs overlap; tighter spacing increases combined drawdown but raises installation effort and header friction. Compare scenarios before finalizing header size and pump duty.

Interpreting design drawdown results

The calculator provides superposed drawdown and a design drawdown using a safety factor. Use the design value for method statements, approvals, and planning. Compare predicted lowering with required excavation freeboard, plus allowances for seepage, uplift, and base stability checks. If the margin is small, add wellpoints, tighten spacing, or increase total discharge, then rerun. For unconfined cases, confirm remaining saturated thickness stays realistic.

Practical constraints on site performance

Field results can differ from theory because of partial penetration, screen losses, filter clogging, and air leaks. Long headers add friction losses, reducing effective discharge at distant points. Measure vacuum and flow at several locations. Layered soils may create perched water that needs supplemental sumps. In fine sands and silts, select appropriate filters and manage discharge to prevent erosion and settlement.

Reporting and quality control deliverables

Professional documentation links assumptions to observations. Record input parameters, baseline groundwater levels, wellpoint spacing, and the safety factor used. Store exports so changes are traceable. Use CSV for calculation logs and PDF for permits, inspections, and client reporting. Update the calculation when spacing changes, pumps are swapped, or monitoring indicates different aquifer behavior. Consistent reporting supports faster decisions and reduced downtime overall.

FAQs

1) What is drawdown in a wellpoint system?

Drawdown is the reduction in groundwater head at a location due to pumping. In wellpoint dewatering, it reflects how much the water level is lowered to keep the excavation dry and stable.

2) When should I use confined versus unconfined analysis?

Use confined analysis when an aquifer is bounded by low-permeability layers and thickness is essentially constant. Use unconfined analysis when the water table is free to fall and saturation thickness changes with pumping.

3) How do I choose the radius of influence?

Prefer pumping test results or local experience in similar soils. If unknown, start conservatively and consider boundaries, recharge sources, and nearby wells. Update the value after monitoring confirms the response.

4) Why does spacing change the result so much?

Spacing changes the overlap of drawdown cones. Tighter spacing increases superposed drawdown at the target point, but also increases installation effort and can raise header losses that reduce effective discharge.

5) What safety factor is reasonable for planning?

Common planning multipliers range from about 1.10 to 1.30, depending on data quality and risk. Higher factors help cover clogging, partial penetration, and uncertainty in soil parameters and boundaries.

6) Can I rely on this for final design?

Use it for preliminary sizing and comparisons. Final design should be calibrated with field data, include system losses, and be checked against uplift, piping, and stability requirements for the excavation and subgrade.