Wellpoint Spacing Calculator

Inputs

Example Data Table

Formula Used

This calculator uses a practical Darcy-flow approximation to estimate seepage inflow toward a wellpoint system:

• Q: estimated inflow (m³/s), converted to m³/h.
• k: hydraulic conductivity (m/s).
• s: target drawdown (m).
• R: radius of influence (m).
• L_layout: spacing length (perimeter or line length) (m).
• t: saturated thickness contributing to seepage (m).

Design flow applies a safety factor and optional redundancy, then sizes wellpoints: N = ceil((Q × SF) / Q_wp), Spacing = L_layout / N.

How to Use This Calculator

1. Enter excavation geometry and choose the wellpoint arrangement.
2. Input soil conductivity, saturated thickness, and target drawdown.
3. Set the radius of influence based on experience or test data.
4. Enter wellpoint capacity at your expected operating conditions.
5. Add a safety factor and redundancy for uncertainties and downtime.
6. Submit to view spacing, count, and pumping estimates above.
7. Download CSV or PDF for submittals and internal reviews.

Professional Article

1) What wellpoint spacing controls

Wellpoint spacing governs how evenly vacuum and drawdown are distributed along an excavation. Tight spacing improves continuity of the lowered groundwater surface, while wide spacing can leave “high spots” where seepage persists. Balanced layouts reduce piping risk, support base stability, and limit pump cycling.

2) Geometry and layout length matter

Dewatering demand scales with the length of the header line you are trying to protect. A perimeter setup uses 2 × (L + W), while a single line is closer to L. Longer layout length increases the seepage area feeding the system and typically requires more wellpoints for the same spacing target.

3) Soil permeability drives inflow

Hydraulic conductivity is often the dominant variable. Fine sands may be around 10⁻⁵ to 10⁻⁴ m/s, clean sands can reach 10⁻³ m/s, and silts may be far lower. Small changes in k can strongly shift inflow, so field checks are critical.

4) Saturated thickness sets the seepage area

The contributing thickness represents the vertical zone feeding flow toward the wellpoints. If a low‑permeability layer caps inflow, the effective thickness may be smaller than the full aquifer thickness. Conversely, thick, uniform permeable strata can sustain higher inflow longer, requiring either higher capacity points or closer spacing.

5) Drawdown target and radius of influence

Drawdown creates the hydraulic gradient that moves water toward the line. Using i ≈ s / R, higher s or smaller R increases gradient and predicted flow. Radius of influence typically varies with soil type, stratification, recharge, and nearby boundaries; calibrate with pumping‑test observations whenever available.

6) Capacity, losses, and redundancy

Rated capacity should reflect actual site conditions: vacuum performance, header losses, elevation changes, and filter resistance. Redundancy for fouling or maintenance helps keep drawdown reliable at the duty point.

7) Practical spacing bands and constructability

Many installations target about 0.75–1.50 m spacing for continuous control, but the best value depends on soil gradation, drilling tolerances, and access constraints. Closer spacing increases labor and header complexity, while wider spacing can require rework, secondary points, or supplemental sump control.

8) Verification during execution

Treat calculated spacing as a starting design. Use piezometers, flow meters, and drawdown readings to confirm performance. If drawdown is uneven, adjust by adding points at problem zones, reducing spacing locally, or increasing capacity. Document changes with exported reports for records.

FAQs

1) What spacing should I start with for a typical excavation?

Start with a practical band around 0.75–1.50 m, then refine using soil permeability, target drawdown, and point capacity. Confirm with monitoring and adjust locally where drawdown is not uniform.

2) Why does permeability change the required number of wellpoints so much?

Permeability controls how easily water flows toward the wellpoints. Higher conductivity produces higher inflow for the same gradient and seepage area, so more points or higher capacity is needed to maintain drawdown.

3) How do I choose the radius of influence?

Use pumping‑test interpretation when available. Otherwise, estimate from local experience considering soil type, recharge, boundaries, and stratification. Sensitivity‑check by trying low and high values and reviewing the impact on spacing and wellpoint count.

4) Should I rely on manufacturer capacity ratings?

Treat ratings as upper limits. Actual capacity depends on vacuum level, header losses, installation quality, and filter clogging. Apply a safety factor and consider redundancy to keep performance stable during fouling or downtime.

5) When should I select a perimeter versus a single line arrangement?

Perimeter layouts improve uniform control and reduce edge seepage. Single or double lines suit long trenches or one‑sided inflow, but may need closer spacing or supplemental control.

6) What if the system achieves drawdown but inflow is still high?

High inflow may indicate recharge, permeable layers, or boundary effects. Check piping losses and filter performance, then consider redundancy or staging pumps. Recheck whether the drawdown target is too aggressive.

7) Can this calculator replace a detailed dewatering design?

No. It provides a practical planning estimate using simplified assumptions. For critical excavations, combine it with geotechnical data, pumping tests, transient analysis where needed, and construction‑phase monitoring to validate stability and performance.