Design Objectives

A well must accomplish five goals simultaneously:

1. Access the aquifer: Penetrate the water-bearing formation sufficiently to achieve the desired yield
2. Maintain borehole integrity: Prevent collapse through proper casing and grouting
3. Protect water quality: Seal out contaminated shallow groundwater and surface water
4. Maximize yield: Optimize screen length, slot size, and gravel pack for the formation
5. Enable maintenance: Design for pump installation, testing, rehabilitation, and eventual abandonment

Open Hole vs. Screened Wells

• Open hole (barefoot): No casing or screen in the aquifer zone — used in hard rock formations where the borehole is self-supporting. Lower cost but no formation control. Risk of collapse in weak zones.
• Screened well: Perforated casing or screen installed across the aquifer — required in unconsolidated formations (sand, gravel). Higher cost but controlled entry, better yield, longer life.

Diameter Selection

Screen Types

Wire-Wrapped Screens

Precisely spaced wire wrapped around a cage of longitudinal support rods. The V-shaped wire creates an inward-enlarging slot that resists clogging — particles that pass the outer opening flow freely inward.

• Slot sizes: 0.10-3.00 mm (0.004"-0.120")
• Open area: 40-60% — excellent hydraulic efficiency
• Material: Stainless steel (304 or 316), galvanized steel, PVC
• Cost: Highest — but best long-term performance

Perforated PVC

PVC casing with factory-drilled holes or slots. Economical for domestic wells where cost is the primary concern.

• Slot sizes: 0.5-3.0 mm (0.020"-0.120")
• Open area: 5-15% — lower hydraulic efficiency
• Cost: Lowest — 30-50% of wire-wrapped cost

Bridge Slot Screens

Heavy-duty screens with stamped bridge-shaped openings. Designed for gravel-pack wells where strength and durability are paramount.

Slot Sizing Rules

Gravel Pack Design

A gravel pack is placed in the annular space between the well screen and the borehole wall in unconsolidated formations. It serves two purposes:

1. Filtration: Prevents fine formation material from entering the well
2. Hydraulic improvement: Increases the effective well diameter and improves flow into the screen

Screen Length & Placement

• Length: Screen should be placed across the full saturated thickness of the aquifer for maximum yield
• Open area: Design for entrance velocity <0.03 m/s (1 ft/s) to minimize turbulence and head loss
• Position: Screen bottom should be 1-3 m above the borehole bottom (sump) to allow sediment accumulation
• Overlap: If multiple screens are used, ensure minimum 3 m overlap with the producing zone

Sanitary Seal Purpose

A sanitary seal prevents surface contamination from migrating down the borehole along the casing. Without a proper seal, surface water carrying bacteria, chemicals, and sediment can flow directly into the aquifer through the annular space between the casing and borehole wall.

Grout Materials

Grouting Procedure

1. Pre-grout inspection: Use a caliper log or CCTV to verify borehole diameter and condition
2. Clean the annulus: Remove drilling mud and debris from the annular space with air or water
3. Place chips (bentonite): Drop dry bentonite chips slowly while maintaining water level — chips settle and swell
4. Pump grout (cement): Pump from bottom up through tremie pipe to displace water and prevent bridging
5. Verify seal: After 24-48 hours, check for water movement through the seal (dielectric test or pipe-log)

Purpose

Development removes drilling damage, fine sediment, and drilling fluids from the well screen and surrounding formation. A properly developed well can yield 2-5× more water than an undeveloped well. Development is one of the most critical steps in well construction — it directly determines the well's long-term performance.

Development Methods

Airlift Development

Compressed air is injected through small-diameter tubing (airline) suspended inside the well. The air creates a two-phase mixture that rises rapidly, pulling water and fine sediment from the aquifer. The most effective development method for most wells.

• Procedure: Inject air for 15-30 minutes, let well rest 30 minutes, repeat 3-5 cycles
• Success indicator: Water clears from turbid to transparent over multiple cycles

Surging

Alternating positive and negative pressure pulses to mobilize fines. A surge block (rubber disk) is rapidly moved up and down inside the casing, creating alternating flow directions through the screen.

Jetting

High-pressure water directed at screen openings from a jetting tool lowered inside the well. Effective for cleaning individual screen sections but time-consuming for long screens.

Over-Pumping

Sustained high-rate pumping (1.5-2× design yield) to draw fines into the well. Requires a temporary surface pump and sediment settling tanks. Effective but generates large volumes of turbid water that must be disposed of properly.

Development Verification

• Specific capacity test: Measure yield vs. drawdown — compare to pre-development values
• Sand content test: Water should contain <1 mg/L of suspended solids (<1 ppm)
• Turbidity: <5 NTU after development (ideally <1 NTU)

Pump Types

Submersible Pump Installation

The submersible pump is the most common choice for drilled water wells. It consists of a motor (hermetically sealed, oil or water-filled) coupled to a multi-stage centrifugal pump.

• Setting depth: Minimum 5-10 m below the lowest anticipated water level (drawdown + safety margin)
• Motor type: Water-filled (lower cost, easier maintenance) vs. oil-filled (better lubrication, higher temp tolerance)
• Power supply: Single-phase (1-3 HP for domestic) or three-phase (3-100+ HP for agricultural/municipal)
• Variable frequency drive (VFD): Adjusts motor speed to match demand — saves energy, reduces wear, prevents water hammer

Overview

Hand-dug wells are the oldest form of groundwater access, still serving billions of people in developing regions. While drilled wells are preferred for deep aquifers, hand-dug wells remain practical for shallow water tables (0-15 m) where large diameter provides adequate storage and low-cost construction is essential.

Construction Methods

Manual Excavation

• Hand digging: Workers excavate using picks, shovels, and buckets — typically 2-person teams
• Cribbing: Timber or concrete ring supports prevent wall collapse during excavation
• Depth limit: Usually 6-12 m due to safety constraints and water table depth
• Diameter: 1-3 m — provides significant water storage (4-7 m³ per meter depth)

Caisson / Shaft Sinking

• Open caisson: Concrete ring sections sink under their own weight as soil is excavated from inside
• Steined shaft: Brick or stone lining built progressively as excavation advances
• Depth capability: Up to 30 m with proper techniques and ground conditions
• Diameter: 1-4 m — larger than drilled wells, provides natural storage

Machine-Assisted

• Backhoe excavation: Large equipment for initial overburden removal, then hand finishing
• Auger bailing: Truck-mounted auger for 0.6-1.2 m diameter holes to 15 m
• Bucket auger: Manual or powered auger for intermediate size (0.3-0.6 m)

Lining Methods

Sanitary Protection

Hand-dug wells are highly vulnerable to surface contamination. Critical protections include:

• Concrete apron: 1.5-2 m radius, sloped away from well, minimum 100 mm thick
• Well head protection: Raised collar (300-500 mm above ground) with removable cover
• Setback distances: Minimum 15 m from latrines, 30 m from septic systems
• Covered top: Secure lid with access hatch — prevents debris and animal entry
• Drainage channel: Directs spilled water away from well head

Advantages & Limitations

• Advantages: Low cost (10-20% of drilled wells), local materials and labor, large storage, easy maintenance, community ownership
• Limitations: Shallow depth only, contamination risk, labor-intensive construction, lower yield, limited to soft formations
• Best for: Rural communities, shallow water tables, developing regions, emergency water supply

What Is an Artesian Well?

An artesian well taps a confined aquifer where groundwater is under pressure sufficient to rise above the top of the aquifer. If pressure is enough to reach the surface without pumping, it is a flowing artesian well.

Conditions for Artesian Flow

• Confined aquifer: Aquifer bounded above and below by impermeable layers (aquitards)
• Recharge area: Outcrop at higher elevation where water enters the aquifer
• Hydraulic head: Potentiometric surface above the ground level at the well site
• Pressure gradient: Water rises due to elevation difference between recharge area and well

Pressure Management

Free-Flowing Wells

When artesian pressure exceeds ground level, water flows continuously without pumping:

• Flow control valve: Gate valve or needle valve to regulate discharge
• Pressure gauge: Monitor residual head — declining pressure indicates aquifer stress
• Flow meter: Track total extraction for water balance management
• Waste prevention: Never allow unrestricted flow — wasteful and can cause erosion

Subsurface Safety Valves

For high-pressure artesian wells, safety valves prevent uncontrolled blowout:

• Tubing-retrievable safety valve: Installed on tubing — wireline serviceable
• Wireline-retrievable valve: Quick replacement without pulling tubing
• Surface safety valve (SSV): Emergency shutdown at wellhead
• Downhole safety valve (DHSV): Closes automatically if surface control is lost

Wellhead Pressure Ratings

Artesian Aquifer Management

• Water level monitoring: Piezometer network tracks potentiometric surface
• Extraction limits: Sustainable yield must not reduce artesian pressure
• Recharge management: Artificial recharge to maintain pressure in stressed aquifers
• Multi-well coordination: Interference between artesian wells reduces individual yields
• Compaction risk: Over-extraction can cause irreversible aquifer compaction and land subsidence

Artesian Well Design

• Casing: Full-length steel casing to surface — withstands artesian pressure
• Wellhead: Flanged connection rated for maximum expected pressure
• Packer: Isolates artesian zone from overlying formations
• Screen: Selective screening in artesian aquifer zone only
• Cement: Full cement job to prevent upward pressure migration through annulus

Fundamentals of Groundwater Flow

Water moves through aquifers driven by hydraulic gradients. Darcy's Law describes this relationship:

Pumping Tests

Constant-Rate Pumping Test

The gold standard for aquifer characterization. A production-size pump runs at constant discharge while water levels are monitored in the pumping well and observation wells at regular intervals.

• Duration: 24-72 hours minimum (some tests run weeks for large-scale aquifer characterization)
• Monitoring: Water level measurements at 1, 2, 5, 10, 20, 30, 60 min, then hourly
• Data analysis: Drawdown vs. log-time curves matched to type curves (Theis, Cooper-Jacob, Hantush)

Step-Drawdown Test

Used to determine well efficiency and optimal pumping rate. The well is pumped at 3-5 progressively higher rates, each sustained until drawdown stabilizes.

Slug Test

A low-cost alternative for estimating hydraulic conductivity. A known volume of water is suddenly added (rising head) or removed (falling head) and the recovery is measured.

Interpreting Results

Depth Categories

Artesian Wells

An artesian well taps a confined aquifer where the potentiometric surface lies above the ground surface. The water rises naturally under pressure without pumping. Flowing artesian wells require a capping device to prevent uncontrolled discharge.

Drilling Challenges at Depth

• Torque and drag: At 500 m, a drill string may weigh 15-25 tonnes in air. Friction against the borehole wall adds 30-50% to the effective weight. Torque requirements can exceed 10,000 N·m.
• Temperature gradients: 25-30°C per km — at 1000 m depth, bottom-hole temperature may be 40-55°C. Affects drilling fluid properties and pump motor life.
• Lost circulation: Fractured zones can absorb entire mud volumes. Lost circulation materials (LCM) — nut plug, mica, cellophane — are required.
• Formation pressure: Deep confined aquifers may be overpressured — requiring heavier mud to prevent blowouts.

Why Dewatering?

Construction below the water table requires groundwater removal to provide a dry, stable working environment. Dewatering is essential for foundations, tunnels, mining operations, and utility installations.

Dewatering Methods

Permeability Estimates