Slotted vs Plain Casing for Borewells — selection guide

What does slotted casing do in a borewell?

Slotted casing lets water enter the borewell while keeping formation material outside the pipe. It works as a passive screen — water flows through the openings, but sand and silt stay in the formation. Plain casing (also called blank casing) seals the hole where water entry is not wanted.

Slotted casing is also called perforated casing, screen casing, or simply "slots". The slots are machine-cut openings in the pipe wall that allow groundwater to pass into the borewell while the solid pipe maintains structural strength. Unlike wire-wound screens used in some municipal wells, slotted casing is a single steel pipe with laser-cut or plasma-cut openings. The slot width, length, spacing, and open-area percentage together control how much water enters and what particle size is admitted.

The key difference between slotted casing and wire-wound screens is maintainability. Wire-wound screens have discrete wire rings wound around a central rod; they can be cleaned chemically or mechanically if fouling occurs. Slotted steel casing is a solid pipe — if the slots clog with mineral deposits or bio-fouling, cleaning is much harder. For this reason, slotted casing is specified once based on formation grain size and is expected to work for the well lifetime without intervention.

Planning the slot placement follows the bore log. The drilling contractor identifies water-bearing zones during drilling by observing cutting returns, loss of drilling fluid, and test-pumping results. The slotted section covers exactly these zones. Blank (plain) casing protects the upper unsaturated zone, any contaminated shallow layer, and the surface conductor pipe. Overlapping the slotted section ensures the entire aquifer is exposed to the pump intake, but it does not extend into non-water-bearing rock or clay that would only bring fines.

How is slot geometry selected for different aquifer types?

Slot width is chosen from the formation grain size — finer sand needs narrower slots. Open-area percentage controls total inflow; higher yield needs more open area. Match the slot to the median grain size (D₅₀) from the bore log or sieving analysis.

Slot geometry has three main parameters: slot width (the narrowest opening dimension), slot length, and open-area percentage (total slot opening area as a fraction of pipe surface). The slot width is selected to exclude the majority of formation particles — too wide and sand enters the pump; too narrow and water entry is restricted, causing turbulence, sand agitation, and reduced yield.

The industry rule of thumb is to select a slot width slightly larger than the D₈₅ grain size of the aquifer material. This allows water and fine particles smaller than the slot to pass through, but the formation naturally packs against the slots and acts as a filter. For very fine sand aquifers (D₅₀ < 0.2 mm), 0.5 mm slots with 3% open area are common. Medium sand (D₅₀ = 0.2–0.5 mm) takes 1.0 mm slots at 5% open area. Coarse sand and fine gravel work with 2.0 mm slots at 8% open area.

For high-yield wells where maximum inflow is priority, continuous-spiral or bridge-slotted casing provides 12–18% open area. This is not common for agricultural bores because the increased.open area weakens the pipe structurally and makes the well more prone to sand entry if the formation is not properly developed. The table below summarizes industry practice from IS 4270 manufacturing tolerances and CGWB drilling guidance.

Slot Width | Slot Length | Open Area | Typical Application

0.5 mm | 75 mm | ~3% | Fine sand aquifer (D₅₀ < 0.2 mm)

1.0 mm | 100 mm | ~5% | Medium sand aquifer

2.0 mm | 150 mm | ~8% | Coarse sand / fine gravel

Continuous spiral | n/a | ~12-18% | High-yield commercial wells

Open-area percentage also affects velocity. Higher open area reduces entrance velocity, which lowers head loss and reduces the risk of drawing fine particles into the pump. If the required yield is high and the aquifer is productive, increasing open area helps. If the aquifer is weak, a smaller open area may actually improve well efficiency by reducing drawdown at the screen.

When is a gravel pack needed with slotted casing?

A gravel pack is needed when the natural formation does not filter effectively — usually in fine sand aquifers where slot width must be small, limiting inflow. The gravel pack is a coarse material (2-8 mm) placed in the annulus to act as a secondary filter with larger slots.

The combination of slotted casing and gravel pack follows the principle of controlled filtration. The gravel pack sits between the slotted casing and the bore wall, creating a coarser filtering layer than the natural formation. Water passes through the gravel (which has large pores) then through the slots (which are sized for the aquifer). This two-stage filtering lets the buyer specify wider slots for higher inflow while keeping sand out.

Gravel packing adds cost — the gravel must be washed, sized, and placed carefully. It is most common where the aquifer is very fine sand and the required yield is moderate to high. If the aquifer is coarse sand or gravel, the natural formation usually filters adequately and a gravel pack is unnecessary. The drilling contractor decides pack necessity based on sieve analysis of the aquifer material.

The gravel pack grain size is selected as roughly 4-6 times the slot width. For 0.5 mm slots, 2-4 mm gravel is appropriate; for 1.0 mm slots, 4-6 mm gravel; for 2.0 mm slots, 8-12 mm gravel. This ratio ensures the gravel does not bridge over the slots while still excluding the aquifer material behind it.

What does IS 4270 require for slotted casing?

IS 4270 covers steel tubes for water wells, including tolerances for outside diameter, wall thickness, length, and slot dimensions. The standard does not prescribe which slot width to use — that is a well-design decision. Common buyer mistakes include ordering too little slotted length and ignoring the overlap requirement with adjacent plain casing.

IS 4270 (2024 revision) specifies dimensions for nominal bore sizes from 100 mm to 300 mm, wall thickness from 3.0 mm to 8.0 mm, and threaded-and-coupled ends. For slotted casing, the standard defines permissible slot width tolerance (±0.05 mm), slot length tolerance (±1.0 mm), and minimum wall thickness in the slotted zone. The slot pattern must be consistent — irregular spacing can cause local stress concentrations.

A frequent procurement mistake is ordering the slotted length exactly equal to the aquifer thickness from the initial bore log, without adding overlap. Because the aquifer boundary is not a clean line — it transitions over a few metres — the slotted section can end up falling partly in non-water-bearing material. Adding 1-2 metres of overlap on each end of the identified aquifer zone ensures full coverage.

Another common error is under-specifying slotted length to save cost, then being surprised when the pump sand-locks during summer drawdown. The screen must expose the entire water-bearing zone to the pump intake, not just the most productive centre. If the slotted section is too short, the pump draws from a limited area, increasing velocity, turbulence, and sand movement. Ordering the full aquifer length plus overlap is the safe procurement practice.

Where should plain casing be used instead of slotted?

Use plain casing above the water table in the unsaturated zone, through surface soil and weathered rock, through any contaminated or non-water-bearing layer, and to overlap the slots at both the top and bottom of the screened interval.

Plain casing serves three functions in a typical borewell. First, it seals the unsaturated zone above the water table — this keeps surface water, pesticides, and shallow contamination from entering the well. Second, it provides structural strength through collapsing or loose overburden. Third, it overlaps the transition at both the top and bottom of the screened section to ensure no untreated formation material contacts the pump intake.

The surface conductor casing (the first few metres) is always plain — it supports the bore opening, guards against surface water runoff, and provides a mounting point for the sanitary seal. This plain section must extend at least 0.5 metres below the lowest expected water table to prevent air entrainment during pumping.

At the lower end of the slotted section, plain casing or a blanking plug closes the bore. This prevents the pump from drawing water from below the screened interval where water quality may differ or where the formation is non-productive. The transition from slotted to plain is usually marked by a factory-welded collar or a threaded coupling that the installer can identify during assembly.

How do I specify slotted vs plain casing in an RFQ?

State the total metres of plain casing (surface + above-aquifer), the total metres of slotted casing (with slot width, length, and open-area requirement), the overlap on each end, and any gravel pack specification separately. Include plain and slotted in separate line items — do not combine them under one vague description.

A clear RFQ separates the requirement by function. One line item says "Plain casing for surface and protection, X metres", another says "Slotted casing across aquifer, Y metres, 1.0 mm slots, minimum 5% open area". This lets the fabricator quote correctly and the site team install the right pipe in the right place. A single vague line like "bore pipe with slots" creates confusion and the wrong metres on site.

The RFQ should also state the slot width explicitly when the buyer has a bore log or Sieve analysis. If no grain-size data is available, state the expected aquifer type ("fine sand", "medium sand", "coarse sand/gravel") and let the fabricator recommend the slot width. For high-yield wells, state a minimum open-area percentage requirement (e.g., "minimum 5% open area").

Include coupling counts, thread protection, and whether the slotted pieces need end protectors. Slotted pipe is more prone to transport damage because the slots create stress concentrations at the ends. Require end caps or timber blockers during loading. RP Sales quotes slotted and plain casing separately with clear slot specifications, so buyers receive the right product mix for the drillers assembly sequence.