← Irrigation Engineering · GSSSB Work Assistant Civil

Chapter 11 of 12

Minor Irrigation & Tubewells

In the GSSSB Work Assistant Civil syllabus under Irrigation Engineering · 2 parts

Groundwater, Aquifers & Well Hydraulics · Tube Wells & Pumping Tests

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (20 sections)

Part 1 of 2

Groundwater, Aquifers & Well Hydraulics

Last reviewed 16 Sept 2026 · 7 min read

Occurrence of groundwater

Below the ground surface, water occurs in two main zones:

Zone	Description
Zone of aeration (vadose zone)	Pores contain both air and water. Sub-zones: soil-water zone (root zone), intermediate vadose zone, capillary fringe (just above the water table, held by capillarity)
Zone of saturation	All pores filled with water under pressure ≥ atmospheric; its water is groundwater

The water table (phreatic surface) is the upper surface of the zone of saturation in an unconfined aquifer, where pressure is atmospheric.

Geological formations

Formation	Stores water?	Transmits water?	Example
Aquifer	Yes	Yes, readily	Sand, gravel, fractured rock, cavernous limestone
Aquitard	Yes	Slowly	Sandy clay
Aquiclude	Yes (porous)	No (practically)	Clay
Aquifuge	No	No	Massive unfractured granite

Types of aquifers

Unconfined (water-table) aquifer — upper boundary is the water table; recharged directly from the surface.
Confined (artesian) aquifer — sandwiched between impervious layers; water is under pressure. The level to which water rises in a well is the piezometric surface. If the piezometric surface is above the ground, the well flows naturally (flowing artesian well).
Perched aquifer — a small body of groundwater held above the main water table by a local impervious lens.
Leaky (semi-confined) aquifer — bounded by an aquitard that allows some leakage.

Aquifer properties

FormulaAquifer parameters

Porosity $n = V_{v} / V$
Specific yield $S_{y}$ — volume of water drained by gravity ÷ total volume
Specific retention $S_{r}$ — water retained against gravity ÷ total volume

n = S_{y} + S_{r}

Hydraulic conductivity (coefficient of permeability) $K$ (m/s or m/day)
Intrinsic permeability $k_{0} = \frac{K μ}{ρ g}$ (m² or darcy) — property of the medium alone
Transmissivity $T = K b$ (m²/day) — flow through a unit-width strip of the full aquifer thickness $b$ under unit gradient
Storage coefficient (storativity) $S$ — volume of water released per unit surface area per unit decline of head
- Confined aquifers: very small, about $5 \times 1 0^{- 5}$ to $5 \times 1 0^{- 3}$ (from compression of the aquifer and expansion of water)
- Unconfined aquifers: $S \approx S_{y}$ (typically a few per cent to about 30%)

Clay has high porosity but very low specific yield (high retention); coarse sand and gravel have high specific yield.

Darcy's law

V = K i, Q = K i A

$V$ = discharge (Darcy) velocity over the gross area; the actual average seepage velocity $V_{s} = V / n_{e}$ ( $n_{e}$ = effective porosity).

Validity: laminar flow — Reynolds number $R e = V d_{10} / ν$ below about 1 (up to about 10 in practice). Not valid for flow through coarse gravel, fractures with turbulent flow or karst conduits.

Steady radial flow to wells (equilibrium equations)

Assumptions (Dupuit–Thiem): homogeneous, isotropic aquifer of infinite extent; well fully penetrating; flow horizontal and laminar; steady state reached; pumping rate constant.

FormulaConfined aquifer

Q = \frac{2 π K b ( h _{2} - h _{1} )}{ln ( r _{2} / r _{1} )} = \frac{2 π T ( s _{1} - s _{2} )}{ln ( r _{2} / r _{1} )}

With well radius $r_{w}$ , drawdown $s_{w}$ and radius of influence $R$ : $Q = \frac{2 π T s _{w}}{ln ( R / r _{w} )}$

FormulaUnconfined aquifer

Q = \frac{π K ( h _{2}^{2} - h _{1}^{2} )}{ln ( r _{2} / r _{1} )} or Q = \frac{π K ( H ^{2} - h _{w}^{2} )}{ln ( R / r _{w} )}

$H$ = saturated thickness before pumping; $h_{w}$ = depth of water in the well while pumping.

Radius of influence — distance beyond which drawdown is negligible; Sichardt's formula: $R = 3000 s_{w} K$ ( $R$ , $s_{w}$ in m; $K$ in m/s).
Cone of depression — the drawdown surface around the pumped well.
Discharge is only moderately sensitive to $R$ (it appears inside a logarithm).
Specific capacity $= Q / s_{w}$ — a measure of well productivity (decreases with time and pumping rate).

Part 2 of 2

Tube Wells & Pumping Tests

Last reviewed 16 Sept 2026 · 7 min read

Types of wells

Open (dug) wells

Large-diameter (usually 1–6 m) shallow wells, masonry lined, yielding from the bottom and sometimes through weep holes in the lining. Suitable for small yields from shallow unconfined aquifers; large storage helps meet peak demand. Classed as shallow (resting on a pervious stratum) or deep (resting on an impervious layer and drawing from a pervious stratum below through a bore).

Tube wells

A tube well is a long pipe (casing + screen) sunk into the ground to tap one or more water-bearing strata.

Type	Description
Strainer type	Screens (strainers) placed against water-bearing layers, blind pipe elsewhere; the most common type
Cavity type	Draws water from a cavity formed below a strong clay (impervious) layer; the pipe ends at the bottom of the clay; flow is spherical rather than radial — requires a firm, thick confining layer
Slotted type	Slotted pipe surrounded by a gravel pack (shrouding)

Shallow tube wells — depth usually up to about 60–70 m, small discharge, often with centrifugal pumps at the surface.
Deep tube wells — deeper (often 100 m and more), larger discharge, with vertical turbine or submersible pumps.

Other groundwater structures

Infiltration gallery — a horizontal perforated conduit laid in permeable material alongside or under a river bed; collects filtered water. For flow from one side: $Q = \frac{K L ( H ^{2} - h ^{2} )}{2 R}$ .
Infiltration well — shallow wells in river beds connected to a jack well.
Collector (radial) well — a large central caisson with horizontal radial laterals driven into the aquifer; very large yields near rivers.
Springs — natural outflow of groundwater at the surface (gravity, artesian, contact springs).

Well drilling methods

Method	Principle	Suitability
Cable tool (percussion)	Heavy bit repeatedly lifted and dropped; cuttings removed by bailer	Consolidated and unconsolidated formations; slow but simple
Hydraulic rotary (direct circulation)	Rotating bit; drilling fluid (bentonite mud) pumped down the drill pipe carries cuttings up the annulus	Fast in unconsolidated formations; mud supports the hole
Reverse circulation rotary	Fluid flows down the annulus, cuttings pumped up the drill pipe	Large-diameter wells in loose alluvium
Down-the-hole (DTH) hammer	Pneumatic hammer with compressed air	Hard rock — fastest in basalt, granite
Jetting	High-velocity water jet washes material out	Small wells in soft sand
Augering / boring	Hand or power auger	Shallow wells in soft soil
Driven wells	Well point driven by hammering	Very shallow sand aquifers

Well screens and gravel packs

Screen (strainer) — lets water in while retaining aquifer material. Types used in Indian practice include slotted pipes, wire-wound (continuous slot) screens and proprietary strainers; material: mild steel, stainless steel, brass, PVC/FRP (corrosion-resistant).
Slot size — chosen from the aquifer grain-size curve so that fines near the well are removed during development while most of the formation is retained.
Gravel pack (artificial pack) — graded gravel placed around the screen where the aquifer is fine and uniform; allows larger slots, increases effective well radius and reduces sand pumping.
Entrance velocity through the screen openings is kept low (commonly not more than about 3 cm/s) to reduce head loss, incrustation and sand movement.

Well design steps

Aquifer data from test bores and logs (lithology, grain size, water quality).
Housing casing diameter to accommodate the pump (normally a few cm larger than the pump bowl).
Screen length — depends on aquifer thickness (confined aquifers: most of the thickness; unconfined: lower portion, commonly one-third to half).
Screen diameter and open area — keep entrance velocity within limits.
Slot size and gravel pack design.
Sanitary seal (grout) near the top.

Well development

Removes fine material from around the screen to increase permeability and yield and to stabilise the formation:

Overpumping — pumping at a higher rate than design.
Backwashing / surging — alternate pumping and reversing flow.
Compressed air — intermittent air surging and air lifting.
Mechanical surging — surge block moved up and down.
High-velocity jetting through the screen.
Chemicals — polyphosphates to disperse clays and drilling mud; acid treatment in limestone aquifers.
Hydrofracturing — in hard rock, to open fractures.

Pumping tests

Test	Procedure	Gives
Constant-rate test	Pump at constant $Q$ ; record drawdown with time in pumped and observation wells	$T$ , $S$ (Theis, Cooper–Jacob), boundaries, leakage
Step-drawdown test	Pump at successively higher rates for equal periods	Well loss, formation loss, well efficiency, optimum pumping rate
Recovery test	Record residual drawdown after pumping stops	$T$ (independent of pumping fluctuations)

FormulaWell losses (Jacob)

s_{w} = B Q + C Q^{2}

$B Q$ = formation (aquifer) loss (laminar); $C Q^{2}$ = well loss (turbulent flow through screen, pack and casing).

Plot $s_{w} / Q$ against $Q$ : straight line with slope $C$ and intercept $B$ .

Well efficiency $= \frac{B Q}{s _{w}} \times 100%$

FormulaTheis recovery method

s^{'} = \frac{2.303 Q}{4 π T} lo g_{10} \frac{t}{t ^{'}}

$s^{'}$ = residual drawdown; $t$ = time since pumping started; $t^{'}$ = time since pumping stopped. Slope per log cycle $Δ s^{'}$ gives $T = \frac{2.303 Q}{4 π Δ s ^{'}}$ .

Downloaded from civilgyantests.com · Not for resale. Verify every code clause and date against the official source.

Finished reading? Test yourself.

Practise Irrigation Engineering with the GSSSB Work Assistant Civil test series.

Open GSSSB Work Assistant Civil tests →