RRB JE Junior Engineer · Technical Abilities (CBT-2)
Civil Engineering (CBT-2)
Building materials, surveying, soil mechanics, RCC and steel, hydrology and environmental engineering, and quantity estimating at diploma depth for RRB JE CBT-2 Technical…
A Junior Engineer on a railway works site does not meet civil as eight separate papers. The same 12 m by 8 m station office needs a mix that will harden, a surveyed rectangle on the ground, soil that can carry the walls, steel where the slab sags, a drain that can take the rain, and a bill that measures what was built. Six ideas, in that site order.
- RRB JE Junior Engineer
- Medium level
- 6 concepts
- 50 practice questions
1A mix is a paste plus stone
A building material is whatever we ask to stay in place and carry load or weather. Cement is a powder that hydrates: it reacts with water and becomes a stone-like paste. Concrete is that paste plus sand (fine aggregate) plus stone (coarse aggregate). The water-cement ratio is the mass of water divided by the mass of cement in the mix — it is the knob that trades strength against workability.
More water makes the mix easier to place and leaves more leftover voids after hydration, so strength falls. A first-class brick and a piece of Fe415 steel are different materials doing different jobs — the brick is a unit of wall, the bar is tension capacity — but both are specified by a grade, not by a brand name.
Figure. The diploma nominal mix for M20 is 1 : 1.5 : 3 by volume — cement, sand, coarse aggregate. Water is a fourth ingredient, set by W/C, and is not a bar on this figure.
How a batch is set
- Weigh the cementThe bag is the cement mass. Everything else in the ratio is measured from this number, not guessed from the look of the heap.
- Choose W/C firstPick the water-cement ratio the grade needs before anyone opens a hose. Water mass is that ratio times the cement mass.
- Do not add site waterExtra water on the platform raises W/C after the mix was designed. Workability improves for a few minutes; the hardened strength does not.
Grade names a property of the substance, not a supplier.
| Material | What it is | Diploma cue |
|---|---|---|
| OPC cement | Hydrating binder | Initial set ≥ 30 min; final ≤ 10 h |
| Concrete | Paste + fine + coarse | M20 nominal mix 1 : 1.5 : 3 |
| First-class brick | Fired clay unit | Modular 190 × 90 × 90 mm |
| HYSD bar | Tension steel | Fe415 means f_{y} = 415 N/mm² |
The extra bucket
The station-office slab is batched from a 50 kg bag of cement at water-cement ratio 0.50. A mason then pours in 5 more litres of water to make placing easier.
- Water at W/C = 0.50: 0.50 \times 5025 kg = 25 L
- Water after the extra 5 L30 L
- New W/C = 30 / 500.60
Pro tip. The mix did not become 'richer'. Cement stayed 50 kg; only water rose, so W/C rose and the hardened strength falls.
A 50 kg bag is already batched at water-cement ratio 0.50. Ten extra litres of water are then added on the platform. What happens?
- W/C stays 0.50; extra water only helps placing
- W/C becomes 0.70 and the hardened strength falls
- W/C falls and the concrete becomes richer
Designed water is 25 L. Ten extra litres make 35 L, so W/C = 35/50 = 0.70. Cement mass did not rise, so the mix is wetter, not richer, and strength falls.
2A larger staff reading is a lower point
A survey is a measured picture of the ground so the 12 m by 8 m office can be set out where the drawing says, and so a drain can fall the right way. A level instrument makes a horizontal line of sight. A staff standing on a point reads how far below that line the point is. A larger staff reading means a lower point.
The height of collimation is the reduced level of that line of sight: add the backsight on a known bench mark. Subtract a foresight to get the reduced level of a new point. A bearing is a different measurement — direction in the horizontal plane, not height.
Figure. The dashed line is the horizontal line of sight. Staff-reading lengths are in the ratio 1.250 : 2.150, so the right-hand ground is lower. The slope of the ground between the two staffs is schematic.
How a level shot is reduced
- Known bench markStart from a point whose reduced level is already known. That is the bench mark.
- Backsight to HIRead the staff on the bench mark (backsight). Height of collimation is bench-mark RL plus that backsight.
- Foresight to new RLRead the staff on the new point (foresight). New RL is HI minus that foresight. If the foresight is larger than the backsight, the new point is lower.
A level shot and a compass bearing answer different questions about the same site.
| Quantity | What it measures | Instrument cue |
|---|---|---|
| Reduced level | Height of a point | Level + staff |
| Height of collimation | Height of the line of sight | RL + backsight |
| Bearing | Horizontal direction | Compass / theodolite |
| Contour | Line of equal RL | Join equal readings |
The office-corner level shot
A bench mark beside the station office has RL 100.000 m. The backsight on it is 1.250 m. The foresight on the proposed floor corner is 2.150 m.
- HI = 100.000 + 1.250101.250 m
- Corner RL = 101.250 - 2.15099.100 m
- Fall = 2.150 - 1.2500.900 m
Pro tip. The foresight is larger, so the corner is lower than the bench mark. Adding the foresight instead of subtracting it is the usual arithmetic trap.
Height of collimation is 52.400 m and the foresight on a drain invert is 1.850 m. What is the invert RL?
- 54.250 m
- 50.550 m
- 52.400 m
RL = HI − FS = 52.400 − 1.850 = 50.550 m. Adding the foresight (54.250 m) treats a staff reading as if it sat above the line of sight.
3Soil is particles plus voids
Soil is not a solid block. It is mineral particles with voids between them. Those voids hold air, water, or both. Void ratio is the volume of voids divided by the volume of solids — e = V_{v}/V_{s} — a property of the packing, not a brand of soil.
Compaction on a site squeezes air out of the voids with a roller; the particles stay the same. Consolidation is slower: a load squeezes water out of a saturated clay over months. A foundation is how the office's load reaches the soil. Safe bearing capacity is the pressure we are allowed to put on that soil; the footing area must be at least load divided by that pressure.
Figure. Void ratio of the office fill before and after rolling, from the worked volumes. The solids volume is the same in both bars; only the voids changed.
How a footing is sized
- Name the loadThe column or wall load is the force the foundation must hand to the soil, in kN.
- Name the allowed pressureSafe bearing capacity is that allowed pressure, in kN/m². It belongs to the soil at that depth, not to the concrete grade.
- Area at least load / SBCFooting area must be at least load divided by SBC. A 1.20 m by 1.20 m pad is 1.44 m² — use it only when that number clears the required area.
The criterion is what leaves the voids, and how fast.
| Idea | What leaves the voids | Time scale |
|---|---|---|
| Compaction | Air, by a roller | Hours on site |
| Consolidation | Water, under a load | Months in clay |
| SBC | Nothing leaves — a pressure limit | A number for sizing |
| Shallow footing | Load spread near the surface | Strip, pad, raft |
The same solids, fewer voids
A 1.000 m³ fill sample under the office has 0.600 m³ of solids and 0.400 m³ of voids. After rolling, the solids are still 0.600 m³ and the voids have become 0.300 m³.
- e_{1} = 0.400 / 0.6000.667
- New total volume = 0.600 + 0.3000.900 m³
- e_{2} = 0.300 / 0.6000.500
Pro tip. Solids did not shrink. Compaction changed only the voids, so e fell. Porosity uses the current total volume: n_{2} = 0.300 / 0.900 = 0.333.
A clay layer under a new embankment loses water over two years and the ground surface settles. Which process is that?
- Compaction — a roller squeezed air out
- Consolidation — the load squeezed water out slowly
- A rise in safe bearing capacity, which is the same thing
Water leaving a saturated clay under load, over years, is consolidation. Compaction is air leaving under a roller in hours. SBC is a pressure limit, not a process.
4Concrete squeezes; steel pulls
Plain concrete is strong when you squeeze it and weak when you pull it. Steel is strong in tension. Reinforced cement concrete is the partnership: concrete takes the compression, steel takes the tension, and they stay together because the steel is ribbed and is given cover so it does not rust.
On the station-office roof — a simply supported slab — the soffit sags, so the bottom fibres stretch and the main steel sits near the bottom. A cantilever is the other way up: the top fibres stretch, so the steel sits near the top. Limit-state design sizes that steel so the section fails in a ductile way (steel yields first) rather than by crushing the concrete with no warning.
Figure. Schematic sagging section. Breadth-to-depth follows 230:400. The two small squares mark the tension bars — not bar diameter. Compression is the top block; the dashed line is the neutral axis.
Where the main steel goes
- Name the spanSimply supported and continuous mid-span sag; a cantilever hog. The face that stretches is the tension face.
- Steel on the tension faceMain bars sit near that face, with cover outside them. Cover is a rust-and-fire thickness, not leftover concrete.
- Prefer under-reinforcedThe steel should yield before the concrete crushes. That is the diploma meaning of a ductile, under-reinforced section.
IS 456 nominal cover (mild exposure) — a starting cue, not a substitute for the exposure table.
| Member | Tension face | Nominal cover |
|---|---|---|
| Simply supported slab | Bottom | 20 mm |
| Simply supported beam | Bottom | 25 mm |
| Cantilever slab / chhajja | Top | 20 mm |
| Footing | Bottom (over soil) | 50 mm |
The 3 m roof strip
A 1 m wide strip of the simply supported office roof spans 3.0 m and carries 8 kN/m² including self-weight.
- Strip load w = 8 \times 18 kN/m
- M = wL^{2}/8 = 8 \times 3^{2} / 89 kN·m
Pro tip. That 9 kN·m is a sagging moment, so the bottom fibres stretch. The main steel for this strip belongs near the soffit, not at mid-depth.
A 2 m cantilever chhajja projects from the office wall. Where do the main bars go?
- Near the bottom, as in every slab
- Near the top, because the top fibres stretch
- At mid-depth, so they can take both faces
A cantilever hogs: the top fibres stretch. Main steel sits near the top. Mid-depth steel is not 'both faces', and copying the simply-supported soffit rule is the trap.
5Discharge is a volume per second
Discharge is the volume of water that passes a section in one second: Q = AV, area times mean velocity. Continuity says that volume is not lost between two sections of the same pipe or drain — if the section halves, the water must run twice as fast. Irrigation duty is that same discharge, asked a land question: how many hectares one cubic metre per second can serve through a crop season.
The station office also uses water and throws water away. Water supply is the same volume cleaned — coagulation, settling, filtration, chlorination — until a residual chlorine of about 0.2 mg/L remains at the tap. Sewage is the volume collected after use; BOD is how much oxygen the microbes in it will demand as they eat the organic load. A drain sized for rain is a hydraulics object; a tap that is safe to drink is an environmental one. Both start from a volume.
Figure. Two frozen sections of one discharge, not a movie of water moving. Widths are 2:1 and the arrows are 2:1, so A_{1}V_{1} = A_{2}V_{2} = 0.192 m³/s.
How a drain becomes a command
- Area of the sectionFor a rectangular drain, area is width times flow depth. That is A, not the full lined depth if the water is lower.
- Discharge Q = AVMultiply by the mean velocity. One cubic metre per second is one cumec — the unit irrigation duty is written in.
- Land from dutyCommand area is Q in cumec times duty in hectares per cumec. Duty is a given for the crop and canal, not a second velocity.
Hydraulics sizes the moving volume; irrigation and environmental engineering decide what that volume is for.
| Job | What we ask | Diploma cue |
|---|---|---|
| Drain / pipe | How much passes | Q = AV; continuity |
| Irrigation | How much land one cumec serves | \Delta = 8.64 B / D |
| Water supply | A safe volume at the tap | About 135 L/person/day; residual Cl ~ 0.2 mg/L |
| Sewage | The used volume, then cleaned | BOD = oxygen the microbes will demand |
The side drain, then the field
The office side drain is 0.60 m wide with water 0.40 m deep, flowing at 0.80 m/s. A canal carrying that same discharge has a duty of 800 hectares per cumec.
- A = 0.60 \times 0.400.24 m²
- Q = 0.24 \times 0.800.192 m³/s
- Command = 0.192 \times 800153.6 ha
Pro tip. The 800 ha/cumec is a land rating of the same 0.192 cumec. It is not a second velocity, and it does not change Q.
A pipe of area 0.20 m² carries water at 1.2 m/s into a section of area 0.10 m². If discharge is unchanged, the velocity in the narrow section is
- 0.6 m/s
- 1.2 m/s
- 2.4 m/s
Q = 0.20 \times 1.2 = 0.24 m³/s. Then V = 0.24 / 0.10 = 2.4 m/s. Halving the area halves the velocity only if someone has already thrown continuity away.
6A road sheds water; a bill names a unit
A road is a surface shaped so rain leaves it. Camber is the cross-fall from the crown to each edge; on the 4 m approach at 2.5 percent the crown sits 50 mm above the edges. Gradient is the fall along the length. Those shapes are design decisions, not decorations — standing water on a bituminous surface is a failure of camber, not of the mix.
Estimation is how we measure the work so it can be paid. Each item has a unit that matches the thing: brickwork and concrete in cubic metres, plastering and painting in square metres, reinforcement in kilograms. The centre-line method walks the wall's heart-line once, then multiplies by height and thickness. Openings are deducted after, not forgotten.
Figure. Approach-road camber: 2.5 percent of each 2 m half-width is 50 mm. The crown rise is exaggerated about 24 times so the fall is visible; a true 50 mm on 4 m would be 0.0125 of the carriageway width and would vanish.
How a wall is billed
- Walk the centre lineFor a rectangular plan, centre-line length is 2(L - t) + 2(B - t), where t is wall thickness and L, B are external sides.
- Multiply by height and tBrickwork volume is that length times storey height times thickness. The unit is m³ because a wall is a solid.
- Deduct openingsEach door or window is width times height times the same thickness, subtracted after. Painting those same faces later is m², not m³.
Bill the surface you built, in the unit of that surface.
| Item | Unit | Why |
|---|---|---|
| Brickwork / PCC / RCC | m³ | A solid volume |
| Plaster / paint / flooring | m² | A finished face |
| Reinforcement | kg | A mass of bars |
| Camber (4 m @ 2.5%) | 50 mm rise | 2.5% of each 2 m half-width |
The office walls, then the doors
The station office is 12 m by 8 m external, walls 0.30 m thick and 3.00 m high, with two doors 1.00 m by 2.10 m. Find the net brickwork.
- Centre line = 2(12 - 0.30) + 2(8 - 0.30)38.80 m
- Gross = 38.80 \times 3.00 \times 0.3034.92 m³
- Doors = 2 \times 1.00 \times 2.10 \times 0.301.26 m³
- Net brickwork = 34.92 - 1.2633.66 m³
Pro tip. Long-wall out-to-out plus short-wall in-to-in is the same 38.80 m: 2 \times 12 + 2 \times (8 - 2 \times 0.30). Use either walk; do not mix them in one bill.
The office inner walls are to be painted after plaster. In which unit is the painting billed?
- m³, because it is a civil item
- m², because painting is a finished face
- kg, because paint is bought by mass
Painting is a finished face, so m². m³ is the trap that bills every civil item as a solid. The tin may be bought in kg; the bill of quantities is not.
Notes
- Concrete is a paste plus aggregate: Cement hydrates with water into a stone-like paste, and concrete is that paste plus fine and coarse aggregate. The water-cement ratio is water mass over cement mass - more water places more easily and leaves more voids after hydration, so hardened strength falls. The M20 nominal mix is 1 : 1.5 : 3.
- Levelling - a larger staff reading is a lower point: Height of collimation is the bench-mark reduced level plus the backsight, and a new point’s reduced level is that collimation minus its foresight. A bearing is a horizontal direction, not a height.
- Soil is particles plus voids: Void ratio is e = V_{v}/V_{s}, a property of the packing. Compaction drives air out of the voids with a roller in hours, while consolidation squeezes water out of a saturated clay over months. Safe bearing capacity is a pressure limit, so the footing area must be at least the load divided by that pressure.
- RCC - concrete squeezes, steel pulls: Main steel sits near the face that stretches, the bottom of a simply supported slab or beam and the top of a cantilever, with cover outside it against rust and fire. Limit-state design prefers an under-reinforced section so the steel yields before the concrete crushes.
- Discharge is a volume per second: Q = AV, and continuity keeps that volume between two sections of the same channel, so halving the area doubles the velocity. The same volume is rated as land by irrigation duty, as a safe supply after coagulation, settling, filtration and chlorination, and as sewage measured by its BOD.
- Camber, gradient and the billing unit: Camber is the cross-fall from crown to edge and gradient the fall along the length, so standing water on a surface is a camber failure rather than a mix failure. Estimation bills brickwork and concrete in cubic metres, plaster and paint in square metres and reinforcement in kilograms, and the centre-line method walks the wall’s heart-line once before openings are deducted.
Formulas
- Water-cement ratio = mass of water / mass of cement. More water places more easily and leaves more voids, so hardened strength falls. M20 nominal mix is 1 : 1.5 : 3.
- Levelling: height of collimation = bench-mark RL + backsight; RL of a new point = collimation - its foresight. A larger staff reading means a lower point.
- Soil: void ratio e = V_{v}/V_{s}. Compaction drives air out in hours with a roller; consolidation squeezes water out of saturated clay over months. Required footing area = load / safe bearing capacity.
- RCC: main steel goes near the face that stretches — bottom of a simply supported span, top of a cantilever — with cover outside it. Under-reinforced sections yield the steel before the concrete crushes.
- Flow: Q = AV, and continuity holds the volume between two sections, so halving the area doubles the velocity. Water treatment order is coagulation, sedimentation, filtration, chlorination.
- Estimation units: brickwork and concrete in cubic metres, plaster and paint in square metres, reinforcement in kilograms. Camber is the cross-fall across the road; gradient is the fall along it.
Exam traps & shortcuts
- Adding water improves workability and lowers hardened strength. Any option that raises both is wrong.
- In levelling, the larger staff reading is the lower point. Reversing that sign flips every reduced level in the table.
- Compaction removes air in hours; consolidation removes water over months. Papers swap the two in one-word options.
- Cantilever main steel goes on top, simply supported main steel at the bottom. Draw the stretched face before choosing.
- Standing water on a carriageway is a camber failure, not a mix failure — camber is across the road, gradient along it.
- Watch the billing unit: plaster and paint are square metres while concrete and brickwork are cubic metres.
Reference tables
One running site. Every concept returns to these numbers rather than inventing a new building.
| Piece | Number | Used in |
|---|---|---|
| Plan, external | 12 m × 8 m | Survey set-out; brickwork centre line |
| Walls | 0.30 m thick, 3.00 m high | Foundation load path; net brickwork 33.66 m³ |
| Roof strip | 3.0 m span, 8 kN/m² | Sagging moment 9 kN·m; steel at soffit |
| Approach | 4 m wide, camber 2.5% | Crown 50 mm above edges |
| Side drain | 0.60 m × 0.40 m at 0.80 m/s | Q = 0.192 cumec |
Recap
If you keep only pegs the night before CBT-2, keep these.
- W/C
- More water, same cement: W/C rises and hardened strength falls. M20 nominal 1 : 1.5 : 3.
- Staff reading
- Larger reading = lower point. RL = HI − FS. HI = BM RL + BS.
- Voids
- e = voids / solids. Compaction drives air out now; consolidation drives water out over months. Footing area ≥ load / SBC.
- RCC steel
- Simply supported: steel at the bottom. Cantilever: steel at the top. Concrete compresses; steel pulls.
- Discharge
- Q = AV. Same Q, half A → double V. Duty rates that Q as hectares. Residual chlorine ~ 0.2 mg/L at the tap; BOD is oxygen demand.
- Units
- Brickwork and concrete in m³; plaster and paint in m²; steel in kg. Centre line × height × thickness, then deduct openings.
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