Transportation and Environmental Engineering — ESE Civil

Weightage: Transportation engineering and environmental engineering are two of the shorter Civil blocks, but both are highly formula-based and predictable. Sight distance, superelevation, overflow rate and BOD calculations recur almost every cycle.

1. Highway geometric design

Highway design rests on driver behaviour and vehicle dynamics.

Stopping sight distance (SSD) is the sum of the distance covered during the perception-reaction time and the braking distance:

$SSD=vt+\frac{v^2}{2g(f\pm n)}$

with $v$ in m/s, $t$ the reaction time (the IRC uses 2.5 s), $f$ the longitudinal friction and $n$ the gradient (plus for upgrades, minus for downgrades).

Worked example. At $v=72\text{km/h}=20\text{m/s}$, $t=2.5$ s and $f=0.35$ on level ground, $SSD=50+400/(2\times 9.81\times 0.35)=50+58.2=108$ m.

Overtaking sight distance is far longer than SSD, because it covers the overtaking vehicle's reaction, its pass and the opposing traffic. Sight distances on vertical curves decide their length: summit curves are designed for sight distance, valley curves for headlight distance and comfort.

2. Horizontal curves

A vehicle on a curve needs a centripetal force from superelevation $e$ and side friction $f$:

$e+f=\frac{v^2}{gR}$

IRC limits superelevation to about 7 percent on plain and rolling terrain, with friction taken as 0.15. Radius is then $R=v^2/g(e+f)$. A transition curve (spiral) introduces the curvature gradually, and extra widening is given on sharp curves.

3. Pavements

California bearing ratio compares the penetration resistance of soil with that of standard crushed stone. A lower CBR needs a thicker pavement. Rigid slabs have joints: expansion, contraction and longitudinal, with dowel bars carrying load across transverse joints and tie bars holding longitudinal ones. Bitumen is graded by penetration (for example 60/70) or viscosity.

4. Traffic engineering

The basic relations are:

$q=kv$

with flow $q$, density $k$ and space-mean speed $v$. Greenshields' linear model gives a capacity of $q_{max}=v_f{k}_j/4$ at half the jam density. Mixed traffic is converted to passenger car units (PCU). Signal design uses Webster's optimum cycle $C_o=\frac{1.5L+5}{1-Y}$, where $L$ is lost time and $Y$ the sum of critical flow ratios.

5. Railway basics

Indian broad gauge is 1676 mm. Track components are rails, sleepers, ballast and subgrade. Creep is the gradual movement of rails in the direction of traffic. A cant (superelevation) on curves is $e=G{V}^2/(127R)$ with $V$ in km/h and $R$ in metres. Points and crossings let trains change tracks.

6. Water supply

The design population and the per-capita demand set the capacity. A common design allowance is 135 litres per capita per day for domestic use in many Indian cities, with extra for fire, industry and losses. Check the current norm in the source provided.

Treatment steps for surface water follow the source quality:

1. Screening.
2. Coagulation and flocculation: alum makes colloids clump.
3. Sedimentation.
4. Filtration: rapid sand (about 5000 to 6000 litres per square metre per hour) or slow sand (about 100 to 200).
5. Disinfection: chlorination, with the residual maintained in the network.

The overflow rate of an ideal settling tank is $v_0=Q/A$. A particle settles out if its settling velocity exceeds this, and the removal does not depend on depth. Stokes' law gives the velocity of a small sphere: $v_s=g(G_s-1)d^2/(18\nu )$.

Worked example. A tank handles $Q=2400{\text{m}}^3/\text{day}$ with plan area $80{\text{m}}^2$. The overflow rate is $30\text{m/day}$, so particles settling faster than 30 m/day are fully removed.

Hardness is removed by lime-soda or ion exchange. The Indian drinking water standard is IS 10500, which sets acceptable and permissible limits for each parameter.

7. Wastewater and its treatment

Sewage strength is measured by BOD. The oxygen demand follows first-order kinetics:

$BO{D}_t=L_0\left(1-e^{-kt}\right)$

with $L_0$ the ultimate demand. The standard test is BOD at 5 days and 20 degrees C.

Worked example. With $k=0.23{\text{day}}^{-1}$ (base e) the five-day BOD is $L_0(1-e^{-1.15})=0.68L_0$, so a five-day BOD of 200 mg/L means $L_0\approx 293$ mg/L.

Treatment levels:

In the activated sludge process the key parameters are the food-to-microorganism ratio $F/M$, the mean cell residence time and the sludge volume index. Sewers are designed for self-cleansing velocity, about 0.6 to 0.9 m/s, and flow at partial depth.

8. Solid waste and air

Solid waste management follows a hierarchy of reduce, reuse, recycle, recover and sanitary landfill. Municipal waste in India has a high organic fraction, so composting and biomethanation suit it. Air pollution control uses cyclones, bag filters, electrostatic precipitators and scrubbers, and for vehicles catalytic converters.

Common traps

• Mixing speeds in km/h and m/s in the SSD equation.
• Forgetting the gradient term or using its sign wrongly.
• Thinking tank depth affects ideal sedimentation. Only plan area does.
• Mistaking $BO{D}_5$ for the ultimate demand.
• Using the wrong constant for cant. 127 applies with $V$ in km/h.

Memory aids

• "Reaction plus braking": SSD.
• "e plus f equals v squared over gR": curves.
• "Surface loading is Q over A": ideal settling.

Summary

Highway design relates sight distance, superelevation and friction to speed, and pavements are sized from subgrade strength and traffic. Traffic flow follows $q=kv$.

Environmental questions follow the treatment train: coagulation, settling, filtration and disinfection for water, and preliminary through tertiary stages for sewage, with BOD kinetics as the main calculation.

Exam protocol

• Convert every speed to m/s before using SSD.
• Write the governing equation, then the numbers.
• Use plan area, not depth, for ideal sedimentation.
• Confirm design norms against the cited standard.