Highway Engineering

A sag gully is a gully installed at a low point in roads and stormwater would pond up the gully. An on-grade gully is a gully installed in a sloping road where any excess flows may bypass the inlet and flow to another one downstream. Generally speaking, the entry capacity of sag gully can be estimated from hydraulic design. But for on-grade gully, owing to complicated hydraulic behaviour, the entry capacity can hardly be predicted in accuracy by mere calculation.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Overflow weirs should be provided for steep roads (longitudinal gradient>5%) , flat roads (longitudinal gradient<0.5%), sag points and blockage blackspots. For steep roads, flow is rapid and overflow weirs should be provided to accommodate the excess flow. For flat roads, the probability of accumulation of rubbish increases. Therefore, overflow weirs should be provided in these locations to bypass the stormwater flow in case of blockage of gullies caused by trapping of rubbish. [wp_ad_camp_1] Basically, kerb overflow weirs suffer from the drawback that it provides another passage for debris to enter the gullies and therefore bars (either horizontal or vertical) should be provided to prevent the entry of debris into the weirs. For steep roads, as the main concern is to provide an alternative route for excess flow, horizontal bars should be provided in this case to maintain better drainage efficiency. For flat roads, the purpose of overflow weirs is to trap rubbish and therefore, vertical bars should be provided because it is more effective in prevention of entry of debris. This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Grease and oils are commonly found in stormwater runoff from catchments. They come from the leakage and spillage of lubricants, fuels, vehicle coolants etc. Since oils and grease are hydrocarbons which are lighter than water, they form films and emulsions on water and generate odorous smell. In particular, these hydrocarbons tend to stick to the particulates in water and settle with them. Hence, they should be trapped prior to discharging into stormwater system. Oil interceptors are installed to trap these oil loads coming from stormwater. In commercial areas, car parks and areas where construction works are likely. It is recommended to establish oil-trapping systems in these locations.

Typical oil interceptors usually contain following compartments:

(i) The first inlet compartment serves mainly for the settlement of grits and for the trapping of floatable debris and rubbish.

(ii) The second middle compartment is used for separating oils from runoff.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The basic mechanism of noise absorptive material is to change the acoustic energy into heat energy. The amount of heat generated is normally very small due to the limited energy in sound waves (e.g. less than 0.01watts). The two common ways for energy transformation are:

(i) Viscous flow loss
The absorptive material contains interconnected voids and pores into which the sound energy will propagate. As sound waves pass through the material, the wave energy causes relative motion between the air particles and the absorbing material and consequently energy losses are incurred.

(ii) Internal fiction
The absorptive materials have some elastic fibrous or porous structures which would be extended and compressed during sound wave propagation. Other than energy loss due to viscous flow loss, dissipation of energy also results from the internal friction during its flex and squeezing movement.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Steel beam barrier consists mainly of horizontal rails and vertical posts. When a vehicle hits the steel beam barrier, the kinetic energy is resolved in three components, namely vertical, normal to barrier and parallel to barrier. The vertical and normal components of kinetic energy are dissipated through deformation and bending of beam and supporting posts. As such, the remaining component (i.e. parallel) guides the vehicle back to the carriageway in a direction parallel to the barrier.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The majority of local parapets are 1.1m high and they are designed to resist impact from a 1.5ton car moving at a speed of 113km/hr. In some locations such as in the vicinity of railway lines, barriers with 1.5m high are provided to contain a vehicle with 24ton at a speed of 50km/hr.

The impact situation for vehicles varies from event to event and they are dependent on the speed, size and angle of incidence of the impacting vehicle. Though full-scale crash test is the simplest way to prove their performance, computer simulation has been used extensively owing to its lower in cost. Based on the results of computer simulation and crash tests, it is established that the said parapets comply with international standard for safe usage.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Parapets are designed to satisfy different containment levels. The containment level represents the magnitude of impact that the parapet is supposed to uphold.

A parapet designed as low containment level can hardly withstand the impact by large vehicles which may even damage the parapet. On the other hand a parapet designed as high containment level can effectively contain safety large vehicle. However, when it is collided by light vehicles, it is expected that it would cause considerable damage to the light vehicles and its passengers on board. Therefore, strong parapets may not necessarily mean a good parapet.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Basically the major problem associated with the collision of double-decked bus lies on the possible overturning of the bus upon collision. The overturning moment is the product of impact force and the difference in the centre of gravity of bus and the height of vehicular parapet. The restoring moment is the product of bus weight and 0.5 times the width of bus.

In fact, owing to the elastic deformation of both the parapet and bus, it is expected that the impact force, and hence the overturning moment may not be larger than the restoring moment for 1.1m high vehicular parapet. Computer simulations have to be conducted to verify if a double-decked bus traveling at a certain speed would roll over the parapet when impacted at a certain angle.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In general there are several main conditions which warrant the installation of safety barriers along roads:

(i) It protects vehicles from hitting a roadside object (e.g. bridge pier, sign post, walls etc). Conversely, it protects the roadside object from damage by collision of vehicles.

(ii) It avoids the crossing over of vehicles over central median.

(iii) It protects the vehicle from falling down a steep slope (more than 3m high)

(iv) A poor record of accidents involving run-off vehicles.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Tensioned corrugated beam is mainly used in high speed road while untensioned corrugated beam is mainly used in low speed road.

Tensioned corrugated beam is designed mainly for use on central reservation. When a vehicle crashes into tensioned corrugated beam, the beam remains in tension while the post gives way to allow for deformation.
Tensioned corrugated beam absorbs impact energy by deflecting as a whole and helps the vehicle decelerate, and at the same time guide it back towards the carriageway in a gradual and controlled manner. Tensioned corrugated beam is normally not used on curves with radius less than 120m.

Untensioned corrugated beam is commonly used for road bends where radius of curvature is small as the use of untensioned beam does not require tensioning device. It is designed to deform in beams and to re-direct impacting vehicles on a course as close as possible parallel to the barrier.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.