The main function of PVC lining is to protect concrete surface against hydrogen sulfide attack. Moreover, it also guards against attack by a wide range of acids and alkalis. Hydrogen sulfide generated in sewerage by bacterial action under anaerobic (absence of oxygen) conditions is converted to sulphuric acid by aerobic bacteria growing on wet sewer walls. This acid reacts with the lime in the concrete, causing breakdown of the concrete pipes. The addition of lining could also improve the flow capacity.

Only the concrete above the line of minimum flow shall be attacked by hydrogen sulfide and therefore PVC lining is needed above this flow line. For medium to large pipe sizes, partial lining of 300 degree with 60 degree pipe invert exposed proves to be effective against attack. Small diameter pipe may warrant complete linings of pipes.

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 market price of vitrified clay pipes is generally less than that of precast concrete pipes with PVC lining. Therefore, for small size of pipes (pipe diameter less than 600mm) it is more economical to use vitrified clay pipes. However, vitrified clay pipes do suffer from the problem of brittleness and its effect is even severe for larger size of pipes. Moreover, it is rather time consuming to deliver clay pipes products because the majority of them are manufactured in Europe. Hence, for larger size of sewer pipes (diameter more than 600mm) it is customary to use precast concrete pipes with PVC lining.

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.

(i) It has no reaction with sewage and seawater and does not suffer from the corrosion problem associated with cast iron.

(ii) No protective coating is required and it is almost maintenance-free.

(iii) HDPE flap valves require very low opening pressure in operation (like 5mm water level difference). For cast iron flap valves, due to its own heavy self-weight, the required opening pressure of cast iron flap valves is higher than that of HDPE flap valves. This criterion is
essential for dry weather flow conditions.

However, the pressure resistance of HDPE flap valves is not as good as cast iron flap valves. For instance, a typical 450mm wide HDPE flap valve can only withstand about 5m water column.

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.

There are three scenarios of such connection arrangement:

(a) a new drainage system is connected to an existing drainage channel
(b) an existing drainage system is connected to a new drainage channel
(c) a new drainage system is connected to a new drainage channel.

For all scenarios, what engineers consider is the total amount of differential settlement or lateral movement to be encountered between the drainage system and main drainage channel. For scenario (b) and (c), it is very likely that substantial differential settlement will occur and this will cause damage to the connecting concrete pipes. Therefore a segment of short pipes are designed so that they serve to provide flexibility to the pipes in case of uneven settlement occurring between drainage system and main drainage channels.

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.

Stilling basins are usually introduced to convert supercritical flow to subcritcal flow before it reaches downstream. A typical stilling basin consists of a short length of channels located at the source of supercritical flow (e.g. end of spillway). Certain features are introduced to the basins like baffles and sills to provide resistance to the flow. As such, a hydraulic jump will form in the basin without having conducting significant amount of excavation for the stilling basin if baffles are installed.

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.