7. METODOLOGÍA
7.3 CARACTERIZACIÓN DE FACTORES Y VARIABLES DE ESTUDIO
1. Why is geotextile filter introduced below the rockfill layer of a typical box culvert? (BC1)
In a typical box culvert, at the interface between rockfill layer and sub-grade, a layer of geotextile filter is usually added to perform separation function. With the addition of geotextile filter layer, it avoids the intermixing of widely different soil granulations so as to reduce long-term settlement.
Moreover, it also prevents interpenetration of rockfill into sub-grade so that the deformed configuration of rockfill may impair its intended function of load spreading.
2. Can dowel bars be omitted in the joints of box culvert? (BC2)
Dowel bars in joint normally serve to maintain structural continuity by transmitting shear forces between adjacent concrete structures. For box culvert, the use of dowel bars in joints is essential owing to the following reasons:
(i) Without dowel bars, differential settlement would result and it leads to the formation of steps in the box culvert. As a result the flow capacity of box culvert would be reduced.
(ii) The steps in the box culvert would also provide locations to trap rubbish and debris.
3. What is the function of waterstops in joints of box culverts and drainage channels?(BC2)
The principal function of waterstops is to prevent liquids (e.g. water), water-borne materials and solids to pass through concrete joints. In essence, it aims at providing watertightness to the drainage channel.
Besides, waterstops in drainage channels or box culverts can also serve two other purposes: (i) to avoid water contacting joints’ dowel bars and causing corrosion. (ii) to avoid water seeping in from the underside of drainage channels or box culverts, thereby washing in soil particles and causing voids underneath these structures and finally leading to their failure. To serve the second purpose, obviously only one waterstop is
required at any depth location.
To serve the first purpose, a waterstop has to be installed on top of dowel bars to prevent water from drainage channels from leaking through. On the other hand, a waterstop has to be provided below dowel bars to avoid underground water from surging upwards.
In fact, the other way out to serve the first purpose is by using corrosion resistant bars.
4. What is the difference between inlet control and outlet control in hydraulic design of box culvert? (BC4)
In the hydraulic design of box culvert, there are two flow controls, namely inlet control and outlet control. In inlet control, the entrance characteristics of the box culvert (e.g. headwater depth and entrance configuration) determine the hydraulic capacity of the box culvert, and the culvert is actually capable of conveying a greater flow than the inlet would allow.
Barrel shapes and tailwater depth are of no significance in determining the hydraulic capacity. Inlet control usually takes place for culverts lying on steep slopes.
For outlet control, the inlet could accept more flow than the box culvert could carry and the hydraulic capacity of the box culvert is dependent on all hydraulic factors upstream from outlet tailwater.
5. Which one is better, single-cell box culvert and double-cell box culvert? (BC3)
The use of double-cell box culverts is preferred to single-cell box culverts for cross-sectional area larger than about 5m2 owing to the following reasons:
(i) Where there is tight headroom requirement, the use of double-cell box culvert can shorten the height of culverts by having a wider base so that the same design flow can be accommodated.
(ii) The invert of one cell can be designed at a lower level to cater for low flow condition so that it reduces the occurrence of sediment deposition and avoid the presence of standing waters.
(iii) The provision of temporary flow diversion can be easily provided for inspection and maintenance of each cell. During routine maintenance operation, water flow can be diverted to one cell and the other one is
open for desilting.
If a choice has to be made between a single-cell box culvert and smaller multiple pipes, it is better to select single-cell box culvert because of the lower risk of blockage when compared with smaller size of multiple pipes.
In addition, the hydraulic performance of a single-cell box culvert is better than multiple pipes system because of the larger hydraulic radius associated with the box culvert for a given cross-sectional area.
6. What is the purpose in providing beveled edge in the inlet of box culvert? (BC4)
The bevel is sometimes introduced at the inlet of box culvert to decrease the flow contraction at the inlet. In fact, the outlook of bevel is similar to a chamfer except that a chamfer is smaller in size and it is mainly used to prevent damage to sharp edges of concrete during construction. The bevels can be designed as plane or round edges.
The addition of bevels to the inlet of box culverts could increase the capacity of box culvert by about 5-10%. In fact, the socket end of precast concrete pipes serves the same function as the bevels in the box culvert.
Fig. Beveled edge in inlet
7. Should screens (trash or security) be always placed at culvert inlet?
(BC4)
Screens are provided at culvert inlet owing to one of the following reasons:
(i) To trap trash or debris which might otherwise accumulate in the
culvert and block the flow subsequently;
(ii) To prevent access into the culvert by children.
However, screens are not always placed at culvert inlet and they should be determined case by case. For trash or security screens installed in place, it would inevitably trap floating debris and rubbish. Unless they are regularly removed, it would eventually lead to rise in upstream water level leading to local flooding.
For new culvert, there is a trend that the use of trash screens is declining.
Trash screens are only placed in culvert inlet where there is a high risk of blockage history. Instead the need of trash screen can be eliminated by adopting the following design features:
(i) There are fewer changes of cross section, fewer bends and smooth transitions into the culvert. As such, it would provide fewer locations to trap debris and trash.
(ii) Provision of good access to the culvert to facilitate regular inspection.
(iii) The culvert should be not designed too long to enhance easy access to clear a blockage.
8. What are the differences in applications between pipe culverts and box culverts?
Basically, a culvert means a covered hydraulic structure which conveys fluid. Therefore in a broad sense, pipe culverts in a small scale represent normal pipes like precast concrete pipes.
In terms of hydraulic performance, circular section is the best geometrical sections among all. Therefore, for relative small discharge, precast concrete pipes and ductile iron pipes are normally used which are circular in shape. But for applications of very large flow, precast concrete pipes and ductile iron pipes may not be available in current market. In this connection, cast-in-situ construction has to be employed. It is beyond doubt that the fabrication of formwork for circular shape is difficult when compared with normal box culvert structures. However, circular shape is the most hydraulic efficient structure which means for a given discharge, the area of flow is minimum. Therefore, it helps to save the cost of extra linings required for the choice of box culverts.
However, box culverts do possess some advantages. For example, they
can cope with large flow situation where headroom is limited because the height of box culverts can be reduced while the size of pipe culverts is fixed.
Secondly, for some difficult site conditions, e.g. excavation of structure in rock, for the same equivalent cross-sectional area, the width of box culverts can be designed to be smaller than that of pipe culverts and this enhances smaller amount of excavation and backfilling.
Fig. Small spatial requirement of box culver than pipes.
9. What are the functions of the following features observed in a typical manhole? (i) groove near benching, (ii) R.S.J. (iii) double seal manhole cover and (iv) u-trap with rodding arm. (M1)
(i) The groove is used to facilitate the maintenance of manholes and sewer/drain pipes. Shutoff boards are erected on the grooves during maintenance operation so that water flow coming from upstream is terminated in the manhole and backwater from downstream is also blocked. In addition, the groove also facilitates water flow diversion for routine maintenance operation.
(ii) R.S.J. is a small-scale size of universal beams and is used for resisting the high stresses incurred by heavy traffic loads acting directly on the upper narrow projected section of manholes.
(iii) Double seal terminal manhole covers are used for sealing off gases emitted inside sewer/drains and prevent them from releasing out of the manhole.
(iv) U-trap with rodding arms is also used for sealing off unpleasant gas smell by the trapped u-shaped water columns. Rodding arm is normally closed with rubber rings during normal operation. However, during maintenance operation, the rubber ring is removed and rodding can be carried out through the rodding arm.
10. How can manholes be adapted to the final height of the pavement?
(M2)
Theoretically speaking, the whole manhole structures can be constructed as a whole instead of splitting into two stages of construction. With detailed calculation of longitudinal fall and cross fall of road pavement, it is possible to place and construct the manhole cover and frame in accurate levels and falls so that the whole manhole can be constructed in a single stage.
However, in actual practice it may not always be possible to accurately predict the inclination and levels of future manhole cover. As such, it is not uncommon that the construction of manholes is split into two stages:
(i) 1st Stage: The lower part of manholes is constructed up to the access shaft (about the level of sub-base). Then a rigid cover is placed over the access opening of the manholes and bituminous materials are placed on its top to construct the pavement.
(ii) 2nd Stage: The bituminous material around the rigid cover is removed and several courses of brickwork are normally placed on the access shaft to adjust the level of manhole cover and frame to fit with the final height of the pavement. Finally, manhole cover and frame are installed in position as shown in the sketch below.
Fig. Typical details of manhole cover and frame
11. Why are some manhole covers designed into two triangular halves? (M2)
Manhole covers are generally made up of two pieces of triangular plates to form a square cover [23]. One may wonder why two rectangular halves are
used for a rectangular cover. To understand this, one should note that a planar surface is usually in contact with three support points. For a triangular plate, the centre of gravity (CG) normally lies within the zone bounded by three support points even when loaded, thus creating a stable support system for the manhole cover. For a rectangular plate, the CG of a loaded manhole cover may lie outside the three support points, causing it to rock. Hence, the potential problem of rocking produced by vehicular traffic by rectangular traffic could be eliminated by using two triangular halves.
Fig. Different types of manhole covers.
12. What are the potential problems of split triangular manhole covers?
(M2)
In some countries, manhole covers are designed into split triangular shapes to reduce the effect of rocking from traffic. However, the two pieces of triangular covers should be bolted together. As for a piece of triangular cover, it is easily dropped into the rectangular hole of manhole during routine maintenance. Therefore, from maintenance point of view, some countries prefer another geometrical shape i.e. circular, as this is the only shape that the cover could hardly be accidentally dropped into the manhole.
On the other hand, for other geometrical shapes such as rectangle or square, they could still be dropped into their formed hole when inclined into proper angles.
13. Why are some manhole covers made of cast iron while some are made of ductile iron? (M2)
Traditionally, manholes covers are made of cast iron. However, in the viewpoint of pipe maintenance, frequent opening of manhole covers has to be carried out. Therefore, it poses potential safety hazard to the workers during the lifting-up process of manhole covers because cast iron manhole covers are very heavy to normal workers. Consequently, research has been conducted and ductile iron is considered as a better choice than cast iron because it can resist the same traffic loads with lower self-weight.
Moreover, as ductile iron is less brittle than cast iron, the traditional cast iron manhole covers are more susceptible to damage and thus requires higher maintenance cost.
However, ductile iron manhole covers do suffer from some demerits. For instance, owing to their relative low self-weight, vehicles passing over these manhole covers would lead to the movement of covers and generate unpleasant noises. To solve this problem, instead of increasing the self-weight of ductile iron manhole covers which similarly causes safety problems to workers during regular maintenance, the covers can be designed to be attached to the manhole frames which hold them in firm position.
14. The spacing of manholes in straight sections for different pipe sizes is stated in Stormwater Drainage Manual. How are these figures arrived at? (M3)
For pipe size < 300mm, rodding is usually adopted in which workers place about 1m long rods through the pipes to the location of blockage and manually operate the rod to clear the blockage.
For pipe size < 700mm, water-jetting is normally employed in which water is supplied from nearby fire hydrants and pressurized water jet is used for clearing blockage.
Winching method is adopted for all sizes of pipes.
For instance, for pipe size exceeding 1050mm, it is stated in Stormwater Drainage Manual that maximum intervals between manholes along straight lengths should be 120m. This is because for sizes over 1050m, the main method of pipe maintenance is by winching whose maximum length of operation is 120m. Similarly, the maximum intervals of manholes for other
straight pipes are derived from their corresponding maintenance methods.
15. What is the difference between road gullies and catchpits?
Both road gullies and catchpits are the two basic types of drainage inlets of drainage system. Though they are designed to catch stormwater, road gullies and catchpits are intended to catch stormwater at different locations.
Catchpits are designed to receive stormwater from slopes and stream courses. There is no standard design of catchpits and they can take different forms and shapes like inclusion of sand trap to improve the quality of collected stormwater and to prevent the blockage of drains. On the other hand, road gullies are intended to receive stormwater from roads only.
16. Is reinforcement needed in precast concrete manhole units? (M1) Precast concrete manholes are normally constructed by placing the bases of manholes firstly. The walls of precast manholes are formed by placing the precast concrete rings one on top of the other up to the required height.
Someone may notice that reinforcement used for resisting the lateral earth pressure and surface loads are not considered in some design. It is discussed in Concrete Pipe Association of Great Britain that analysis of soil pressures shows that standard unreinforced precast units are capable of resisting uniformly distributed pressures (e.g. loading condition in a manhole) down to a depth of 150m. If very severe road traffic and side loads are encountered, an additional concrete surround of about 150mm may be provided.
Fig. Precast concrete manhole
17. What is the difference between road gullies and catchpits?
Both road gullies and catchpits are the two basic types of drainage inlets of drainage system. Though they are designed to catch stormwater, road gullies and catchpits are intended to catch stormwater at different locations.
Catchpits are designed to receive stormwater from slopes and stream courses. There is no standard design of catchpits and they can take different forms and shapes like inclusion of sand trap to improve the quality of collected stormwater and to prevent the blockage of drains. On the other hand, road gullies are intended to receive stormwater from roads only.