Watertightness in joints of drainage channels and box culverts
By Vincent T H Chu
The question of the need for watertightness in the joints of drainage channels is often raised by graduate engineers. In particular, many graduate engineers and assistant engineers have asked me about the need to provide a waterstop in the joints of drainage channels. This article serves to clarify the issue and eliminate their uncertainty when confronted with this issue again during their day-to-day design work.
In designing the joints for box culverts and drainage channels, watertightness is usually required to maintain water flow in the channel and is normally achieved by the installation of a waterstop across the joints of the drainage structure. The plain, dumb-bell type waterstop is used for joint locations where small movements are anticipated. Therefore, this type of waterstop is desirable for construction joints. On the other hand, the centre-bulb type waterstop is suitable for expansion joints or locations where lateral and shear movements occur due to settlement or deflection.
It is a common engineering practice to design weepholes along the walls of drainage channels and box culverts to relieve the water pressure so that its effect can be ignored during the design of channel walls and box culverts. Given the presence of weepholes, it would appear that the watertightness achieved by installing waterstops across the joints is meaningless because water flowing in the channel can escape through the weepholes on the walls of the drainage structures.
If the purpose of maintaining watertightness in the joints of drainage structures is to reduce the amount of water loss through joints, the above argument would be correct. However, the main reason for achieving watertightness across the joints in drainage structures is not directly related to the amount of water lost in drainage channels and box culverts.
Watertightness in the joints of drainage channels and box culverts is considered desirable under the following two scenarios:
(i) Where there is a high water table in the vicinity of the box culverts/channels, waterstops should be installed across the joints. The high groundwater level and rainwater seepage through the embankment may cause water to pass through the joints and into the adjacent soil. The loss of soil may lead to failure of the structure when it is subject to severe loads.
(ii) Where the box culvert/channels are designed in such a way that water flowing through the joints washes out the bedding materials, joint watertightness has to be fulfilled. Otherwise there may be a structural failure as in the scenario mentioned above, when the box culvert/channel is subject to heavy loads.
Apart from the concern that foundation materials may be washed away, watertightness in the joints of drainage channels and box culverts is considered beneficial because it eliminates the potential problem associated with the corrosion of dowel bars across the joints. In essence, dowel bars serve to maintain the alignment of the structure when the structure experiences contraction and expansion throughout its design life; as well as to transfer shear forces across the joints.
If the dowel bars malfunction due to corrosion, the joints would be subject to additional stresses which may lead to the structure's failure. Where a drainage channel/box culvert is expected to be continually in contact with seawater, which contains various corrosive agents, watertightness in joints is recommended, to avoid corrosion in the dowel bars. In cold countries salt is sometimes applied on roads above box culverts or at channel crossings to prevent freezing and thawing. The leaching of road salts into the joints may cause corrosion of the joint reinforcement.
About the author: Vincent T H CHU is a civil engineer with a government works department.