A trench drain consists of drain wrapped in geotextile, see Figs. 13.13-13.15. The drain is made of a mineral material such as a rounded or crushed aggregate. Originally either no carrier pipes orun-jointed pottery pipes were employed at the bottom of such drains. Nowadays, several materials are used for this type of pipe, from perforated or porous concrete, to PVC and fibreglass, the last ones with grooves or perforations. The pores, joints, perforations or grooves are designed to allow water collection. Typical diameters vary from 150 to 200 mm, with a longitudinal gradient that satisfies the self-cleaning condition (> 0.25%). Whenever these drains reach
|
Fig. 13.13 Conventional trench drain |
their maximum capacity, a lateral pipe with the adequate discharge capability should be placed underneath the drains to take away water from the trench. The geotextile is employed as a filter which prevents migration of fine soil particles into the drain and its silting-up. The water permeability of the geotextile should allow water to flow freely from the surrounding ground into the drain. The characteristic pore opening size, 090, of the geotextile used for the trench drain is selected to prevent mixing of soil with aggregate (see Section 13.3.9). Procedures for selection are provided by (e. g.) Christopher & Holtz (1985), Joint Departments (1995) or Koerner (2005).
2 Readers may like to identify violations of safe working practice which can be seen in this picture. The inclusion of this photograph does not mean that the authors condone such practice!
The cross-sectional area of drain is determined depending on the amount of water which ought to be carried away and on the grain-size distribution of the mineral material in the drain. Determining the dimensions is usually performed according to empirical procedures as the materials, climate, groundwater conditions and materials all have a pronounced influence upon the water that is to be conveyed and that can be carried by the drain.
The pavement layers must be shaped so as to ensure that water in them moves towards the drain and the base of the trench must be substantially lower than the layer to be drained:
• to ensure a suitable hydraulic gradient towards the drain which will drive drainage action;
• to aid entry across the geosynthetic liner which may require a small head difference before wetting is achieved and water passage possible; and
• to ensure that here is adequate capacity within the drain to hold exceptional water flow events without a risk of water flowing from the drain into the layer.
Thus a distance from the base of carrier pipe to the bottom of the layer to be drained of about 0.5 m is typical. Successfully designed and properly made, trench drains have a lot of advantages. Among other things there are (Uzdalewicz, 2001):
• a lengthy life in which it works effectively;
• low construction and operating costs;
• the possibility of managing the “area above the drain”, for example as a footway. The following are the stages of construction of a trench drain:
i) digging of a narrow trench excavation;
ii) cleaning out the narrow trench;
iii) lining of excavation surfaces with geotextile (along the excavation or cut set across excavation) (e. g. Fig. 13.14);
iv) placing a covering of aggregate at the bottom;
v) installing the drain pipe, if needed;
vi) filling the drain volume with aggregate;
vii) closing the drain and jointing of geotextile edges (e. g. with “U” shaped clamps) (e. g. Fig. 13.15a &b); and
viii) covering the closed drain surface with 3-5 cm (or more) of top soil or other low permeability cover (except where surface runoff is also to be collected — see Section 13.3.5).
The minimal length of overlap for geotextiles should be at least 30 cm (Edel, 2002). The longitudinal direction of overlaps should be consistent with the direction of water flow through the drain. The aggregate filling the trench drain should be compacted (in layers).
