Box culverts and pipe culverts: the steps of preliminary design for a hydraulic crossing

Design flow, choosing between a pipe and a box culvert, fill and traffic loads, checks: the steps of preliminary design for a hydraulic crossing.

Under a road, a railway or a platform, water flows must be restored by hydraulic structures: circular pipe culverts, or box culverts, which are rectangular reinforced concrete frames. Some include a chamber, for example at a junction, a change of direction or for maintenance access. Preliminary design sets the section, the type of structure and its main thicknesses before detailed studies.

1. Gather the input data

• The design flow, from the hydrological study, for the return period set by the client.
• The geometry of the watercourse or ditch: bed, slope, upstream and downstream levels.
• The longitudinal profile of the infrastructure and the fill height above the structure.
• The traffic loads to be considered.
• Geotechnical data and groundwater level.
• Any environmental constraints, such as maintaining ecological continuity.

2. Choose between a pipe and a box culvert

Pipe culverts, often precast, suit moderate flows and are quick to install. Box culverts provide a large flow area for a limited height, which makes them attractive when the available fill is low or the flow is large; they can be precast or cast in place. The choice also depends on programme, lifting equipment and the founding soil.

3. Hydraulic preliminary design

The aim is to pass the design flow without excessive headwater upstream or excessive velocities downstream:

• determine whether the flow is controlled by the inlet or the outlet, depending on slope, length and tailwater levels;
• check the upstream water level and the freeboard available below the road;
• check the outlet velocity and, if needed, provide scour protection, for example with riprap.

4. Structural preliminary design

The structure must then resist the loads acting on it:

• the weight of the fill, which depends on the cover depth and on the installation (embankment or trench);
• lateral earth pressures, often close to at-rest pressure for a rigid frame;
• traffic loads, spread through the fill;
• water pressure and, with a high water table, uplift.

For a precast pipe, a manufacturer's strength class compatible with these loads is selected. A box culvert is analysed as a closed frame resting on the ground; modelling ground stiffness with springs gives a more realistic distribution of forces than a rigid support. Backfilling stages, which must remain symmetrical, also need to be checked.

5. Configurations with a chamber

A chamber adds points of attention: checking the walls and roof slab, reinforcement around pipe openings, and stability against uplift. An empty, lightly loaded structure can be lifted by a high water table; this flotation check should be part of preliminary design.

6. What a preliminary design note contains

• The assumptions: flow, levels, loads, geotechnical data.
• The sections and types of structure selected, with their main thicknesses.
• The results of the hydraulic and structural checks.
• The points to be confirmed in the following design stages.

In short

Sound preliminary design avoids having to rework the design of a hydraulic structure at construction stage. ROKENG carries out preliminary design of pipe culvert hydraulic crossings, including configurations with chambers, and the design of buried structures.