KNOWLEDGE BASE

Flood wall design

Flood walls are designed structurally in the same way as a traditional retaining wall. The load, instead of soil, is water.

The main failure modes are overturning, bearing capacity, sliding, stability, shear, seepage, and piping.

Flood wall failure modes

Overturning – The material being retained creates a force (moment) that causes toppling of the wall. The weight of a gravity retaining wall resists this moment.

Bearing capacity – The weight of the wall and earth is supported by the base. The ground-bearing capacity needs to be checked to ensure any settlement is within limits.

Sliding – The material being retained creates a horizontal force that can cause the retaining wall to slide. This is resisted by the friction between the wall and the earth beneath and the embedment of the wall.

Stability – The earth around the wall requires adequate strength to avoid an overall slip failure.

Shear – The blocks can shear at the base or between courses. We introduce a kicker at the base and interlocking buttons to resist these forces.

Seepage – Water seeping underneath the foundation

Piping – When seeping become faster and flowing, this then becomes piping that can cause the wall to slide and rotate. The solution is to build a cutoff. Depending on the scale of the project this can be constructed using sheet piles or, for smaller projects, an underground wall constructed by excavating a deep slim trench and filling it with concrete.

Flood wall design for homes

Flood defence walls for domestic properties tend to be below 2m in height. We prefer to design a hollow block system using stepoc blocks.

We have two basic flood wall designs one for clay soils that do not require a cut off wall and one for sandy, gravelly soils. See the images below.

A water pumping system may be required when seepage may slowly get under the wall due to the soil condition.

For any seepage and pump design calculation, we need to know the porosity of the soil. This is a job for a geotechnical engineer and then a mechanical engineer to size the pumping system.

Typical flood wall designs

Flood defence wall design

The process of building flood defence walls

The process of building a flood defence wall is as follows:

Planning
Design
Construction

The planning phase

Obtaining local flood maps, a Google Earth map and spot levels will help determine the wall’s layout and height.
Carry out a soil investigation and record the findings.
Carry out a utility search to find the location of all underground services, including gas, electric, telephone, and internet cables.
Carry out a manhole survey to find the depth and location of all foul and surface water drains.
Draw a plan for the wall, including dimensions, openings, and services.
Design the foundation for depth and type using the soil and wall height data.
Create a fully priced bill of quantities to give an accurate budget from the foundation and wall plan.
Create a design brief for the structural engineer by pulling all the relevant information into a file.
Use the soil investigation data to design the sump pump sizes and layout.
Ensure the finance is in place including any grants that may be available before starting the construction phase.

Structural design phase

The structural engineer will design the flood wall and deliver a structural report, calculations, and drawing showing the section through the wall, together with a specification.

Construction and project management phase

Adjust the bill of quantities using the structural engineer’s flood wall design.
To place orders, create a shopping list that includes the steel reinforcement bar schedule, reinforcement spaces and blocks, concrete mixes with quantities, and the stepoc block quantities.
Produce a tender document to go out to tender to obtain competitive prices to build the wall.
Assess the tender prices to ensure the right contractor is selected for the best value.
Create a time plan bar chart for buying and on-site production to build the wall.
Monitor on-site works to check the work is being carried out to the structural engineer’s specifications.
Value the work to pay the contractor the correct value without overpaying.

We can help you with the full process if required. Just tell us about your project by completing the form below.