Retaining wall design basics
Retaining wall design basics video content
There are many types of retaining walls, and they all fall into three major categories. Gravity, cantilever and reinforced soil designs. You can see examples of a cantilever king post and many L shape designs. The gravity design can be improved using reinforced soil when the wall height increases beyond the economical gravity design.
Our structural engineer designs retaining wall to Eurocode 7. The Eurocode give design guidelines and standards to ensure the wall passes design tests with factors of safety.
Retaining walls can fail in a number of ways.
Overturning – The material being retained creates a force (moment) that causes toppling of the wall..
Bearing capacity – The weight of the wall and earth is supported by the ground beneath. The wall can settle and fail.
Sliding – The retaining earth creates a horizontal force that can cause the retaining wall to slide. This is resisted by the friction between the wall and 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 and masonry can shear at the base or on courses.
Bending – King post walls can bend if the steel section is not adequate for the load case.
The video in the video shows how a retaining wall slips and overturns. The leg of the retaining wall is facing outwards and glass marbles are place behind it. You can see how the wall slides. A rubber mat is now put down. The rubber mat represents friction. The retaining wall is placed on the rubber mat and is again loaded with marbles. The mode of failure this time is overturning because the rubber mat is resisting the sliding. The height of the marbles at failure is 8 cm. When the wall is turned around, so the leg faces inwards, and the retaining wall is loaded with marbles. You can see the marbles now sit on top of the retaining wall leg and the height of the marbles is 11cm before failure. You can now see why it is vital that your retaining wall project is engineered in accordance with Eurocode 7
Structural engineer’s need soil information before they can deign a retaining wall. Different soils have different prosperities. There are three quick ways to get soil information for small projects. Bore hole, trial pit and a desk top study using information from the geographical survey bore hole logs.
You can hire a bole tool or a JSB to dig into the ground to obtain the information. You take sample from a number of depths and record the depth. You can then squeeze the soil sample and explain what happens. Video the process to help the engineer. A soft soil will squeeze through your fist, where as a hard soil will stay in tack.
The engineer can also access bore hole data from the British geological survey.
The video shows what a bore hole log looks like from a professional site investigation report. On larger projects the architect or engineer will commission a full site investigation survey. This includes looking at flood planes, soil chemical analysis and a number of bore holes carried out by geologist technicians.
Simple field tests to determine the ground condition. The table shows a number of soil types and a simple field test to find out the condition in terms of hardness. From the descriptions the engineer can determine a value for the bearing capacity of the ground the retaining wall will sit on. Download the slides and study the table, then carry out your own field test from the excavated material from your trial pits or bore holes. Video your results when trying to squeeze the material.
Key benefits of structural design
- The retaining wall will not fail subject to being built to the design.
- You will have the correct material specifications
- You will have a copy of the calculations for third parties
- You will have a section through drawing of the wall
- You will be protected by the engineer’s professional indemnity insurance
An example a king post design for a small house project
An example of gabion basket requirement for a housing development
An example of a inclined concrete block for a car park on a commercial project
A cast in L shape and a concrete Criblock design
A hollow block (Stepoc) and a Geoweb reinforced soil retaining wall design
Every project is different with different soil types and site constraints. We highly recommend your retaining wall project begins with a structural design.
We hope you liked retaining wall design basics.