There are simple procedures covering Embedded Sign Post Foundation Design within BS EN 40-2 and Highways Authority BD94/17. The CivilWeb Sign Post Foundation Design Spreadsheet can be used to complete these calculations.
First the wind loads acting on the sign post foundation must be determined in order to determine the overturning moment. Then the below procedure calculates the resistance of the ground which can then be compared to the destabilising overturning moment.
Embedded sign post foundation design is one standard type of foundation for sign posts. The other is to bolt the post down onto a concrete pad foundation. A simple procedure is used for determining the minimum planting depth for embedded sign post foundation design in BS EN 40-2 and HA BD 94/17. This involves calculating the moment resistance of the ground and comparing this with the destabilizing moment caused by the sign post.
Embedded Sign Post Foundation Design - Ground Resistance Moment
First the Ground Resistance Moment of the embedded sign post foundation is calculated using the below equation. Mg = ground resistance moment, D = minimum diameter of the sign post foundation (the sign post and its surround), P = enbedded depth (this must exceed 2xD to ensure foundation behaves as a embedded foundation and not as a spread foundation) and G is a factor dependent on the ground conditions.
Straightforward guidance on choosing a value for G in embedded sign post foundation design is given in both BS EN 40-2 and HA BD 94/17 as shown below. It is rare for any detailed site investigation information to be available and rarer still for the information to be gathered prior to the installation of sign post foundations. It is typical that a value for G is taken from the below table based on previous experience of the site. If no experience of the site is available it is typical to choose the relatively conservative value for poor soils for embedded sign post foundation design. Note that while the poor soil value for G is quite conservative where soil conditions are normal but unknown, further investigation will need to be completed in areas where the soil is known to be especially poor.
Embedded Sign Post Foundation Design - Destabilizing Moment
The destabilizing moment of the embedded sign post is calculated about a fulcrum point located at a depth of 1/√2 times the embedded sign post depth below ground. This lever arm is then multiplied by the Wind Force and by a Model Factor (recommended as 1.25 in HA BD 94/17).
The destabilizing moment is then checked against the ground moment resistance to ascertain whether the foundation is suitable.
Minimum Embedment Depth in Sign Post Foundation Design
Minimum embedment depths depending on ground conditions are given in BS EN 40-2 as shown below. HA BD 94/17 recommends that the central values are used for signs, though this can be relaxed to 600mm for embedded foundations for sign posts less than 2m height. As well as the overturning moments, the CivilWeb Sign Post Foundation Design Spreadsheet also checks that the proposed embedded sign post foundation depth is in accordance with the minimum planting depths as specified in BS EN 40-2.
Embedded foundations for sign posts require the mobilization of the passive resistance of the surrounding soils. For this reason surrounding soils must be well compacted and the moment resistance can be affected by nearby slopes or excavations.
Completing these calculations by hand can be time consuming as the destabilising moment and ground resistance moment both change with each change of embedment depth and each design must be checked that it exceeds the minimum embedment depth as specified in BS EN 40-2. This often leads to the use of standard embedment depths and foundation designs which are inherently conservative. The CivilWeb Sign Post Foundation Design Spreadsheet instantly completes calculations for all normal embedment depths and then presents this in a graph allowing the user to complete an optimised design at a glance. This saves both time for the designer and money for the client.