Calculation of the Required Strength of Geosynthetics for a Retaining Wall - Following Whitcomb, Bell and Jewell recommended by Koerner

Height, measured vertically (H) in m
Thickness surcharge material (D) in m
Slope angle in degrees
Spacing between the layers (T) in m
Dynamic surcharge
Equivalent road (20 kPA) or train/tram (80 kPa) in kPa
Angle of internal friction of backfill in degrees
Density of backfill in kN/m3
Density of soil surcharge in kN/m3
Level of layer to be designed
Distance from top level in m
Safety factors geosynthetic
Safety factor production
Safety factor installation damage
Safety factor durability
Safety factor against creep
Safety factor overall
Safety factor design
Safety factors overall
Tensile strength required at level z from top level

Calculation method

This procedure is based on a combination of the work of Whitcomb & Bell and Jewell, both of which are recommended by Koerner in his text book, modified to take account of modern Eurocode partial factor design concepts. Valid for all slopes from 90 degrees to 30 degrees, using free-draining, compacted granular fill having an internal angle of friction phi of 30 degrees or greater. The anchoring length calculation takes in account the requirement for installation.

This Applet covers internal stability of the structure only. It does not ensure that the foundation itself will be stable. To do that, slip circle analysis will be necessary, which often results in the bottom two layers being increased in strength or number, and extending up to twice H length back under the slope. If the wall is being placed adjacent to existing higher ground, then this may well mean excavating back a distance of 2H in order to lay the basal reinforcing layers to intercept critical slip circles.

About the calculation method

This is a straightforward calculation procedure* that has been well used for many years*. It involves calculating the outward thrust developed by the slope construction soil plus any surcharge soil envisaged. For full design purposes, it is possible to also take into account dynamic loads as well, but since these primarily affect the reinforcing layers in the upper third of the slope, and since these are the lowest stressed by static loads, it has not been considered necessary to introduce them in this approximate geosynthetic assessment app.

* Whitcomb W and Bell J R, ‘Analysis techniques for low reinforced soil retaining walls’ Proceedings of the 17th Geological Soils Engineering Symposium, Moscow ID, Idaho, 1979.

How to use the Applet

It is for the user to fix whichever parameter he/she requires and then calculate the outcome parameters for the design assessment. The Whitcomb method has been added to in this procedure by allowing the user to specify the required life of the structure, and then allowing for site damage, chemical deterioration, and of course, for creep that would take place if the geosynthetic were to be overstressed. This is calculated for different types of polymer including polyester, polyethylene and polypropylene.