Slope stability calculations form the most significant part of a successful safe embankment design.
Design of safe steep batter slopes to the design criteria set out in Eurocode EC7 can achieve considerable cost savings, by eliminating the requirement for large scale sheet piling or other retaining wall structures.
OGI’s approach is to use the available Site Investigation (SI) data, together with site geometry to make an initial assessment as to whether the steep batter approach is feasible and to identify areas where existing data is insufficient to enable a robust design to be produced. This data may include geological logs, in situ ground strength testing, laboratory soil testing and ground water level measurements.
A first stage conceptual model (CM1) is then produced which identifies important geology, water table levels, site geometry and any relevant constraints such as existing structures, services, permanent and temporary roads etc.
The second stage conceptual model (CM2) is produced to develop ground strength parameters which will be used later in the mathematical model (MM). CM2 develops the CM1 model by assigning appropriate ground strength parameters to each geological unit, using in-situ and lab testing data and application of partial factors according to Eurocode 7 (EC7). CM2 also takes into account site specific loads to be applied to each of the batter slopes due to existing structures, site traffic etc.
The MM uses the parameters developed in CM2 as inputs into the finite element model, which is able to calculate the activating forces and resisting forces for hundreds of potential slip surfaces. The ratio of the resisting forces to the activating forces provides the factor of safety (FOS). If the factor of safety exceeds 1.0 the batter is deemed to satisfy the safety criteria of EC7.
Having completed the slope stability calculations, OGI undertake internal design checks (Category 2), and issue a design certificate, together with all supporting documentation which includes all calculations including assumptions made, source data and parameters used in the model.
An Inspection, Testing and Monitoring (ITM) Plan is issued, forming an essential part of the overall embankment design. The purpose of this is to enable the Contractor to demonstrate that on-going checks have been made to ensure that the batter strength is as predicted, and to allow OGI’s design to be validated.
The ITM Plan includes procedures to deal with any degradation of the slope observed for a range of possible circumstances in order to maintain the integrity of the batter throughout the works.