A basal heave assessment in DeepEX is performed as part of the excavation stability evaluation using limit-equilibrium–based methods. The process involves first creating a complete excavation model, defining soil stratigraphy, groundwater conditions, and construction staging, and then explicitly enabling the basal heave (basal stability) check within the Stability+ module.
Basal heave is primarily a short-term undrained stability problem, most relevant for deep excavations in cohesive soils. For this reason, the accuracy of the assessment depends strongly on the correct definition of undrained shear strength for clay layers and on the excavation stage being evaluated. DeepEX allows the basal heave check to be applied selectively to specific stages and to individual walls, recognising that basal stability often governs at intermediate excavation depths and may differ from one side of the excavation to the other.
Once enabled, the software evaluates basal stability using established analytical formulations and reports the factor of safety for each selected stage. These results allow the user to identify critical stages, assess whether basal heave governs the design, and determine whether mitigation measures or changes in excavation sequencing are required.
Step-by-step
Step 1 — Create the base model (Figure 1)
Define the soil stratigraphy, ensuring that cohesive layers beneath the excavation base include appropriate undrained shear strength parameters.
Set the groundwater conditions in line with the design assumptions.
Define the excavation staging, including excavation depths and support installation. Basal heave is assessed stage-by-stage, so a realistic construction sequence is essential.
Step 2 — Enable basal heave assessment (Figure 2)
Navigate to Stability+ → Basal stability and Clough method.
Open the basal stability calculation options and activate the basal stability (basal heave) factor of safety check.
Step 3 — Select the basal heave method (Figure 3)
Choose the basal heave method or methods to be evaluated (for example, standard or Prandtl-based approaches). Multiple methods may be enabled to support comparison or internal design checks.
Step 4 — Define the applicable stages and wall (Figure 4)
Specify whether the basal heave assessment should apply to:
the current stage only,
all stages, or
a selected range of stages.
Select the wall to which the check applies, particularly important in asymmetric excavations or non-uniform ground conditions.
Step 5 — Run the analysis and interpret results
Run the analysis and review the reported factor of safety against basal heave for each selected stage.
Identify the critical stage and evaluate whether basal stability governs the excavation design.
Example of basal heave results in DeepEX: how to read and interpret them (Figure 5)
This example illustrates how basal heave assessment results are presented in DeepEX, combining outputs from both limit equilibrium–based checks (LEM) and finite element analysis (FEM). The two result types serve different but complementary purposes.
FEM results – surface displacements (||du||)
The FEM output shows surface and subsurface displacement contours, typically expressed as vertical displacements (||du||). These results provide a deformation-based view of the excavation response:
Zones of higher displacement near the excavation base indicate stress relief and upward movement tendencies.
The displacement pattern reflects soil–structure interaction, wall stiffness, and excavation staging.
FEM results help identify whether basal instability is developing progressively, even when a clear failure mechanism is not yet formed.
It is important to note that FEM does not directly report a basal heave factor of safety. Instead, it provides insight into the mechanisms and deformation trends associated with base instability.
LEM results – basal heave factors of safety
The LEM output presents a tabulated summary of basal heave factors of safety, calculated using multiple analytical approaches. These typically include:
Basal standard method
Prandtl-based methods (average and code-based formulations)
Circular failure mechanisms
Additional basal stability indices where applicable
Each method evaluates base stability using an idealised failure mechanism and reports a factor of safety for the selected wall and excavation stage. This allows:
direct verification of basal heave stability,
comparison between different analytical formulations, and
identification of the governing (lowest) factor of safety.
The governing value is normally used for design checks and compliance with project or code requirements.
How FEM and LEM results should be used together
LEM basal heave checks provide an explicit and conservative measure of base stability and are typically used for formal design verification.
FEM results support interpretation by showing how deformation develops in the ground and whether instability is likely to manifest progressively.
A low or marginal basal heave factor of safety in LEM should be examined alongside FEM displacement patterns to understand the failure mechanism and sensitivity to assumptions.
Conversely, acceptable FEM deformations do not eliminate the need for an explicit basal heave safety check.
Practical takeaway
In DeepEX, basal heave assessment is best interpreted by combining LEM and FEM outputs:
use LEM tables to verify safety against basal heave using recognised analytical methods, and
use FEM displacement contours to understand deformation behaviour and support engineering judgement.
Together, these results provide a robust basis for assessing excavation base stability and identifying whether mitigation measures or changes in staging are required.
Summary
Basal heave assessment in DeepEX is a structured, stage-dependent process that integrates a well-defined excavation model with established limit equilibrium stability checks. By activating basal stability evaluation for the relevant excavation stages and walls, users can reliably assess short-term base stability in cohesive soils and identify critical conditions early in the design process.
The results are most effectively interpreted by combining limit equilibrium methods (LEM) and finite element analysis (FEM). LEM provides explicit factors of safety against basal heave using recognised analytical formulations and therefore forms the primary basis for stability verification and design compliance. FEM complements this by offering a deformation-based perspective, illustrating stress relief and upward ground movements as excavation progresses.
Used together, these approaches allow engineers to both verify stability and understand the governing mechanisms. The controlling basal heave factor of safety from the LEM analysis should be adopted for design checks, while FEM displacement patterns support assessment of response realism, identification of progressive instability, and informed decisions on excavation staging and mitigation measures.
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