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  • Home
  • Site Investigation
  • Foundation Geotechnics
    • Shallow Foundations
    • Driven Piles
    • Suction Piles
  • Pipeline Geotechnics
    • Surface Laid
    • Buried
    • Product Lowering / Burial
    • Rockdump
  • Blog
  • CONTACT

Shallow Foundation Design

Introduction
​Shallow foundations in the context of this website are defined as a skirted or un-skirted mud mat, if a skirt is adopted then it should be fully penetrated under the self weight of the structure.   The foundations could comprise single or multiple footings.
Design Codes - Spreadsheet Checks
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Any design code will aim to check the suitability of the design under design loading depending upon their likelihood to occur which as a minimum may include:
  • ​Ultimate Limit State (ULS)
  • Accidental Limit State (ALS)
  • ​Serviceability Limit State (SLS)
  • ​Seismic (ELE / ALE)
The godfather of foundation design codes is DNV Classification Notes 30.4 "Foundations".   The guidance found in the most recent  ISO or API codes is similar to that which was produced  in 1992 by DNV.  For shallow foundation design the main goal of any code is to construct a diagram which described the capacity of the foundation under various load cases (as shown below for drained and undrained soil response).  The inputs to the analyses need to be selected carefully particularly where skirted foundations are involved such that the expected mode of failure is accurately represented (it may be prudent to check a few potential failure modes picking the most conservative result - see examples below from DNV).

There are many uncertainties and limitations with the simplified methods for calculation of shallow foundation bearing / sliding capacity:
  • Multiple soil layers (methods available for uniform sand, uniform strength clay and clay linearly increasing strength)
  • Non-standard foundation geometry (methods available for rectangles and circles)
  • Multiple footing foundations (limited codified guidance on load distribution and footing interaction)
  • Conservatism inherent in the 'effective area' concept adopted for the moment loading component
  • Foundation interaction with rock or nearby seabed depressions (jack-up rig spuds for example)
There are several industry standard ways of estimating the impact of these however some uncertainty on behaviour exists.  A degree of clarity may be obtained by adopting an advanced analyses technique.
VHM envelope for sand
VHM envelope for clay
Foundation failure mechanisms
Advanced Analyses
Optimisation of the capacity or settlement assessment of shallow foundations does not necessarily mean using advanced software (but it helps).  The following publications over the past few years from Cathie Associates, BP and UWA may be of use depending on the soil conditions / loads governing your design:
  • A framework for the design of sliding mudmat foundations (ISFOG 2012)
  • Hybrid Subsea Foundations for Subsea Equipment (ASCE)
  • Whole life assessment of subsea shallow foundation capacity (OSIG 2017)

The final paper presents a summary of recent R&D at UWA which can lead to significant mudmat/skirted mudmat size reduction or indeed the use of foundations which are designed to slide across the seabed.  See the blog for a review of this paper.

Limit State Analyses and / or 2D / 3D FEA are increasingly used tools to better understand foundation response under a range of loading.  Of critical importance is selection of the correct soil model and input parameters to correctly represent the analyses being performed.

The VHM interaction envelopes (such as those generated above using simplified approaches in DNV)  may be created in such software for a given foundation configuration and soil type.  If used correctly, this software should allow optimisation of geotechnical foundation design.

Some validation of 3D FEA in very soft clay and recommendations for simplifying the analyses into a spreadsheet assessment (presumably for use a quick foundation capacity screening assessment) has recently been published (see blog 'favourite shallow foundation design papers').
Plaxis 2D foundation displacements
Picture
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