Shallow Foundations

Shallow Foundations

1. Pad Foundations

   • Overview
     • more commonly used under point loads such as columns but can be used under ground beams to transfer continous loads
     • Width & Depth of each pad foundation will depend on the soil conditions
     • Reinforcement in the pad foundation typically adds tensile strength allowing the loads to be distributed over a larger area, preventing shear failure
   • Construction Sequence
     • String lines used to mark out the centre lines for the bolts
       • Holding down box is positioned when the concrete is poured
         • Steel reinforcement cages added
           • Reinforcement starter bars are evident which are used to tie to the ground beam into the foundation
   • Distributing the load
     • Large load point -load is distributed over a large area, load per unit area is reduced
   • Column Base
     • Shims are used to level the column to the correct height
     • Base of the column fastened to the pad foundation by holding down the bolts
       • Wedges hold the column in position intil the bolts and column base is grouted
   • Advantages
     • Load from column is distributed across the full area of the foundation
   • Shoe Column Connections
     • connection between the column & beam

2. Raft Foundations

   • Solid slab raft foundation
     • reinforced concrete raft
     • well compacted hardcore
   • Functional Requirements
     • all materials must be sufficient detail to enable competent individual of approved level of workmanship to be able to assemble the structure without error or reason to cause damage, deteriotation in manner or method of approach used in assembly
     • All materials & workmanship must be within tolerances as defined in respective Codes of Practice
   • Used on....
     • Highly shrinkable soil
     • Overall objective is to avoid pin pointing load
     • Usually designed for sites with low ground bearing capacity or pockets of filled ground
     • Where it is necessary to limit the load applied to the underlying soil or to reduce the effects of differential foundation movements as a result of variable soil conditions or variations in loading
   • Do not use on...
     • ground which is susceptible to heave or shrinkage of depths up to 3m
     • When trees are present or have been removed
   • Damp Proof Membranes/Courses
     • Installed where raft foundation and wall meet
     • positioned under the raft and wrapped around external toe and lapped into the interal DPC

3. Factors to consider

   • Chemical Composition
     • Where soils are known to have impurities they can lead to shear or differential movement leading to the subsequent failure of the foudnations
   • Granular Infill
     • placed beneath raft foundations
     • at least 50% of foundation depth
       • not more than 1.25m deep
     • extend beyond the edge of the foundation by a distance equal to its natural angle of repose +0.5m
   • Sulfates
     • voids at the sides of the structural elements such as beams/piles which are then filled with a compressiviel material as EPS
       • Short term
         • barrier between soil and structure
       • Long term
         • EPS provides additional protection