Ground Conditions, Foundations and Basements

Ground Conditions, Foundations and Basements

1. Ground investigations

   • Site history
   • Environmental assesment
     • For projects of a significant size a EIA (environmental impact assessment is required
       • Project description (key components)
       • Environment - populations, air, soil, water humans, landscape
       • Mitigation
       • Significant effects
   • Topography
   • Ground conditions (trial pits, bore holes)
   • Water table
     • Dewatering may be required on sites near rivers or water ways
       • Area should be excavated to 1m, sheet piles to 6m should be insertedd, water is them pumped out from internal area, excavation can begin
   • Interpretation

2. Foundation types

   • Shallow
     • Pad
     • Strip
       • Various forms are used dependant on ground conditions bearing capacity
         • Normal strip on good ground
           • Wide strip on low bearing capacity ground
             • Deep strip on reasonable ground (2/3m)
     • Raft, Base slab
   • Deep
     • Pile/Pile caps
       • Driven Piles are made of site and hammered into the ground using a piling rig
       • Cast insitu pile foundations are poured on site
         • Bored piles - Pre drilling, drill pile shaft and temporary casing, construct bell out, clean pile shaft, install reinforcement cage, pour concrete
         • CFA (continuos flight auger) - Bore pile shaft and remove soil via excavator, concrete is poured as auger is removed, concrete cage is dropped in after
           • Simple and effective
       • Pile caps
         • Excavate top of pile caps (some support for earth may be required)
           • Blind pile caps to show reinforcement
             • Bend reinforcement into pile cap base
               • Suspend holding down bolt into position
                 • Add concrete base, clean out bolt holes
     • Pile walls
     • Piled rafts

3. How can sites become well organised?

   • Clear un-propped walls
   • Sequenced working
   • Managed access
   • Managed materials and storage

4. Structures and Temporary works

   • Basement walling
   • Excavation
   • Retaining wall propping
   • Method statements
   • Piled walls
     • Continuous - simple reinforced concrete piled walls
     • Secant - reinforced concrete piled walls with two types of concrete alternated to create a stronger bond
   • Diaphragm walls
     • Excavation of the ground using hydraulic cutter, hydraulic grab, cable grab or kelly grab on left side, then right then centre of wall
       • Bentonite mud is pumped in to stop collapse
         • Reinforcement cage is added
           • funnels are suspended and filled with concrete, when lifted out the concrete spreads evenly at the base, this is repeated until concrete reaches the desired level
             • Primary panels first, (left and right) secondary panel (right of left primary), closing panel (left of right primary)
   • Steel Walls
     • Interlocking steel panels are driven into the ground
       • Vibration driven into sand and clay, hammer driven into rock

5. Excavating the basement

   • Excavation of more than a single storey basement requires co-ordination and sequencing for the excavated material
     • Basement walls and slabs need to form a watertight enclosure so casing against the walls needs to be done carefully
       • Basements are often large thick poured concrete slabs, it is important to reduce any cracking due to thermal activity during setting and drying out so sequence is key
         • Deeper basements will require temporary supports in the form of props to support the lateral load which is usually taken up by the floor slab
           • The props are often hydraulic so that they can be changed to support the wall and correct any movement, the temporary propping is arranged in a way that the slabs can be formed and then the propping can be removed
             • Propping is formed away from floor slabs, columns and internal wall locations

6. Ground bearing floor slab make up

   • 1. Geotextile
     • Hardcore layer (150mm)
       • Waterproof membrane
         • Cement blinding (50mm)
           • Insulation (100mm)
             • Reinforced concrete slab
               • Vapor barrier
                 • Screed
                   • Floor finish

7. Deep basement works

   • When time or space is very limited the basement slabs will be built from top down
   • The slabs will often require a continous pour, this can be a logistics problem when trying to get masses of concrete mix through london
   • Top down construction
     • 1. Drill piles and plunge columns into wet concrete, backfill with granulated material
       • 2. Cast ground floor slab and excavate below through opening
         • 3. Complete excavation and cast lower slab, tied into plunge columns
           • 4. Excavate base slab through opening in slab above, ensure there is plenty of ventilation
             • 5. Cast base slab, with pile caps and any lift its
               • Complete all slabs and infill openings

Images and files in the map