L3: High-Rise Foundations Flashcards
What is the purpose of foundations?
To safely transfer building loads to the ground
What is the type of foundation based on?
Building size
Occupied weight
Need for stability
Soil/ground support capabilities
What are the foundation loads?
Dead, live, wind, horizontal pressures underground, structural member thrust, uplift, earthquake
What are foundations supported by?
Pressure bulbs (spheres of soil)
What can happen if buildings are in close proximity and how is it overcome?
Pressure bulbs overlap causing a failure as it cannot support both buildings
Caisson - produces its own pressure bulb
End bearing piles - individual pressure bulbs which combine into one similar to that of a caisson
What are the requirements of a foundation?
Safe against structural failure
Settlement not an impairing function
Technically and economically feasible
What is settlement and what is it dependent on?
Sinking of building as soil compresses; dependent on soil type (eg bedrock vs clay)
What are the two types of settlement?
Uniform and differential
What are signs of differential settlement?
Sloped floors, cracked walls/windows, door opening/closing improperly
What building is an example of uniform settlement and how much did it settle?
The Palace of Fine Arts; 3m
What are the two types of matter under buildings?
Rock (continuous)
Soil (particulate)
What is the difference between cohesive and non-cohesive soils?
Cohesive soils retain shear resistance in confining forces (stick together) and non-cohesive soils rely on friction between particles to support loads
How does level of cohesion affect angle of repose
Cohesive - steeper
Non-cohesive - shallower (cannot maintain vertical form)
How is soil tested?
Bearing capacity (driving hammer) Test pits (less than 2.5m) Test bores (deeper)
What does bearing capacity test?
How much force the soil pushes back (number of blows per metres driven)
What do test pits/bores show?
Strata (layers)
What is soil stability?
Soil’s ability to retain its engineering properties under varying conditions
What are examples of stable and unstable soils?
Stable - rock, gravel, sands, some silts
Unstable - clay
Why is clay considered an unstable soil?
Subject to changing water content - swells when wet and shrinks when dry; wet clay put under pressure will reduce in volume
How is the water table detected?
Shows in test pit/bore in course-grained soils but requires special devices in clay soils (impermeable)
What is compaction?
Process of increasing soil density in thin layers (lifts), to squeeze out air between particles and rearrange particles to improve soil stability
When is compaction useful?
For shallow foundations supported uniformly by large surface of soil
What are the two main types of excavation?
Benched and sheeted
When is benched excavation used?
When the site is sufficiently larger than the building area
How is benched excavation sloped?
Shallow in frictional soils; steeper in cohesive soils
What does sheeted excavation require and why?
Bracing, to resist soil pressure
What are the three options for sheeted slope supports and when are they used?
Soldier beams and lagging (deeper excavation) Sheet piling (shallowed excavation) Slurry wall (used as permanent foundation wall)
What is soldier beams and lagging?
H shaped soldier beams (steel) driven into ground with lagging placed between (wood)
What is sheet piling?
Sheet material driven into ground (timber, steel, precast concrete)
What is slurry and how is a slurry wall formed?
Mixture of bentonite clay and water
Concrete guide walls laid out
Trench dug to desired depth and filled with slurry
Cage of steel installed
Concrete filled into trench using a tremie and slurry is removed
Slurry wall tied back
What are the methods to brace sheeted slope support and when are they used?
Crosslot - used in tight urban areas
Rakers - used when width is too large for crosslot
Tiebacks - most common
Why is tiebacks the most common compared to crosslot and rakers?
Crosslot and rakers take up space in the excavated area
How are tiebacks installed?
Steel tendon/cable placed in a hole drilled in the wall and cured at the end with grout;
Walers installed to horizontally connect tiebacks
When can a sheeted slope support be unbraced?
When the excavation is very shallow compared to the sheeted slope support (cantilevered)
What are two other options for excavation?
Soil nailing and shotcrete
Secant pile walls
What is soil nailing and shotcrete and when can it be used?
Soil nails (tiebacks) installed
Grid of rebar placed over soil
Shotcrete (sprayed concrete) applied over rebar and cures
Nails tightened with anchor plate
Used in cohesive soils because needs to hold before shotcrete is applied
What is a secant pile wall?
A series of interlocking poured, reinforced concrete piles; alternating concrete and concrete with steel rebar
What are the advantages of a secant pile wall?
Stiffer than sheeted slope support and can be installed in difficult soil
What is a tangent pile wall?
A secant pile wall without overlap between piles
Why is dewatering done?
Because excavation must be done in dry conditions
What are the methods for dewatering and when are they used?
Well points - most common
Watertight barrier - used when reachable impermeable layer
What are well points?
Wells placed around perimeter to pump from water table; can affect surrounding buildings
What is a watertight barrier?
A slurry wall around the perimeter to an impermeable layer, isolating the water table; does not affect surrounding buildings
What are the three main parts of buildings?
Foundation
Substructure
Superstructure
What are the types of shallow foundations?
Mat/raft foundation
Floating foundation
What is a mat/raft foundation?
Where the soil cannot support the building weight so column footings are merged together
What is a floating foundation?
Where soil is dug and replaced by building; 1 storey of soil supports 5-8 building storeys
What are the types of deep foundations?
Caissons
Piles
Site cast concrete piles
What is the purpose of deep foundations?
To reach through the weak soil
What are caissons?
Large, column-like members poured into the ground
What are piles?
Mechanically pounded then cut level at top; 2-24 piles together with pile caps and clusters joined by grade beams
What are the types of caissons?
Belled ending (on top of good soil) or socketed (into good soil)
What are the types of piles?
End bearing (on top of good soil) or friction pile (cannot reach good soil so uses friction against weak soil to stay up)
What are site cast concrete piles?
Steel casing driven into soil and then filled with concrete
What are the types of site cast concrete piles?
Cased (casing remains) or uncased (casing removed)
What is seismic base isolation?
Flexible joints to allow for shifting at connection between foundation and sub/super structure
What makes up base isolators?
Column base plate, lead core, layers of steel plates and rubber, isolate base plate
What is underpinning?
Process of strengthening and stabilising foundations of an existing building eg. fixing failing foundation, adding levels
What are the three approaches to underpinning?
Digging trenches at intervals and underpinning bit by bit
Exposing entire portions of foundation by temporary supports (needle beams)
Installing mini-piles without excavation
What is up-down construction?
Building substructure and superstructure are built simultaneously
What is the up-down construction process?
Perimeter slurry wall
Drill holes and fill with slurry
Steel columns set in concrete footing poured using a tremie
Ground floor slab poured with opening
Superstructure is built while excavation takes place (each storey individually with controlled low strength material placed to support formwork above - excavated with next storey)
