excav & deep foundations fall 09
TRANSCRIPT
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Foundations&Excavation
Leaning Tower of Pisa
Professor Richard Luxenburg, AIA
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Foundations
Purpose; Transfer Building Loads to the
Earth
Basic TypesShallow: Transfer Load @ Base of
Substructure Deep: Penetrate Unsuitable Soil to
Reach Competent Soil or Rock
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Deep Foundations - Purposetransfer building loads deep into the earth
Basic typesDrilled (&
poured)
Driven
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Caissons
Similar to a column footing -
only deep
Drilled to required bearing
capacity
Point bearing (exception -
socketed)
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Caisson Installation Sequence
Hole drilled with a large drill rig
Casing installed (typically)
Bell or Tip enlargement (optional)
Bottom inspected and tested
Reinforced
Concrete placement (& casing removal)
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casing being removed
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Driven Piles
Two basic types of Piles
End bearing pile - point loading
Friction pile - load transferred by friction resistance between the pile and the earth
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Pile material
Steel; H- piles, Steel pipeConcrete; Site cast or PrecastWood; TimberComposite
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Pile Driver
Massive Rig - Crane w/Leads (Guide Rails)
Logistics and Sequence Considerations (Level Site)
Noise , Vibration
Single Acting - Lift and Drop
Double Acting - Steam, Compressed Air or Hydraulics, Diesel
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Pile Caps
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Site Cast Concrete Piles
Cased Piles Uncased Piles
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Deep Foundations
Units of MeasureDrilling & Driving
#, Lineal foot, & Size
Major Productivity IssuesType of Material (Soil)Material & Equipment
AccessQuantity (Total & # per
cap/cluster)
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Differential Settlement
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shallow foundation with perimeter spread footing
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Soils
What Architects & Engineers look for:
• Structural Properties
• Particle Size
• Drainage / Water Retention
• Cohesiveness
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ClaysPorous(sandy)
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Soil Distribution
Rarely one type - often a Mixture and/or Different Strata
Distribution of soil type and Particle Size Important in Predicting:Load Bearing CapacitySoil StabilityDrainage Characteristics
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Loadbearing Capacity
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Soil Borings / Penetration Tests
Common on projects with high loadings
Bearing Capacity - # Blows/Unit (ft)
Soil Strata & Water Table Data
Soil Samples & Testing
Sample Holes Strategically Located
Information used to prepare Soils Report
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Foundation Design & Building Codes
Sample IBC Provisions Maximum loadbearing values for
soil types Design criteriaSoil investigation & testing
requirementsMinimum foundation dimensions
Purpose: to ensure an adequate building foundation system.
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Test boring equipment
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Rocks & Soil
RocksMineral particles are firmly bonded
togetherLocated beneath layer(s) of soil
SoilUnconsolidated mineral particles or
conglomerates located at the top layer of the earth’s crust
Most foundations are supported on soil, except when the soil strength is not adequate
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Soil classification
Particle size Coarse grained (gravel & sand) Fine grained (silt & clay)
Particle shape Spherical or ellipsoidal, shaped by mechanical
weathering Gravel, sand & silt
Flat, plate-like, large surface area to volume ratio Behavior influenced by electrostatic forces and
presence of waterCohesion Cohesive soils are fine grained and particles are
attracted to each other in the presence of water. Non-cohesive soils are course grained and are not
attracted to each otherOrganic soils consist of fully or partially decayed plant
matter, compressive and unsuitable for foundations
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Soil characteristics by particle size
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Soil properties &
Foundation design• Engineering properties
• Soil bearing capacity• Particle size distribution
• Stability & drainage• Nature of excavation• Depth of water table
• Drainage and dewatering• Compressivity
• Foundation settlement
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Soil sampling and testing
Soil samplingTest pit methodTest boring method
Also allows for standard penetration test on site
Laboratory testingSieve analysis
Determines particle size distribution Soil classification
Moisture contentDry densityLiquid limit, plastic limitCompressive strengthShear strength
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Test boring log
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Earthwork
Units of MeasureExcavation & Backfill Cubic
Yards (CY)Grading Sq. Ft. or Sq. Yd.
Major Productivity Issues
Type of Operation (Mass to Ltd./Confined))
Type of Material (Soil)Material TransportationExpected Environmental
Conditions
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Slope SupportDeep Excavations
Types of Site Conditions
Unrestricted Site – area sufficiently larger that the building footprint
Restricted Site – area constricted / limited
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Unrestricted Site
Bench and/or Angle of Repose
Must have perimeter clearance
ConsiderationsBank ErosionWater DiversionSafetyStorage of Backfill (& cost)
Most likely - least expensive
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Sloped Excavation@ the ‘Angle of Repose’
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Benched Excavation
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Excavation in Clay(A Cohesive Soil)
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Restricted Site
Types of Sheeting
Soldier Beams and Lagging
Sheet PilingWoodSteel Precast
Slurry Wall
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Supports for deep excavations
Sheet pilesInterlocking vertical steel
sheets driven into soil prior to excavation
Lower part of sheet remains buried, providing cantilever
Deep excavations braced (> 15 ft.)
BracingCross-bracingDiagonal bracingTieback
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Profile of steel sheet piles
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Sheet piles driven into soil with diesel hammer
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Drilling for tiebacks
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Close-up: drilling for tiebacks
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Tieback hole
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Tendon as tieback
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Connection of lagging to soldier piles
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Plan: contiguous bored piles
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Contiguous bored piles support deep urban excavation
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Secant piles
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Reinforced concrete wall using bentonite slurry
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Section: soil nailed excavation support
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View of soil nailed excavation
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Solder Beam & Lagging
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with tiebacks
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Sheet Pile Options
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Steel Sheeting
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Steel Sheeting Interlock
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Slurry Wall
StepsLayout
Excavate the soil
Interject Slurry to
prevent Collapse as
Excavation Continues
Install Reinforcing
Place Concrete
(replaces the slurry mix)
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Restricted Site
Sheeting Support
Unbraced - CantileveredBraced
Crosslot BracingRackers and Heel BlocksTiebacks
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Bracing
Crosslot
Rackers
Tiebacks
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Crosslot Bracing
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Tieback Installation
Rotary Drill Hole
Insert & Grout Tendons
Tendons Stressed & Anchored
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Dewatering
A process of removing Water and/or lowering the Water Table within a construction site
Purpose: To Provide a Dry working platform - (typically required by Code and Specification)
If the Water Table is above the working platform;Options:
Keep water out Let water in & remove it Combination
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Sump Pumps
Sump Pump ConsiderationsPump(s) Placed in Low points
Water collected & Pumped Out
Requires clearance around
excavation
Most Common
Often - Least expensive
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Well Points
Keeping water out of the excavation by
Lowering the water table
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Suction Pumps
Discharge Line
Well Point
Header Pipe
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Watertight Barrier Walls
Keep Water OutBarrier must reach an impervious strataTypesSlurry WallsSheeting w/
pumpsMust resist hydrostatic pressure
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Major Building Parts
Superstructure
Substructure
Foundation
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Seismic Base Isolation
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Underpinning
a process of strengthening and/or stabilizing the foundation of an existing building
Reasons it may be requiredFailure of existing foundationChange in building useNew construction adjacent to
existing
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Underpinning Methods and Problems
MethodsEnlarge existing foundationInstall a new foundationStabilize surrounding soil
Problems/ConcernsDangerousTemporary support and existing
conditionsLimited work space, slow process,
expensive
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Retaining walls Design Elements to Prevent
Failure
Relieve H2O pressureCrushed stone Weeps
OverturningCantilevered
FootingReinforcing
SlidingKey
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Waterproofing
Two basic approaches to WaterproofingWaterproof Membranes, orDrainageGenerally - both used in tandem
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Waterproofing Membranes
MaterialsLiquid or Sheet (Plastic, asphaltic,
synthetic rubber)Coatings (asphaltic)Cementitious Plasters & admixturesBentonite clay
AccessoriesProtection BoardWaterstop
Unit of Measure - SF, Mils (thickness)
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DampproofingTypically, a liquid asphalt
applied with a roller or sprayer
Not an effective barrier for water under pressure.BUT, will prevent ground
‘moisture’ from migrating through a wall.
Typically used in conjunction will drainage pipe.
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Site preparation prior to construction
Fencing site
Locate and mark utility lines
Demolish unneeded structures and utility lines
Remove trees, brush, topsoil
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ExcavationThe process of removing soil or rock from its original location in preparation for construction
Depth of excavation depends on Foundation type (deep or shallow)Soil type
Types of excavationsOpen. Large and sometimes deep
excavationsTrenches. Linear excavations for
utilities or footingsPits. Excavations for footing of one
column, elevator shaft, etc.
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Controlling groundwater
Prevent surface water from entering excavation
Draining soil around the foundationSumpsWell points
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Sumps collect and pumps discharge groundwater after it
enters the excavation
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Wellpont system prevents water from entering
excavation
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Footing types
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Presumptive allowable bearing capacity of soils
Class of Material Allowable bearing capacity
Crystalline bedrock 12.0 ksf
Sedimentary and foliated rock 4.0 ksf
Soil types GW and GP 3.0 ksf
Soil types SW, SP, SM, SC, GM, & GC
2.0 ksf
Soil types CL, ML, MH, and CH
1.5 ksf
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Commonly used deep foundations
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Grade beam and drilled piers
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Formwork: Grade beam and drilled piers
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Grade beam and drilled piers
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Wood light frame on grade beam and drilled piers
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Structural slab on drilled pier, note void boxes below slab
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Grade beams and under slab utilities in place
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Void boxes being placed prior to constructing structural slab
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Interior of structural slab on ground supported on pier caps
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Below grade waterproofing
Prevent water from reaching foundationDirect surface run-off away
from building
Waterproof below grade structureApply waterproof layerIncorporate a foundation
drainage system
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Waterproofing layer
Applied to outside surface ofbasement wall basement floor
Cold systems most commonly used todayRubberized asphalt or
thermoplastic sheets60 mil thick, self-adhering rolls
Liquid applied elastomeric compoundsSprayed on or rolled
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Drainage system
Purpose of system is to collect, drain and discharge subsoil water away from buildingSystem includes Drainage mats
Placed against the waterproofing or insulation layer
Open structure allows water to drain downward Reduce or eliminate water pressure on below
grade wall Protect waterproofing
Foundation drain pipe sloped to carry water away from foundation to storm drain or sump pit
4 in to 6 in perforated pipe at base of foundation Laid in bed of crushed rock Protected by filter fabric that prevents clogging
from soil
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Typical foundation waterproofing and drainage
system for subgrade wall
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Positive-side, negative-side and blind-side
waterproofing
Positive-side added to exterior wall Most effective and commonly used Protects wall against water seepage, mold growth,
corrosion Backfilled after completion
Negative-side applied to interior side of wall Used in remedial applications, not as effective as
other optionsBlind-side is applied to the outside of a wall that becomes inaccessible after construction Structural wall placed directly against excavation
support system that acts as formwork. Applied directly to excavation support system
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Blind-side waterproofing
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Constructing a basement foundation using blind-side
waterproofing
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Reinforcement placed in preparation for interior
formwork.
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Symbols used is USCS
G Gravel
S Sand
M Silt
C Clay
Pt Peat
O Organic
W Well-graded
P Poorly-graded
L Low plasticity
H High plasticity
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Increasing moisture content in fine-grained soil
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Definitions
Plastic limit (PL)- moisture content at which soil becomes plastic (putty-like)
Liquid limit (LL)- moisture content at which soil becomes liquid (slurry)
Plasticity index (PI) - difference between Liquid limit and Plastic limit
LL - PL = PI
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Combination Spread & Strip Footing