Foundations&Excavation

Leaning Tower of Pisa

Professor Richard Luxenburg, AIA

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

Deep Foundations - Purposetransfer building loads deep into the earth

Basic typesDrilled (&

poured)

Driven

Caissons

Similar to a column footing -

only deep

Drilled to required bearing

capacity

Point bearing (exception -

socketed)

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)

casing being removed

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

Pile material

Steel; H- piles, Steel pipeConcrete; Site cast or PrecastWood; TimberComposite

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

Pile Caps

Site Cast Concrete Piles

Cased Piles Uncased Piles

Deep Foundations

Units of MeasureDrilling & Driving

#, Lineal foot, & Size

Major Productivity IssuesType of Material (Soil)Material & Equipment

AccessQuantity (Total & # per

cap/cluster)

Differential Settlement

shallow foundation with perimeter spread footing

Soils

What Architects & Engineers look for:

• Structural Properties

• Particle Size

• Drainage / Water Retention

• Cohesiveness

ClaysPorous(sandy)

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

Loadbearing Capacity

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

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.

Test boring equipment

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

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

Soil characteristics by particle size

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

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

Test boring log

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

Slope SupportDeep Excavations

Types of Site Conditions

Unrestricted Site – area sufficiently larger that the building footprint

Restricted Site – area constricted / limited

Unrestricted Site

Bench and/or Angle of Repose

Must have perimeter clearance

ConsiderationsBank ErosionWater DiversionSafetyStorage of Backfill (& cost)

Most likely - least expensive

Sloped Excavation@ the ‘Angle of Repose’

Benched Excavation

Excavation in Clay(A Cohesive Soil)

Restricted Site

Types of Sheeting

Soldier Beams and Lagging

Sheet PilingWoodSteel Precast

Slurry Wall

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

Profile of steel sheet piles

Sheet piles driven into soil with diesel hammer

Drilling for tiebacks

Close-up: drilling for tiebacks

Tieback hole

Tendon as tieback

Connection of lagging to soldier piles

Plan: contiguous bored piles

Contiguous bored piles support deep urban excavation

Secant piles

Reinforced concrete wall using bentonite slurry

Section: soil nailed excavation support

View of soil nailed excavation

Solder Beam & Lagging

with tiebacks

Sheet Pile Options

Steel Sheeting

Steel Sheeting Interlock

Slurry Wall

StepsLayout

Excavate the soil

Interject Slurry to

prevent Collapse as

Excavation Continues

Install Reinforcing

Place Concrete

(replaces the slurry mix)

Restricted Site

Sheeting Support

Unbraced - CantileveredBraced

Crosslot BracingRackers and Heel BlocksTiebacks

Bracing

Crosslot

Rackers

Tiebacks

Crosslot Bracing

Tieback Installation

Rotary Drill Hole

Insert & Grout Tendons

Tendons Stressed & Anchored

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

Sump Pumps

Sump Pump ConsiderationsPump(s) Placed in Low points

Water collected & Pumped Out

Requires clearance around

excavation

Most Common

Often - Least expensive

Well Points

Keeping water out of the excavation by

Lowering the water table

Suction Pumps

Discharge Line

Well Point

Header Pipe

Watertight Barrier Walls

Keep Water OutBarrier must reach an impervious strataTypesSlurry WallsSheeting w/

pumpsMust resist hydrostatic pressure

Major Building Parts

Superstructure

Substructure

Foundation

Seismic Base Isolation

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

Underpinning Methods and Problems

MethodsEnlarge existing foundationInstall a new foundationStabilize surrounding soil

Problems/ConcernsDangerousTemporary support and existing

conditionsLimited work space, slow process,

expensive

Retaining walls Design Elements to Prevent

Failure

Relieve H2O pressureCrushed stone Weeps

OverturningCantilevered

FootingReinforcing

SlidingKey

Waterproofing

Two basic approaches to WaterproofingWaterproof Membranes, orDrainageGenerally - both used in tandem

Waterproofing Membranes

MaterialsLiquid or Sheet (Plastic, asphaltic,

synthetic rubber)Coatings (asphaltic)Cementitious Plasters & admixturesBentonite clay

AccessoriesProtection BoardWaterstop

Unit of Measure - SF, Mils (thickness)

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.

Site preparation prior to construction

Fencing site

Locate and mark utility lines

Demolish unneeded structures and utility lines

Remove trees, brush, topsoil

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.

Controlling groundwater

Prevent surface water from entering excavation

Draining soil around the foundationSumpsWell points

Sumps collect and pumps discharge groundwater after it

enters the excavation

Wellpont system prevents water from entering

excavation

Footing types

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

Commonly used deep foundations

Grade beam and drilled piers

Formwork: Grade beam and drilled piers

Grade beam and drilled piers

Wood light frame on grade beam and drilled piers

Structural slab on drilled pier, note void boxes below slab

Grade beams and under slab utilities in place

Void boxes being placed prior to constructing structural slab

Interior of structural slab on ground supported on pier caps

Below grade waterproofing

Prevent water from reaching foundationDirect surface run-off away

from building

Waterproof below grade structureApply waterproof layerIncorporate a foundation

drainage system

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

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

Typical foundation waterproofing and drainage

system for subgrade wall

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

Blind-side waterproofing

Constructing a basement foundation using blind-side

waterproofing

Reinforcement placed in preparation for interior

formwork.

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

Increasing moisture content in fine-grained soil

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

Combination Spread & Strip Footing