Foundation notes

Foundations

Foundations

• Feature common to bridge and building structures

• Deep vs. shallow• Safely transfer dead

and live loads with acceptable levels of settlement

Foundations

Shallow Foundations– spread footings: column,

combined, mat– strip footings: walls

• placed on undisturbed or well-compacted soil

• below frost line• consider effects of

groundwater• typically reinforced or plain

concrete

Foundations

Deep Foundations• Piles

– driven or drilled– bearing capacity from end-bearing or skin

friction: usually combination of both– driven plum or battered (1:6 to 1:3)– driven to attain minimum tip elevation or

required resistance in blows per foot (bpf)

Foundations

Piles• Types

– timber piles– precast/prestressed

concrete piles– cast-in-place concrete– steel H-piles– steel pipe piles– composite– minipiles or micro piles

Foundations

Pile Driving

• Equipment– mobile crane (usually crawler)– leads: fixed or swinging– hammer, helmet, & cushion– compressor (if air driven)– hydraulic unit (if vibratory)– steam plant (becoming

obsolete)

Foundations

Pile Driving

• Equipment– follower– mandrel

• Pile accessories– point attachments– splices– pile to cap connections

Foundations

Pile Driving • Hammers

– steam or air: single-acting, double-acting, differential

– diesel hammer: single-acting (open end) or double-acting (closed end)

– vibratory

Pile Driving

• Dynamic analysis - wave equation “WEAP”– based on wave propagation theory with input

from the static analysis – provides relationship between pile capacity

and driving resistance i.e.: equates capacity to blow counts

– predicts pile driving stresses

Pile Driving

• Benefits of using wave equation– verify piles can be driven to required depth &

capacity– determine minimum required pile section– avoid overstressing pile during driving– optimize cost

Pile Driving

• Dynamic testing - Pile Driving Analyzers (PDA) and CAPWAP– uses strain gauge &

accelerometers to measure force & velocity (converts strain to force and acceleration to velocity)

– field tests the driving hammer and system

Pile Driving

• Dynamic testing - Pile Driving Analyzers (PDA) and CAPWAP– cost effective method of

optimizing the driving system and estimating static pile capacity

– using CAPWAP, hammer blows can be analyzed to estimate skin friction, end bearing, dynamic damping factors, and soil stiffness

Pile Driving

Pile Load Test (static)• many tests have been used over the years to

evaluate or confirm axial and lateral capacities• Quick pile load tests (QPLT) are most common• QPLT consists of constructing a reaction frame

around the test pile • the frame is anchored to reaction piles• pile is incrementally loaded with an hydraulic jack

Pile Driving

Pile Driving

Pile Load Test (static)• the test load is measured by a load cell• deformation is measured with dial gauges, wire-

mirror-scale, and surveying instruments• apply load in 10 to 15% increments, maintaining

a 2½ minute interval• check deformation readings immediately before

and after applying load

Pile Driving

Pile Load Test (static)• incrementally increase load until reaching 200%

of the design load or until continuos jacking is required to maintain the test load

• remove the load in 4 equal decrements• plot the load-settlement data using the Davisson

method• the point where the load-settlement curve

crosses the failure criterion is the failure load (Qf)

Pile Driving

Other Factors

• soil setup or “freeze” - common in clays

• relaxation - much less common, found in some silts or fine sand

• restrike pile to verify strength gain or loss

• 120 bpf is practical, safe, upper limit driving blow count

• 200 to 240 bpf is practical refusal

Pile Driving

Inspection

• position, plumb, or batter

• check cushion; change as necessary

• record blow counts

• verify correct operation of hammer

• check splices

• check piles for damage

• check for heave

Pile DrivingDriving Records

• prepare a sketch of foundation to identify piles and order of installation

• Drive records include:

– pile ID and order of installation

– ground elevation – cut-off elevation

– tip elevation – final length

– blows per foot (final foot recorded in bpi)

– driving duration – location of splices

Foundations

Retaining Walls• gravity or cantilever • components

– stem wall– horizontal base

includes:

toe, heel, & key– weep holes for

drainage

Foundations

Temporary Support Structures and

• sheeting w/wales and tie-backs

• soldier beams (piles) w/lagging

• pipe sleeves• tremie concrete

Foundations

Underpinning• remedial or

precautionary• remedial adds

foundation capacity to inadequately supported structures

• precautionary - higher loads or changes in ground conditions

Foundations

Underpinning• Temporary supports

– shores– needle beams– grillage

• Pit underpinning - used for installing or extending foundation walls or footings

• Pile underpinning & various other methods