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 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 (Q f )

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 Driving Driving 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