Driving control: sets, refusal, restrike and splicing

How bridges get built · Driven piles · Driving control: sets, refusal, restrike and splicing

Also called: driving criteria, set per blow, refusal criteria, pile splicing

How it is done

Before production, set the driving criterion (a minimum toe level plus a set or blow count against hammer stroke) from a wave equation analysis and the design capacity, including the expected setup or relaxation.
Drive and test the first (indicator or test) piles: dynamic tests with signal matching at end of driving and on restrike, and static or bi-directional tests where required; revise the criterion before production piling.
Log every pile: blows per 250 mm over the full length, hammer stroke or energy, cushion changes, interruptions and final set; plot logs against the soil profile to spot anomalies.
Stop when the pile reaches the minimum toe level and the required set; if refusal (about 10 blows per 25 mm) occurs early, stop to protect the pile and refer to the engineer; never accept refusal on a boulder as bearing.
Restrike 10-20 % of piles after setup has developed (about a day in sand, several days to two weeks in silt and clay); clays usually gain capacity, dense fine sands and some weak rocks can lose it; adjust the criterion.
Splice when needed with full-penetration butt welds (steel) or welded end plates or mechanical joints (PHC), inspected before driving resumes, and keep splices out of the zone of maximum bending.
Handle length differences: build up piles that drive below cut-off with a lapped or welded extension, and saw off long piles, checking that the remaining head is undamaged.

Key numbers

Practical refusalabout 10 blows per 25 mm over a short final distance.
Restrike timingabout 1 day in sand, 3-14 days in silt and clay.
Setup in clay can raise capacity by 50-100 % or more over days to weeks; relaxation can reduce it in dense fine sand and shale.
AASHTO resistance factor 0.65 where criteria come from dynamic testing with signal matching and at least 2 % of production piles are tested.
Blow counts recorded per 250 mm over the full length of every pile.

Where it fits

Every driven pile job.

Where it does not

Driving to a fixed length regardless of set; accepting refusal on a boulder as bearing on rock.

Choosing it

Rely on wave-equation criteria confirmed by dynamic testing rather than dynamic formulae, which are unreliable and carry low resistance factors.
Use static or bi-directional load tests where the design depends on large setup, piles are long and heavily loaded, or a higher resistance factor will pay for the test.
Specify a minimum toe level as well as a set: a set alone can stop a pile on a thin hard layer above a soft one.

Plant, pace and money

PlantBlow counter, PDA, welding inspection.

ProductivityGoverns the piling programme more than the hammer does.

CostSmall; poor criteria cost far more.

Risks and controls

What goes wrong

Overdriving through the bearing layer, underdriving into a soft layer below the target, splice failure.

Quality assurance

Driving log for every pile, restrike programme, splice inspection records.

Origins

Dynamic formulae such as the Engineering News formula (1888) and Hiley's (1920s) were the first driving criteria; E. A. L. Smith's wave equation method (1960) and the Case Method and pile driving analyser developed at the Case Institute of Technology from the 1960s made driving control a measured, analysable process.

Examples

The game's load-test step models this: a test pile turns a weaker-than-expected layer into a re-sized group instead of a surprise.

Case studies

New Edison Bridge, Fort Myers (Florida)USA · 1990sEleven test piles at five sites, with dynamic monitoring during driving and restrikes plus uplift tests by layer, were used to quantify setup and to write the production driving criteria.

San Diego-Coronado BridgeUSA · 1969Prestressed concrete piles were taken through dense clay and sand in three controlled stages: pre-jetting with driving, side jetting with driving, then hammer only to the final set.