PHC spun concrete piles

How bridges get built · Driven piles · PHC spun concrete piles

Also called: prestressed high-strength concrete pile, spun pile, PHC pile

PHC spun concrete piles — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Before driving, run a wave equation analysis to choose the hammer and cushion and to set limits on driving stress in compression and, through soft layers, in tension.
Check delivered segments for cracks, end-plate squareness and length; set out the positions and pitch the first segment in the leader with a plywood or timber cushion on the head.
Drive with a hydraulic hammer, reducing the stroke in soft layers to avoid tension cracking; record blows per 250 mm and hammer energy for the full length.
Splice by welding the end plates with a circumferential groove weld (or with a mechanical joint); let the weld cool before driving resumes and inspect it.
Drive to the set at or below the minimum toe level in the bearing layer; stop and refer to the engineer if refusal comes well above the expected level or the resistance suddenly drops (a possible break).
Restrike selected piles after setup and test them dynamically with signal matching; run low-strain integrity tests on a sample to find cracked piles or failed joints.
Cut the head with a saw rather than a breaker, fill the hollow with a reinforced concrete plug or weld on a head plate with anchor bars, and embed in the cap.

Key numbers

Diameter300-1,200 mm; concrete strength 80 MPa or more (over 100 MPa now common in Japan).
Effective prestress classes (JIS)about 4, 8 and 10 MPa for types A, B and C.
Segment length5-15 m; spliced piles of 50-60 m are routine and 70-80 m possible.
Working loadabout 1-4 MN per pile.
Driving stress limits (AASHTO)compression 0.85 f'c minus effective prestress; tension about 0.25 sqrt(f'c) plus effective prestress (MPa).
Verticalitywithin about 1:75-1:100.

Where it fits

Alluvial and coastal sites with a bearing layer of dense sand or gravel; viaduct approaches, low piers, wetland viaducts on trestles.
Loads of 1-4 MN per pile; groups of 4-20 under a cap.

Where it does not

Boulders, cobbles and shallow rock: the pile refuses, cracks or breaks (the game's boulder event).
Very stiff clay over the full length: hard driving damages the joints.
Urban sites with noise and vibration limits.

Choosing it

Choose PHC over bored piles when a dense sand or gravel bearing layer lies within about 40 m and loads are moderate (1-4 MN per pile): much cheaper and faster per unit of capacity.
Prefer steel pipe piles where ship impact, large lateral loads, liquefaction or hard driving demand bending capacity and ductility, or where piles must be spliced to great length.
Prefer SIP or another pre-bored method near housing, or where dense upper layers would cause hard driving.
Prefer bored piles or rock sockets where boulders, cobbles or rock are expected above the bearing level.

Plant, pace and money

PlantPiling rig with a 7-15 t hydraulic hammer, service crane, welding set; barge-mounted rigs for shallow water.

Productivity6-15 piles of 20-30 m per rig per shift.

CostUSD 60-140 per metre installed for 600 mm; the cheapest deep foundation where it works.

Risks and controls

What goes wrong

Tension cracking during easy driving in soft clay, joint failure, pile running off line, heave of neighbouring piles in clay.

Quality assurance

Driving records (blows per 250 mm), PDA on 5-10 % of piles, integrity tests after driving, verticality within 1:75.

Origins

Spun reinforced concrete piles were made in Japan from 1934 and prestressed spun piles from 1962; high-strength PHC piles (80 MPa and above) came into use in the 1970s, were standardised in JIS in 1982, and since the late 1980s have made up over 90 % of Japanese precast pile production, spreading to Korea, China and Southeast Asia.

Examples

Most Korean expressway viaducts on alluvium; the reference optimiser picks PHC where a dense layer is reachable.

Case studies

Metropolitan Expressway Route 1, Higashi-Shinagawa, TokyoJapan · 1962Marine piers of the expressway were the first use of prestressed spun concrete piles in Japan, chosen for better corrosion resistance than steel pipe piles.

Lake Pontchartrain CausewayUSA · 1956Spun, prestressed hollow cylinder piles of 1.37 m diameter and about 69 MPa concrete, forerunners of the modern PHC pile, carried the 38 km of twin-pile bents; built in about 14 months from the first pile.