Secant-pile and diaphragm-wall cofferdams

How bridges get built · Shallow foundations, cofferdams and dewatering · Secant-pile and diaphragm-wall cofferdams

Also called: secant pile cofferdam, diaphragm wall cofferdam, contiguous pile wall, embedded retaining wall cofferdam, slurry wall shaft

How it is done

Build reinforced concrete guide walls about 1 m deep along the wall line to fix its position and support the top of the trench or pile bores; divert services.
Secant walls: bore and cast the primary (female) piles first, usually unreinforced or of lower strength, then bore the reinforced secondary (male) piles cutting into them with casing or CFA rigs; hard/firm walls need the cut made at the right age, hard/hard walls need rock-capable tools.
Diaphragm walls: excavate panels 2.5-7 m long under bentonite or polymer slurry with a hydraulic grab or trench cutter, desand the slurry, lower the cage and tremie the concrete; cutters mill into the neighbouring panels to make watertight joints.
Toe the wall into clay or rock to cut off groundwater, or form a jet-grouted or grouted base plug where no cut-off stratum is within reach.
Excavate inside in stages, installing the capping beam and then walings and props or ground anchors at each level; circular shafts carry earth pressure by hoop compression, often with an inner ring lining cast as excavation proceeds. Pump only the water that enters through the base.
Monitor wall deflection (inclinometers), prop loads, groundwater levels inside and outside, and settlement of nearby roads, rails and buildings against agreed trigger levels.
Cast the footing, cap or anchorage block against the wall, which often becomes part of the permanent structure, and remove props in sequence as the permanent works take over.

Key numbers

Secant piles600-1,500 mm diameter with 100-250 mm overlap.
Diaphragm walls0.6-1.5 m thick routinely (2 m and more for large shafts); panels 2.5-7 m long; depths of 20-60 m common, over 100 m with trench cutters.
Verticalityabout 1:100 for grabs and secant piles; 1:200 or better with trench cutters.
Circular shafts of 30-90 m diameter work in hoop compression without internal props.
Lateral deflection of a stiff propped walltypically 0.1-0.3 % of excavation depth in stiff ground, more in soft clay.

Where it fits

Foundations beside rivers, roads and railways where vibration and ground movement must be limited and a sheet-pile wall would deflect too much.
Gravels, cobbles and weak rock that sheet piles cannot penetrate; excavations deeper than about 10-12 m.
Suspension-bridge anchorages and pylon foundations on deep soft ground, built as circular shafts of 50-90 m diameter.

Where it does not

Shallow temporary cofferdams in open water: sheet piles are faster and recoverable.
Sites where rigs cannot work from land or a stable platform, or where there is no space for slurry plant.
Contiguous (gapped) pile walls below the water table in sand without grouting: they leak.

Choosing it

Choose secant piles or a diaphragm wall over sheet piles when the ground holds cobbles, boulders or weak rock, the excavation exceeds about 10-12 m, vibration is not allowed, or the wall can become part of the permanent structure.
Choose a diaphragm wall over secant piles for depths beyond about 25-30 m or where very high stiffness and watertightness are needed; secant walls are cheaper and need less plant for shallower boxes.
A contiguous pile wall is acceptable above the water table or in clay; below the water table in sand, make it secant or grout the gaps.
In open water, prefer sheet-pile, double-wall or box cofferdams: embedded walls need land or a stable platform to work from.

Plant, pace and money

PlantGuide-wall formwork; rotary rigs with casing or CFA rigs for secant piles, or hydraulic grabs and trench cutters with slurry mixing and desanding plant for diaphragm walls; crawler cranes for cages; tremie equipment; props or anchor rigs; inclinometers and survey.

ProductivitySecant walls: about 4-10 piles per rig per shift depending on depth and ground. Diaphragm walls: roughly one panel per rig per day in soil (of the order of 100-300 m2 of wall), much less in rock.

CostIndicative only: secant pile walls about USD 600-1,200 per m2 of wall face and diaphragm walls about USD 800-2,000 per m2, excluding props, excavation and dewatering; a large anchorage shaft runs to tens of millions of USD.

Risks and controls

What goes wrong

Windows or defective joints in the wall (pile deviation, slurry inclusions) leaking under high head; verticality and overlap at depth decide watertightness.
Base heave or hydraulic uplift where the wall does not reach a cut-off layer.
Prop or anchor overload and excessive wall movement causing settlement of adjacent roads, rails and buildings.
Sudden slurry loss into open gravel or cavities causing trench collapse.

Quality assurance

Guide-wall survey; pile and panel verticality measured by sonic calliper or cutter inclinometers; slurry density, viscosity and sand content before concreting; concrete volume per element; cross-hole sonic logging of selected panels; inclinometer, prop-load and settlement readings against trigger levels; leakage inspection at every excavation stage.

Origins

Diaphragm walls built under bentonite slurry were developed in Italy in the early 1950s and spread worldwide within two decades; trench cutters from the 1970s and 1980s, and hard/firm secant piling with casing and CFA rigs, made deep watertight walls routine.

Examples

The anchorages of the Akashi Kaikyo, Yangluo and Maputo-Katembe bridges were built inside circular diaphragm-wall shafts; bridge foundations beside railways, highways and river embankments use secant-pile boxes where sheets cannot be driven.

Case studies

Akashi Kaikyo Bridge (Kobe-side anchorage 1A)Japan · 1998The anchorage was built inside a circular diaphragm wall about 85 m in diameter and about 75 m deep, excavated inside and then filled with concrete to carry the cable pull of the 1,991 m span.

Yangluo Yangtze River Bridge (south anchorage)China · 2007Circular diaphragm wall of 73 m outer diameter, 1.5 m thick and 61.5 m deep, with an inner lining, excavated to 45 m to found the anchorage of a 1,280 m suspension span.

Maputo-Katembe BridgeMozambique · 2018Both anchorages were built in circular diaphragm-wall shafts of 50 m outer diameter, with wall panels to 56 m and excavations of 15 m and 36 m, for Africa's longest suspension span (680 m).