Box jacking and pipe-roof jacking under railways and roads

How bridges get built · Heavy lift, moving and accelerated construction · Box jacking and pipe-roof jacking under railways and roads

Also called: jacked box tunnel, box pushing, pipe roof method, jacked underpass, tubular roof, front jacking

How it is done

Investigate the ground and the asset above: soil profile and groundwater, track or pavement tolerances, utilities, and the settlement and heave limits agreed with the railway or highway authority.
Build the jacking pit and thrust (reaction) wall beside the embankment, with a jacking base slab on line and level, and prepare the reception pit on the far side.
Treat the ground where needed (dewatering, grouting or ground freezing) and install a pipe roof by jacking or auger-boring steel pipes over the box line, sometimes interlocked or filled with concrete.
Cast the box, as one unit or several, on the base slab with a cutting shield at the front, and install anti-drag cables or sheets between box and ground where cover is shallow, so the overlying soil is not dragged along with the box.
Jack in cycles: excavate a short advance at the shield face, push with the main jacks against the thrust wall, use intermediate jacking stations between units for long strings, and lubricate the sides with bentonite or polymer.
Control line and level continuously by survey or laser guidance, steering with differential jack strokes and face excavation; monitor track or pavement levels in real time and impose speed restrictions or stop if trigger values are reached.
On arrival, seal the joints, grout voids around the box, deal with the anti-drag cables as designed, build wingwalls and approaches, and restore track geometry by tamping.

Key numbers

Box sizeabout 3–25 m wide and 3–12 m high (Boston I-90 boxes up to about 24 m wide and 11.5 m high)
Length pushedabout 10–120 m in one or several units (Boston strings up to about 116 m)
Coverfrom about 1 m (with pipe roof or anti-drag system) to 10 m
Advanceabout 0.5–3 m per day
Jacking forcehundreds of tonnes for small boxes to over 10,000 t for the largest (Boston about 11,000–12,000 t)
Track movementtrigger levels agreed with the railway, typically a few tens of millimetres, with tamping crews on standby

Where it fits

New road, rail and pedestrian underpasses and culverts under operating railways, motorways and embankments; lengths of about 10–120 m; sites where possessions or closures are unavailable or too costly.

Where it does not

Ground with boulders or obstructions at the face unless probed and planned for, very shallow cover without a pipe roof or anti-drag system, high water tables in granular soils without dewatering, freezing or grouting, assets above that cannot tolerate any movement.

Choosing it

Choose box jacking over cut-and-cover with temporary track bridges when the railway or road cannot be closed for the weeks that cut-and-cover would need.
Prefer cut-and-cover, or a box built alongside and slid into an open cut, when a closure of a few days is available; it is usually faster and cheaper.
Add a pipe roof when cover is less than about one box height or the ground is loose; add freezing or grouting when the face is below the water table in granular soil.
Prefer pipe jacking or microtunnelling for small openings (under about 3 m) and long drives.

Plant, pace and money

PlantJacking pit with thrust wall, main jacks of a few hundred tonnes each and intermediate jacking stations, front shield, excavators or roadheaders at the face, pipe-jacking or auger-boring rig for the pipe roof, anti-drag cables, freezing or grouting plant, real-time track monitoring.

ProductivityBox casting in the pit over weeks to months; jacking at about 0.5–3 m per day, so a 30 m underpass is pushed in a few weeks.

CostIndicative: USD 50,000–200,000 per metre of box for typical road and footpath underpasses beneath railways; large multi-lane boxes with ground freezing cost much more (the Boston South Station contract was about USD 400 million).

Risks and controls

What goes wrong

Track or pavement settlement or heave beyond limits, face collapse or running ground, the box diving or yawing off line, drag of the overlying soil and track with the box, frost heave and thaw settlement where freezing is used, failure of the thrust wall.

Quality assurance

Monitoring of track or pavement levels with trigger and action values agreed with the owner, survey of box line and level every cycle, jack force records, face inspection, verification of ground treatment (freeze temperatures, grout takes), checks of post-jacking grouting.

Origins

Box jacking grew out of pipe jacking; John Ropkins developed the jacked-box technique with his anti-drag system, used with Edmund Nuttall for several UK railway and road underpasses, and at large scale for the I-90 tunnels under Boston's South Station rail yard around 2000. Pipe-roof and front-jacking variants became standard under railway embankments in Japan and Korea.

Examples

The I-90 tunnels beneath the South Station rail yard in Boston (around 2000), UK railway and motorway underpasses built with the Ropkins system, and many Japanese and Korean underpasses beneath railway embankments using pipe-roof and front-jacking variants.

Case studies

I-90 tunnels under the South Station rail yard (Central Artery/Tunnel project), BostonUSA · 1999–2001Three boxes up to about 24 m wide, 11.5 m high and 116 m long were jacked beneath seven active rail lines using ground freezing (over 1,700 freeze pipes), the Ropkins anti-drag system and thrusts of 11,000–12,000 t.

Jacked culvert under the Great Western main line near MaidenheadUK · 1990sA large box jacked under the main railway line, believed to be the first jacking project to use ground freezing.