A boring machine is only as good as the ground it enters and the preparation behind it. Machine selection and guidance technology are covered elsewhere; this article deals with what happens on site, in sequence, from the first walk-over survey to the CCTV survey of the completed pipeline.

Site Investigation and Utility Locating

Nothing damages a trenchless project faster than an unrecorded service. Before the machine is mobilised, the corridor is walked over and the drawings are treated as a historic record rather than a truth. Four sources are combined: owner records, electromagnetic locators, GPR scans, and trial pits at every crossing and every point of doubt. Trial pits are typically 1.0 m by 1.0 m and 1.2 m deep, backfilled and reinstated to the original surface specification.

Electromagnetic locators are fast but fail on plastic, asbestos cement and empty ducts. GPR gives depth and position of both metallic and non-metallic objects, but needs a clean survey line and an experienced operator to read the hyperbolas. Trial pits remain the only arbitration tool: dig at every crossing and at any anomaly the instruments disagree on.

Clearance is not negotiable. Maintain a minimum horizontal separation of 0.5 m from any live service, and 1.5 m where the service is a gas main, a high-voltage cable or a large trunk sewer. If the path cannot achieve this, redesign the path, not the clearance. Vertical clearance for gravity sewers is 0.5 m crown to invert as a minimum, 1.0 m preferred where the new pipe is smaller than the existing. Log every crossing with coordinates, depth and a photograph, signed off by the client liaison.

Bore Path Design and Utility Crossings

The drill path is drawn on a longitudinal profile showing every known service, manhole and chamber, with cover depth plotted against chainage. At a crossing the bore should pass clearly above or clearly below the obstacle; a shallow angled pass through the uncertain zone is what creates risk. Acute-angle crossings are avoided because the head has little directional authority over a short distance.

Pit and Shaft Construction

The entry and receiving pits are small excavations but hold most of the programme risk. A typical entry pit for a 1.2 m machine is 4.0 m by 2.5 m in plan with a working depth of 2.5 to 3.5 m, sized so the machine can be lowered and pipe fed. Below 1.2 m depth in granular ground, shoring is required: a trench box with 10 mm shield plate for light soil, stepping up to 20 mm with walings in cohesionless ground below 3 m.

The thrust wall takes the jacking load. For a 1.2 m machine jacking 120 m of concrete pipe at 20 kN per metre average friction, total thrust approaches 2,400 kN, with peaks 30 to 40 percent higher on curves. The wall is designed for peak thrust with a factor of safety of 2.0 on ultimate capacity, which in practice means a reinforced concrete block around 1.0 m thick, 3.0 to 4.0 m wide and 2.5 to 3.0 m high, founded in competent ground and doweled into the shoring so the load bypasses the surrounding soil. Rapid-set grout between machine base and wall prevents the base bedding down and shifting the machine line.

Machine Setup and Assembly

Plan the lift before the machine arrives. The drive unit for a 1.2 m machine weighs 800 to 1200 kg and the assembled length including the thrust frame can exceed 6 m, so a crane of 25 to 40 tonne capacity with 12 m outreach is required, with a lifting plan and a banksman.

Assembly sequence is normally: position the base frame and skid, install the thrust reaction structure, set the boring machine and align it with the pipe axis, bolt the drill head and mixing chamber to the machine, attach mud lines, then connect guidance and control systems. Two to three shifts is typical for a first-time erection, while a repeat setup with the same crew takes one shift.

Levelling matters more than crews expect. The machine must be level in both axes to within 1 mm per metre across its length; an out-of-level machine makes the head wander and changes steering response. The base must be fully supported on shim packs, with the holding-down bolts torqued to specification.

The Drilling Sequence

The pilot bore comes first, using a steerable head with a bent housing, typically 300 to 450 mm for a 1.2 m product pipe. It is the accuracy-critical stage: tolerance is normally held to 10 mm vertically and 20 mm horizontally per 10 m of run, with the target at the receiving pit no worse than plus or minus 30 mm. A pipe laser with survey-grade GNSS above the machine gives 3 to 5 mm per 30 m and a 10 to 20 mm position fix.

Rigs of this type generally have a theoretical maximum pull of 60 to 100 kN at the head and drilling thrust of 80 to 120 kN. Realistic values are far lower: thrust of 40 to 70 kN and torque of 8 to 20 kN.m during a 300 mm pilot, rising to 30 to 50 kN.m reaming full diameter. Loss of torque while depth increases means the head is being pushed rather than steered; back off and re-survey before pushing harder. Reaming proceeds in two or three passes, each removing no more than 150 mm of diameter; enlarging in one step causes machine damage and bore deviation.

Pullback and pipe pull is the most force-sensitive operation. The pull must cover pipe weight, friction and machine capacity. For 120 m of 1.2 m concrete pipe with 120 mm walls, the weight in air is roughly 47 kN and in a flooded bore about 24 kN; the flooded figure is correct because the pipe is never lifted free of the water column once submerged. Add a friction allowance of 20 to 30 kN per 100 m and a 1.5 factor, giving a design pull of roughly 80 to 90 kN. Jacking pipes of 3 m length are then pushed in sequence, the jack ram inside the pipe and a thrust ring spreading the load, each joint jacked to within 10 mm of the previous.

Spoil Disposal and Slurry Management

Boring slurry lubricates the bore, carries spoil and gives buoyancy to the incoming pipe. Volume is not trivial: for a 1.2 m bore of 120 m, theoretical annular volume is close to 130 m3, and 1.5 to 2.0 times that is pumped to support the cut and transport solids.

A separation plant is normally required, a desander and desilter deck sized for the flow. Flow for a 1.2 m machine is normally 25 to 45 m3/h at 2 to 6 bar line pressure at the machine. Mud properties are checked each shift and logged: density, marsh funnel viscosity of 35 to 45 seconds, sand content and pH. Pressure climbing while penetration stays constant indicates a blocked line or a rising head; pressure collapsing while torque rises indicates a loss of face or hole collapse behind the head.

Separated spoil is wet, bulky waste needing disposal at a licensed site, and the volume belongs in the contract price from day one. Dewatering on site cuts haulage weight, but the discharge point must be confirmed with the client because solids may be contaminated.

Jacking Pipe, Lubrication and Joints

Jacking pipe needs a straightness of 1 mm per metre maximum, joint ovality of no more than 1 percent of diameter, and joints able to transmit jacking thrust without distortion. Each pipe is pushed against a thrust ring on the joint collar, the gap filled with a sealed flexible joint forming the permanent flexible point.

Lubrication is the primary control on friction, and therefore on the maximum thrust the wall must take. Bentonite is injected at the joints through a monitor tube running the length of the pipe string, forming a lubrication ring at every 3 m joint. The injection rate is adjusted to keep pace with the pipe being pushed, and lube pressure logged against chainage. Losing lube pressure while thrust rises means stop and re-inject before the joint seals take damage.

Unexpected Ground Conditions

Boulders and cobbles above 150 mm are the classic problem: a boulder deflects the head and can stall the pilot. Stop, re-survey, and if the obstruction cannot be passed, divert slightly where the target is clear or ream through it in a slow, high-torque pass.

Running sand and loose saturated ground are more dangerous because the bore can collapse behind the head and trap the machine. The defence is discipline: keep the head full of slurry, maintain pressure, keep the reamer behind the head, and never leave a hole open too long. If the head is trapped, relieve pressure, attempt to reverse the machine, and in the worst case drill from the far end for a rescue drive.

Hitting an existing utility is the worst case, and it is prevented entirely by the investigation stage. The rule is absolute: on any indication of an obstruction, stop drilling immediately, do not push through, and locate the object first. Repairs to a damaged gas or power main are the single largest cost item on trenchless contracts, and almost always follow a decision to push a few more metres.

Operator Skill and Monitoring

The operator is the instrumentation. Depth, inclination, azimuth, thrust and torque are read continuously, and the operator correlates the trend rather than the single value. The standard signals: thrust up with torque steady means hard material or a layer change; torque up with thrust steady means the head is binding or the reamer is cutting unevenly; both up means the machine is losing ground control; both down with falling mud pressure means the hole is collapsing.

Safety Hazards

Three hazards dominate. Shaft collapse is prevented by shoring design, exclusion zones, and material stored at least 1 m back from the edge. Entanglement in the drill head, mixing chamber and rotating parts requires isolation and lockout before any entry or hand clearing, and no one reaches into an open pit while the head is turning. Confined space in the reception pit during jacking requires a permit, atmospheric testing for oxygen, hydrogen sulphide and methane, and ventilation with a standby person on the surface. Define emergency response before the first drive: a shaft rescue plan and first aid and oxygen equipment on site.

Quality Assurance of the Finished Bore

The as-built survey is the first checkpoint. Every pipe invert is surveyed by total station or GNSS and compared to the design profile, with 20 mm vertical tolerance at any station and gradient deviation not exceeding 25 mm per 30 m. Where gravity sewer performance is specified, gradient is the controlling criterion rather than invert level, and any failing section is remediated by re-survey or client acceptance.

Joint integrity is checked by examining each joint as it is made and by CCTV inspection of the completed pipeline, covering every joint with pan and tilt views recorded and defects logged against a severity scale. Where the client requires a formal standard, CCTV survey to EN 1613 or the local equivalent is applied with a written report.

Leak testing is the final proof. Non-pressure pipelines are tested with low-pressure air, generally 3.0 kPa held at least 2 minutes, pressure drop not exceeding 50 Pa across the section. Pressure pipelines are tested hydrostatically at 1.5 times design working pressure, held at least 15 minutes with no measurable loss. Sections are isolated with inflatable pipe plugs, and the test record names the section, pressure and duration.

Conclusion

A boring machine contract succeeds or fails in the ground, not in the plant. The controlling discipline is to know where every utility is, to design a reaction wall and shored pits that carry the load actually generated, to hold the bore within tolerance from the first metre, to read instrument trends rather than isolated values, and to stop the moment the ground behaves unexpectedly.