Underground, Bridge and Infrastructure Techniques

Introduction

Modern construction increasingly takes place in constrained, congested and environmentally sensitive locations. Urban railways, utility corridors, highways, river crossings and major interchanges must often be constructed while existing roads, buildings, services and communities remain operational. Underground, bridge and accelerated infrastructure techniques provide methods for delivering such projects safely and efficiently.

This chapter examines six connected areas: cut-and-cover construction, tunnelling, trenchless technology, retaining systems, bridge erection, and segmental and accelerated construction. The emphasis is on method selection, sequencing, temporary works, quality control and risk management. Although specialist design must be undertaken by qualified engineers, construction supervisors and diploma-level engineering personnel need to understand how these systems work and how site decisions affect safety, cost, programme and durability.

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Explain the principles and construction sequence of cut-and-cover and tunnelling methods.
2. Distinguish among common trenchless technologies and identify suitable applications.
3. Describe retaining systems and the main factors governing their selection.
4. Explain bridge erection methods, including launching, lifting, balanced cantilever and incremental launching.
5. Describe precast segmental construction and accelerated bridge construction.
6. Identify major underground and bridge-construction risks and propose practical controls.
7. Apply method-selection criteria to a constrained infrastructure project.

1. Method Selection for Infrastructure Works

Construction method selection is a technical and managerial decision. The most economical method is not necessarily the one with the lowest direct cost; it is the method that provides an acceptable balance among safety, constructability, environmental impact, programme, quality, maintenance and whole-life cost.

Important selection variables include:

• ground type, groundwater pressure and geological variability;
• depth, alignment and cross-section of the proposed work;
• proximity of buildings, roads, railways and buried services;
• available working space and access for plant;
• traffic and pedestrian-management requirements;
• allowable settlement and vibration;
• construction duration and possession restrictions;
• availability of specialist equipment and skilled labour; and
• regulatory, environmental and community requirements.

A method statement should explain the selected process, temporary works, plant, personnel, inspection points, emergency arrangements and interfaces with other contractors. It should be supported by design calculations, drawings, risk assessments and inspection-and-test plans.

2. Cut-and-Cover Construction

Cut-and-cover is an underground construction method in which a trench or excavation is formed from the surface, the underground structure is constructed, and the excavation is then backfilled or roofed. It is commonly used for shallow transport tunnels, underpasses, culverts, station boxes and utility structures.

2.1 Basic forms

Two principal arrangements are used:

Bottom-up construction. The excavation is supported, the base slab is placed first, followed by walls and roof. The excavation is then backfilled and the surface reinstated.

Top-down construction. Perimeter walls and columns are installed first, followed by a roof slab. Surface activities can resume while excavation continues beneath the roof. Intermediate slabs are constructed as excavation proceeds downward.

Bottom-up construction generally provides better access for placing reinforcement and concrete. Top-down construction can reduce surface disruption and wall movement, but it requires accurate construction of diaphragm walls, plunge columns, temporary propping and restricted underground working.

2.2 Typical sequence

A typical bottom-up sequence is:

1. survey the site and locate existing services;
2. install traffic, pedestrian, environmental and utility controls;
3. construct a retaining system or battered excavation;
4. install groundwater-control measures where necessary;
5. excavate in stages, installing struts, walers or anchors as designed;
6. prepare and waterproof the formation;
7. construct blinding concrete, base slab, walls and roof;
8. install drainage, mechanical and electrical systems;
9. waterproof, protect and test the structure;
10. backfill in controlled layers and reinstate the surface.

Excavation must proceed according to the designed support sequence. Removing soil below a strut level before installing the next support can cause excessive wall movement or collapse.

2.3 Groundwater and settlement

Groundwater may enter through permeable soils, joints, construction interfaces or defects in cutoff walls. Control options include sump pumping, wellpoints, deep wells, grout curtains, secant piles, diaphragm walls and impermeable membranes. Pumping can lower the groundwater table outside the excavation and cause consolidation settlement. Consequently, pumping systems should be designed with monitoring wells and trigger levels.

Settlement may result from ground loss, wall deflection, dewatering, vibration, inadequate backfill compaction or leakage. Survey prisms, settlement markers, inclinometers, piezometers and crack gauges provide information for observational control. Monitoring is useful only when action levels and response procedures have been established in advance.

2.4 Advantages and limitations

Cut-and-cover is often economical for shallow structures and permits direct visual inspection. It can use conventional excavation and concrete equipment and is comparatively straightforward where land is available. Its disadvantages include substantial surface disruption, traffic diversion, utility relocation, noise, dust, groundwater risk and potential impact on adjacent structures. At greater depths, the volume of excavation and temporary support may make tunnelling more attractive.

3. Tunnelling Techniques

Tunnelling creates an underground passage without exposing the entire surface alignment. Tunnels may be excavated through soil or rock by mechanical excavation, tunnel boring machines, roadheaders, drill-and-blast methods or controlled excavation systems.

3.1 Tunnel boring machines

A tunnel boring machine (TBM) excavates at the face while providing support and, in many systems, installing a permanent lining. Common categories include:

• Earth pressure balance TBMs, which use conditioned excavated soil to balance face pressure in cohesive or mixed ground.
• Slurry shield TBMs, which use pressurised slurry to support the face in granular or water-bearing ground.
• Hard-rock TBMs, which use cutting discs and may operate in open or shielded modes.
• Double-shield TBMs, which can install segments while advancing in suitable rock conditions.

A TBM requires launch and reception facilities, segment supply, spoil handling, ventilation, power, survey control, maintenance access and emergency arrangements. Cutterhead selection, face pressure, advance rate and conditioning must match the geology.

3.2 Conventional tunnelling

Conventional methods include drill-and-blast excavation in competent rock and sequential excavation methods in soil or weak rock. In sequential excavation, the ground is excavated in stages and immediately supported using measures such as sprayed concrete, lattice girders, rock bolts, steel ribs or forepoling. The ground-support system is adapted to observed conditions, but any observational approach requires reliable monitoring and predefined limits.

Drill-and-blast work requires blast design, exclusion zones, vibration control, explosives security, ventilation, misfire procedures and post-blast inspection. It is unsuitable where vibration or settlement cannot be tolerated.

3.3 Tunnel lining and waterproofing

A tunnel lining may be primary, secondary or composite. Precast segmental linings are bolted or dowelled and sealed with gaskets. Cast-in-place secondary linings may improve durability, fire resistance, hydraulic performance or appearance. Waterproofing systems include gasketed joints, membranes, waterstops and injection hoses.

Segment erection requires correct ring geometry, bolt tightening, gasket cleanliness and control of tail void grouting. Inadequate grouting can permit ground movement and uneven loading behind the lining.

3.4 Tunnel risks and controls

4. Trenchless Technology

Trenchless technology installs, replaces or rehabilitates underground infrastructure with limited surface excavation. It is particularly valuable beneath roads, railways, rivers, airports and developed urban areas.

4.1 Horizontal directional drilling

Horizontal directional drilling (HDD) normally involves pilot-hole drilling, reaming and pulling a product pipe through the enlarged bore. A steerable drill head permits a curved alignment. HDD is suitable for water, gas, telecommunications and electrical ducts where ground conditions and alignment tolerances are manageable.

Key concerns are drilling-fluid pressure, inadvertent returns or “frac-out,” bore stability, pullback force, pipe buoyancy, minimum bend radius and obstruction detection. Drilling-fluid properties must be monitored and recovered or disposed of legally.

4.2 Microtunnelling and pipe jacking

Microtunnelling uses a remotely controlled boring machine and jacking system. Pipes are installed behind the cutting head while spoil is removed mechanically or hydraulically. It provides accurate line and level control and is useful for gravity sewers, where gradient is critical. Jacking forces, lubrication, intermediate jacking stations, reception shafts and pipe joint capacity must be considered.

4.3 Auger boring and pipe ramming

Auger boring installs a casing while an auger removes spoil. It is suited to relatively short crossings with predictable ground. Pipe ramming drives a steel casing through the ground using an impact hammer. It can be effective in suitable soils but offers less steering accuracy and may generate vibration.

4.4 Rehabilitation methods

Trenchless rehabilitation includes cured-in-place pipe (CIPP), pipe bursting, sliplining and spray-applied liners. CIPP forms a new pipe inside an existing one using a resin-impregnated liner cured by heat, steam or ultraviolet light. Pipe bursting fractures the old pipe and draws in a replacement pipe, but it requires assessment of nearby services and ground heave.

4.5 Selection considerations

Trenchless methods should not be selected solely because they reduce open excavation. A proper assessment considers bore length, diameter, depth, soil abrasivity, cobbles or boulders, groundwater, entry and exit space, allowable settlement, utility records, pipe material and emergency recovery options. Accurate utility locating and trial holes are essential; records alone are not sufficient.

5. Retaining Systems and Excavation Support

Retaining systems resist lateral earth pressure, groundwater pressure, surcharge and construction loads. They may be temporary, permanent or dual-purpose.

5.1 Common systems

• Sheet piles: interlocking steel sections driven or vibrated into the ground; useful for water-bearing soils and repetitive walls.
• Contiguous bored piles: discrete piles with small gaps; economical where water inflow is limited.
• Secant piles: overlapping reinforced and unreinforced piles; provide increased stiffness and improved groundwater cut-off.
• Diaphragm walls: reinforced concrete panels constructed in a bentonite or polymer-supported trench; suitable for deep excavations and permanent basement walls.
• Soldier piles and lagging: steel sections with timber, precast or sprayed-concrete infill; flexible and often economical in relatively dry ground.
• Soil nails: passive reinforcement installed into an excavation face and combined with mesh and sprayed concrete; generally used in suitable stiff soils above the groundwater table.
• Anchored walls and internally propped walls: anchors transfer forces into competent ground, while struts or slabs provide internal support.

5.2 Earth pressure concept

For a simplified, level backfill of cohesionless soil, the active earth-pressure coefficient in the classical Rankine model is:

where φ is the soil friction angle. If wall friction is neglected and the backfill is homogeneous, the active lateral pressure at depth z is:

where σ_h is lateral effective stress in kPa, γ is soil unit weight in kN/m³, and z is depth in m. The resultant active force on a wall of height H is:

with units of kN per metre length of wall. These equations assume drained, cohesionless, homogeneous soil, level backfill, no surcharge, no groundwater pressure and sufficient wall movement to mobilise active conditions. They are not substitutes for a complete retaining-wall design. Water pressure, surcharge, cohesion, wall friction, seismic loading and construction stages may significantly change the result.

Illustrative calculation. Let φ = 30°, γ = 18 kN/m³ and H = 4 m. Then K_a = (1 − 0.5)/(1 + 0.5) = 0.333. Therefore:

The force acts at approximately H/3 above the base for the triangular pressure distribution. This result excludes water and surcharge; including either would increase the design demand.

5.3 Stability and construction control

Retaining systems must be checked for structural strength, overturning, sliding, bearing, basal heave, piping, global stability and excessive movement. For excavations, basal heave can occur when the weight of soil outside the excavation exceeds the resistance of the base soil. Piping may occur when upward seepage reduces effective stress.

Construction controls include staged excavation, timely installation of supports, controlled dewatering, restrictions on plant loading near the crest, daily inspections, survey monitoring and clear stop-work criteria. Temporary works should be designed, checked and managed with the same seriousness as permanent works.

6. Bridge Erection Techniques

Bridge erection method is influenced by span length, deck type, site access, navigation requirements, traffic, environmental restrictions, available cranes and the capacity of temporary supports.

6.1 Crane erection

Precast beams, steel girders or deck units can be lifted by mobile or crawler cranes. The lift plan must verify crane capacity at the actual radius, ground bearing pressure, wind limits, lifting points, exclusion zones, delivery sequence and temporary stability. Tandem lifts require coordinated communication and engineered load sharing.

6.2 Incremental launching

In incremental launching, deck segments are assembled behind an abutment and pushed or pulled longitudinally over piers. A temporary launching nose reduces the cantilever moment during launching. The method minimizes work over obstacles and can reduce the need for falsework, but it requires suitable geometry, repetitive deck construction and careful control of temporary reactions.

6.3 Balanced cantilever construction

Balanced cantilever construction erects segments or casts concrete in successive lengths on both sides of a pier. The near balance limits unbalanced moments and reduces the need for ground-supported falsework. Temporary pier heads, prestressing, wind controls and construction-stage analysis are important because the partially completed bridge may be more vulnerable than the completed structure.

6.4 Full-span launching and self-launching gantries

A launching gantry can place precast girders or segmental spans over existing roads, railways or water. Full-span erection reduces the number of operations at ground level and is well suited to repetitive viaducts. Gantry movement, temporary support reactions, locking systems, wind speed and rescue arrangements require specific procedures.

6.5 Temporary works and erection stability

A bridge member that is stable in its final configuration may be unstable during lifting, transport or temporary support. Erection engineering should address lateral torsional buckling, lateral restraint, connection completion, temporary bearings, wind, seismic effects where relevant, collision protection and accidental load cases. No load should be released from a crane until the member is secured and its stability has been verified.

7. Segmental Construction and Accelerated Infrastructure

Segmental construction divides a structure into repeatable units manufactured in a controlled environment or cast near the site. Segments may be erected using cranes, gantries, launching systems or balanced cantilever methods. Post-tensioning tendons, epoxy or dry joints, shear keys and bearing interfaces transfer forces between segments.

7.1 Precast segmental bridges

A typical cycle includes mould preparation, reinforcement and embedded items, concrete placement, curing, dimensional inspection, storage, transport, erection, temporary alignment, joint treatment and post-tensioning. Match-casting produces adjacent segments with complementary faces and helps achieve accurate alignment.

Quality controls include concrete strength and durability testing, tendon identification, duct inspection, grout records, bolt or anchorage torque records, joint cleanliness and dimensional surveys. Post-tensioning must follow the approved stressing sequence. Grouting protects tendons from corrosion and must be completed using suitable materials and documented procedures.

7.2 Accelerated bridge construction

Accelerated bridge construction (ABC) reduces closure time by using prefabricated components, modular systems, rapid-setting materials, staged construction, temporary works and carefully planned logistics. Common strategies include:

• prefabricated bridge units installed during short possessions;
• partial or complete bridge replacement using slide-in or lateral-slide methods;
• precast deck panels and integral connection systems;
• rapid-curing concrete for closures;
• off-site reinforcement cages and utility assemblies; and
• digital modelling and rehearsed installation sequences.

Acceleration should not mean uncontrolled working. The method must preserve inspection, curing, connection, waterproofing and load-transfer requirements. A short road closure is not a success if premature opening creates durability or safety defects.

7.3 Productivity and schedule logic

A simple activity-duration estimate can be expressed as:

where T is duration, Q is quantity and R is production rate, using consistent units. In practice, total duration also includes setup, access, inspection, curing, plant changes and weather allowances. For repetitive segment erection, cycle time is often more useful than an average daily rate. Bottlenecks may arise in mould availability, transport, crane access, tendon stressing or inspection rather than in concrete placement itself.

8. Integrated Risk Management

Infrastructure construction involves interacting hazards rather than isolated risks. A risk register should identify the hazard, initiating event, consequence, existing controls, responsible person, further action, trigger level and emergency response.

8.1 Principal risks

Collapse and ground movement. Control through adequate investigation, engineered temporary works, staged excavation, support installation, monitoring and competent supervision.

Buried services. Obtain current records, use electromagnetic locating and ground-penetrating radar where appropriate, expose services by safe methods, mark them physically and maintain exclusion zones.

Lifting and temporary stability. Use engineered lift plans, certified equipment, competent operators, wind limits, tag lines where suitable and exclusion zones. Never rely on visual judgement alone for crane capacity or ground stability.

Traffic and public interface. Use physical barriers, approved traffic plans, lighting, trained marshals, impact protection and frequent inspections. Protect pedestrians from falling objects, plant movements and uneven surfaces.

Water and flooding. Provide pumps with backup power, alarms, standby capacity and protected discharge routes. Define evacuation routes and inspect weather forecasts.

Fire, confined space and air quality. Control ignition sources, test atmospheres, ventilate, provide rescue equipment and ensure that personnel understand permit-to-work arrangements.

Concrete and post-tensioning. Control formwork pressure, curing, tendon stressing, anchorage zones and grout operations. Establish exclusion zones during stressing because stored energy can cause fatal injuries.

8.2 Monitoring and trigger-action plans

A trigger-action-response plan links measurements to decisions. For example, an amber settlement threshold may require increased survey frequency and engineering review, while a red threshold may require stopping excavation, evacuating the affected area and implementing stabilisation measures. Thresholds must be based on design predictions, sensitivity of nearby assets and contractual or regulatory requirements.

9. Applied Case: Illustrative Urban Rail Underpass

Background

A city must construct a 180 m long, 9 m wide rail underpass beneath a heavily trafficked road. Existing water, gas, electrical and telecommunications services occupy the corridor. Adjacent buildings have shallow foundations, and the groundwater table lies approximately 3 m below road level.

Problem

A fully open excavation would require a long road closure and could cause settlement of adjacent buildings. A bored tunnel would reduce surface disruption but would require expensive launch and reception shafts and complex interfaces at the shallow approaches.

Analysis

The central 100 m section is suitable for a top-down cut-and-cover box using diaphragm walls, while the approach sections can be constructed by staged bottom-up excavation. A geotechnical investigation identifies permeable sand layers and variable fill. Groundwater control and wall stiffness are therefore critical. Utility records are incomplete, so trial excavations and service proving are required.

Approach and solution

The project team adopts the following sequence:

1. establish traffic diversions and protected pedestrian routes;
2. survey and expose utilities, relocating only those that conflict;
3. construct diaphragm walls and plunge columns under guide walls;
4. cast the roof slab in short traffic-management stages;
5. reopen the road over completed roof sections;
6. excavate below the roof using small plant and remove spoil through access openings;
7. install temporary props and construct the base slab and internal walls;
8. apply waterproofing and complete drainage, lighting and fire systems;
9. backfill approach zones and restore the road permanently.

Piezometers, inclinometers, settlement points and building crack gauges are installed. The plan includes standby pumps, emergency power, a flood barrier and a procedure for rapid propping if wall movement exceeds the action level.

Outcome

The top-down section maintains partial road operation and limits wall movement. The approach works require more traffic control but permit efficient concrete placement. The project’s success depends less on any single technology than on coordinated sequencing, service verification, monitoring and rapid response.

Learning points

• Method selection must consider interfaces, not merely excavation cost.
• Top-down construction can reduce surface disruption but demands precise early works.
• Groundwater and utility risks must be addressed before excavation begins.
• Monitoring is valuable only when linked to predefined actions.
• Temporary works and emergency arrangements are central to infrastructure safety.

10. Contemporary Practice and Professional Responsibilities

Building information modelling supports coordination of underground utilities, temporary works, reinforcement, segment numbering and erection sequences. Three-dimensional ground models can combine borehole data, utility surveys and monitoring results, but the model remains uncertain where data are sparse. Digital tools do not eliminate the need for field verification.

Remote monitoring, machine guidance, drones, automated total stations and digital inspection records improve information flow. Artificial intelligence may assist anomaly detection, progress measurement and predictive maintenance, but decisions affecting public safety require competent human review, traceable data and validated procedures.

Sustainable construction includes reuse of excavated material where suitable, low-carbon concrete, reduced transport, energy-efficient pumping, noise control, protection of waterways and planning for inspection and maintenance. Ethical practice requires honest reporting of defects, near misses and monitoring results. Pressure to meet an opening date must never justify bypassing inspection, curing, design approval or emergency controls.

Chapter Summary

Cut-and-cover provides an effective solution for shallow underground structures but creates significant surface and groundwater impacts. Bottom-up and top-down sequences have different access, programme and temporary-support characteristics. Tunnelling reduces surface disruption and may use TBMs or conventional excavation, but it requires control of face stability, water, ventilation, lining and settlement. Trenchless methods such as HDD, microtunnelling, auger boring, pipe ramming and pipe rehabilitation reduce open excavation but require careful assessment of ground, alignment, utilities and recovery arrangements.

Retaining systems include sheet piles, bored piles, secant piles, diaphragm walls, soil nails, anchors and internal props. Their performance depends on design, construction sequence, groundwater control and monitoring. Bridge erection methods include crane lifting, incremental launching, balanced cantilever construction, launching gantries and full-span erection. Segmental and accelerated construction improve productivity and reduce disruption through prefabrication, repeatable cycles and planned logistics, but quality and temporary stability must remain controlled.

Across all methods, the essential professional principles are adequate investigation, engineered temporary works, competent supervision, controlled sequencing, verification of existing services, monitoring with trigger-action plans, effective public protection and ethical reporting. Technology assists these principles; it does not replace engineering judgement.

Key Terms

Accelerated bridge construction: Methods that reduce site duration and traffic disruption through prefabrication, modular systems and planned installation.

Active earth pressure: Lateral soil pressure mobilised when a retaining wall moves sufficiently away from the retained soil.

Balanced cantilever: Bridge erection method in which construction proceeds on both sides of a pier to reduce unbalanced moments.

Cut-and-cover: Construction of an underground structure from a surface excavation followed by roofing or backfilling.

Diaphragm wall: Reinforced concrete wall constructed in supported trench panels for retention and often groundwater cut-off.

Face pressure: Pressure applied at a tunnel excavation face to maintain ground stability.

HDD: Horizontal directional drilling, a steerable trenchless method involving pilot drilling, reaming and pipe pullback.

Incremental launching: Longitudinal pushing or pulling of a bridge deck from an assembly area across its supports.

Microtunnelling: Remotely controlled trenchless excavation combined with pipe jacking.

Plunge column: Column installed through a deep foundation element before concrete has fully set, commonly used in top-down construction.

Pipe bursting: Trenchless replacement method that fractures an existing pipe while drawing in a new one.

Secant piles: Overlapping bored piles forming a relatively stiff retaining and cutoff wall.

Segmental construction: Construction from separate precast or cast-in-place units joined by structural connections.

Soil nail: Passive reinforcement installed into soil and usually combined with facing to stabilise an excavation slope.

Temporary works: Non-permanent structures or systems required to construct, support, protect or access permanent works.

Top-down construction: Underground construction in which the roof and upper slabs are built before excavation is completed below them.

Trenchless technology: Installation, replacement or rehabilitation of underground infrastructure with limited surface excavation.

Trigger-action-response plan: Monitoring system that links measured conditions to predefined actions and escalation procedures.

Tunnel boring machine: Mechanised equipment that excavates and supports a tunnel face, often while installing lining.

Review Questions

Short Answer Questions

1. State two differences between bottom-up and top-down cut-and-cover construction.
2. What is the purpose of face pressure in a shielded tunnel boring machine?
3. Identify two situations in which trenchless construction may be preferable to open excavation.
4. Name four common retaining systems used for deep excavations.
5. Why are monitoring thresholds important in an excavation project?

Long Answer Questions

1. Explain the complete construction sequence for a bottom-up cut-and-cover underpass, including groundwater and traffic controls.
2. Compare HDD, microtunnelling and pipe jacking with respect to applications, accuracy, risks and site requirements.
3. Describe the principal bridge erection methods and explain how temporary stability is maintained during erection.
4. Discuss the quality-control requirements for precast segmental bridge construction and post-tensioning.
5. Explain how a risk register and trigger-action-response plan should be used on an urban underground project.

Analytical / Application Questions

1. A proposed utility crossing must pass beneath an active railway with only a small working area at each side. Recommend a construction method and justify your choice using ground, settlement, access and risk criteria.
2. For a 4 m high wall retaining level, cohesionless backfill with φ = 30° and γ = 18 kN/m³, calculate the simplified active earth-force resultant using P_a = ½K_aγH². State the assumptions and explain how groundwater would alter the result.
3. An urban bridge replacement must keep a major road open except for one weekend. Develop an accelerated construction strategy and identify at least six critical risks and their controls.

Multiple Choice Questions with Answers

Question 1

Which cut-and-cover sequence constructs the roof before most of the excavation below it?

A. Bottom-up construction

B. Top-down construction

C. Open-cut spoil replacement

D. Drill-and-blast construction

Correct Answer: B. Top-down construction

Question 2

Which tunnel boring machine commonly uses conditioned excavated soil to balance face pressure?

A. Earth pressure balance TBM

B. Open gripper TBM

C. Roadheader

D. Raise-boring machine

Correct Answer: A. Earth pressure balance TBM

Question 3

What is a principal advantage of microtunnelling for gravity sewers?

A. It requires no launch or reception shafts

B. It provides accurate line and level control

C. It is limited to dry rock

D. It eliminates the need for pipe joints

Correct Answer: B. It provides accurate line and level control

Question 4

For level cohesionless backfill with friction angle φ, the simplified Rankine active coefficient is:

A. Kₐ = (1 + sin φ)/(1 − sin φ)

B. Kₐ = sin φ

C. Kₐ = (1 − sin φ)/(1 + sin φ)

D. Kₐ = tan φ

Correct Answer: C. Kₐ = (1 − sin φ)/(1 + sin φ)

Question 5

Which retaining system is formed from overlapping bored piles?

A. Soldier pile and lagging wall

B. Secant pile wall

C. Timber trench shield

D. Soil-cement slope only

Correct Answer: B. Secant pile wall

Question 6

What is the main principle of balanced cantilever bridge construction?

A. Placing all deck units from one abutment only

B. Maintaining approximate balance on both sides of a pier

C. Removing all temporary supports before erection

D. Using only cast-in-place foundations

Correct Answer: B. Maintaining approximate balance on both sides of a pier

Question 7

In horizontal directional drilling, what normally follows pilot-hole drilling?

A. Immediate permanent backfilling without reaming

B. Reaming and product-pipe pullback

C. Blasting the entire alignment

D. Installing diaphragm wall panels

Correct Answer: B. Reaming and product-pipe pullback

Question 8

What is the purpose of grout behind a tunnel segmental lining?

A. To increase surface traffic capacity

B. To fill the tail void and improve ground contact

C. To replace all face-pressure controls

D. To remove the need for segment seals

Correct Answer: B. To fill the tail void and improve ground contact

Question 9

A wall has φ = 30°, γ = 18 kN/m³ and H = 4 m. Using Kₐ = 0.333, what is the simplified active force per metre length?

A. 12.0 kN/m

B. 24.0 kN/m

C. 48.0 kN/m

D. 96.0 kN/m

Correct Answer: C. 48.0 kN/m

Question 10

Which statement best describes a trigger-action-response plan?

A. It records only completed payments

B. It links monitoring results to predefined project actions

C. It replaces the temporary-works design

D. It permits excavation without investigation

Correct Answer: B. It links monitoring results to predefined project actions