Foundations, Ground Improvement, and Underground Methods

Introduction and context

Foundations and underground works transfer structural loads, create below-ground space, retain soil and water, and provide routes for utilities and transportation. Their construction is strongly controlled by geology, groundwater, access, adjacent structures, environmental constraints, and the consequences of movement. Equipment selection is therefore not simply a matter of choosing the largest machine. It requires matching the construction method to ground behavior, production requirements, tolerable vibration and noise, available working space, safety obligations, and quality-control procedures.

This chapter examines shallow foundations, driven piles, drilled shafts, diaphragm walls, retaining systems, ground improvement, trench support, tunneling, and trenchless construction. It emphasizes the relationship between method, equipment, soil response, risk, and project performance.

Learning objectives

After studying this chapter, the reader should be able to:

1. Distinguish shallow-foundation, piling, drilled-shaft, and underground construction methods.
2. Select appropriate equipment for different soil, groundwater, access, and production conditions.
3. Explain the operation and limitations of diaphragm walls, retaining systems, trench support, tunnels, and trenchless methods.
4. Compare ground-improvement techniques using performance, cost, environmental, and constructability criteria.
5. Interpret basic production and capacity calculations for foundation equipment.
6. Develop safe, monitorable, and environmentally responsible construction plans.

1. Site investigation as the basis for method selection

A construction method should follow a ground model rather than precede it. The model should describe soil and rock strata, groundwater levels and seasonal variation, strength, compressibility, permeability, obstructions, contamination, and the presence of existing foundations or utilities.

Common investigations include boreholes, standard penetration tests, cone penetration tests, laboratory classification and strength tests, geophysical surveys, trial pits, and groundwater monitoring. The investigation must also identify construction constraints: overhead power lines, restricted headroom, traffic, vibration-sensitive equipment, neighboring buildings, and disposal requirements for spoil or drilling fluid.

Important terminology includes:

• Bearing capacity: resistance of soil to foundation loading without unacceptable shear failure.
• Settlement: vertical movement caused by soil compression, distortion, or rearrangement.
• Heave: upward ground movement, often caused by excavation unloading, swelling soil, or installation displacement.
• Liquefaction susceptibility: tendency of saturated loose soil to lose effective strength during cyclic loading.
• Working platform: engineered temporary surface supporting cranes, rigs, and haulage equipment.
• Temporary works: structures or systems needed during construction, such as bracing, shoring, slurry walls, and access ramps.

Ground conditions are spatially variable. Consequently, trial sections, observational monitoring, and contingency plans are often more reliable than assuming uniform conditions from a few investigation points.

2. Shallow foundations

Shallow foundations transfer load to soil near the ground surface. Typical forms are isolated pad footings, strip footings, combined footings, raft foundations, and ground beams. They are generally economical where competent soil occurs at modest depth and settlement is acceptable.

Construction sequence and equipment

A typical sequence is setting out, service verification, excavation, trimming and inspection, blinding where specified, reinforcement placement, formwork, concrete placement, curing, and backfilling. Equipment may include excavators, loaders, dumpers, trenchers, compactors, pumps, concrete trucks, placing booms, vibrators, and surveying instruments.

Excavators are selected by bucket capacity, reach, breakout force, operating weight, and access. A narrow bucket is useful for strip footings, while a grading bucket improves formation trimming. Final formation should not be excessively disturbed or left exposed to rain and freezing. Over-excavation should be corrected with approved engineered fill or concrete, not loose soil.

Concrete placement must avoid segregation and excessive free fall. Internal vibrators consolidate concrete, but over-vibration can cause segregation and should be avoided. Reinforcement must be supported to maintain specified cover.

Bearing and settlement concepts

A simplified gross bearing-capacity expression for a centrally loaded, drained strip footing is often written as:

Here, q_u is ultimate gross bearing pressure, c is soil cohesion, γ is unit weight, B is footing width, q is effective surcharge at foundation level, and N_c, N_q, and N_γ are bearing-capacity factors related to the soil friction angle. This expression assumes a simplified strip footing, uniform soil, a defined failure mechanism, and appropriate drainage conditions. Shape, depth, inclination, eccentricity, groundwater, and layered soil require correction factors or more advanced analysis.

Allowable pressure is not obtained by dividing blindly in every design situation. In a basic allowable-stress approach:

where F is the factor of safety. The allowable value must also satisfy settlement criteria and any code requirements. In modern limit-state design, resistance and load factors are applied according to the governing code.

Raft foundations

A raft spreads loads over a large area and can reduce differential settlement where columns are closely spaced or soil bearing capacity is low. Construction requires accurate formation preparation, reinforcement fixing, concrete logistics, temperature-control planning, joints, and waterproofing. Large pours may require multiple pumps, continuous supply, thermal monitoring, and a carefully sequenced placement plan.

3. Driven piles

Driven piles are prefabricated steel, concrete, or timber elements installed by impact, vibration, or pressing. They transfer load through shaft friction, end bearing, or both. Installation can densify loose granular soil, but can also generate noise, vibration, ground heave, and stress on adjacent structures.

Equipment and operation

Impact hammers include diesel, hydraulic, and drop hammers. Hydraulic hammers offer controllable energy and are adaptable to many pile types. Vibratory hammers use oscillatory motion and are particularly effective in granular soils, sheet piles, and temporary works; final static capacity may require separate verification. Press-in systems install piles with hydraulic reaction forces and generate very low vibration, but need sufficient reaction capacity and may have lower production in hard ground.

A pile rig generally includes a leader or mast, power unit, hammer, pile handling system, and alignment controls. Auxiliary equipment includes cranes, welding systems, water jets, extraction devices, and instrumentation.

Driving records should include pile identity, depth, hammer type, blow count, energy or stroke, refusal observations, interruptions, and unusual behavior. Dynamic testing can estimate installed capacity, while static load tests provide direct performance evidence. Correlations between blow count and capacity are method- and soil-dependent and should not replace engineering judgment.

Advantages and limitations

Driven piles provide factory-controlled quality and immediate installation. They may be installed through soft deposits and can be load-tested during production. Limitations include vibration, noise, restricted headroom, possible damage during driving, and difficulty controlling exact toe level in variable strata. Pile cut-off, splicing, and handling also affect productivity.

4. Drilled shafts and bored piles

Drilled shafts are cast-in-place elements formed by excavating a large-diameter hole and placing reinforcement and concrete. They can carry high axial and lateral loads and are useful where low vibration is essential. Their principal risks are bore collapse, groundwater inflow, necking, inclusions, segregation, and defects caused by poor concrete placement.

Drilling systems

Rotary rigs use augers, core barrels, drilling buckets, and casing oscillators. Continuous-flight augers can be efficient in cohesive soils. Temporary casing supports unstable zones and can be installed with rotary or oscillator equipment. Slurry systems use bentonite or polymer fluid to balance groundwater and stabilize the bore. In rock, core barrels, rock augers, chisels, and percussion tools may be required.

Concrete is commonly placed by tremie when water or slurry remains in the excavation. The tremie outlet must remain embedded in fresh concrete so that concrete displaces the fluid from the bottom upward. Reinforcement cages require sufficient stiffness, centralizers, lifting points, and controlled insertion.

Quality assurance may include bore-depth and cleanliness checks, slurry-density and viscosity tests, concrete delivery records, cross-hole sonic logging, thermal integrity profiling, and low-strain integrity testing where appropriate. No single test detects every defect.

5. Diaphragm walls and retaining systems

A diaphragm wall is a reinforced-concrete wall constructed in panels excavated under stabilizing slurry. It can function as a temporary excavation support system, a permanent basement wall, or a groundwater barrier. A guide wall establishes alignment and supports the upper trench. Panels are excavated using hydraulic grabs or hydromills, with slurry maintaining trench stability. Stop-end systems form joints between panels; reinforcement cages are lowered, and concrete is placed by tremie.

Diaphragm walls provide high stiffness and can reduce ground movement, but they require specialized equipment, slurry treatment, spoil management, accurate panel sequencing, and sufficient site space. Hydromills are effective in hard ground but have higher capital and operational complexity than grabs.

Other retaining systems include:

Internal bracing may use steel struts, rakers, or reinforced-concrete slabs. Anchored walls use ground anchors installed by drilling, grouting, stressing, and testing. Anchors require legal rights beyond the site boundary and careful verification of bond and lock-off loads.

6. Ground improvement

Ground improvement modifies soil behavior rather than merely bypassing weak ground. The selected method should identify the target mechanism: increased strength, reduced compressibility, increased permeability, densification, liquefaction resistance, or improved bearing performance.

Principal methods

• Compaction: Rollers, vibratory rollers, tamping equipment, and plate compactors densify suitable fill. Moisture content and lift thickness strongly influence results.
• Dynamic compaction: A heavy tamper is repeatedly dropped by crane onto loose granular ground. It is unsuitable near vibration-sensitive facilities unless carefully controlled.
• Vibro-compaction: A vibrating probe densifies clean granular soils, often with water assistance.
• Stone columns: Aggregate columns installed by vibratory probes improve drainage, stiffness, and bearing behavior. They may be unsuitable where surrounding soil cannot provide adequate lateral confinement.
• Deep soil mixing: Binders are mechanically mixed into soil to form improved columns, panels, or blocks. It is useful for strength improvement, cutoff walls, and excavation support.
• Jet grouting: High-velocity grout erodes and mixes with soil to create soil-cement columns or panels. It can work in confined spaces but requires careful spoil and groundwater management.
• Grouting: Cement, chemical, or particulate grout fills voids, fractures, or pores. Selection depends on permeability, particle size, strength requirements, and environmental compatibility.
• Preloading and vertical drains: A surcharge accelerates consolidation, while prefabricated vertical drains shorten drainage paths. Settlement and pore-pressure monitoring are essential.
• Geosynthetics: Geotextiles, geogrids, and geomembranes provide separation, reinforcement, filtration, and containment.

Ground improvement must be verified by field tests, such as density tests, cone penetration tests, plate-load tests, column integrity checks, strength testing of mixed samples, or settlement monitoring. The improvement target must be measurable; “better ground” is not an acceptance criterion.

7. Trench excavation and support

Trenches are among the most hazardous excavations because collapse can occur suddenly and bury workers. Safe systems include sloping, benching, shielding, or shoring. Selection depends on soil, depth, water, surcharge, adjacent structures, utilities, and duration.

Excavators perform most trench excavation, while trenchers are efficient for long, relatively narrow utility alignments in suitable soil. Bedding and pipe placement may require laser-controlled grades, lifting devices, pumps, and compactors.

A trench box protects workers by providing a shield, but it does not necessarily prevent soil movement outside the box. Shoring supports the excavation sides; sloping removes unsupported vertical faces by creating a stable inclination. The competent person or authorized engineer must inspect excavations, support systems, access, water accumulation, and atmospheric hazards. Spoil piles and equipment should be kept away from edges because surcharge increases lateral pressure.

De-watering may use sumps, wellpoints, deep wells, or eductor systems. Pumping can cause piping, erosion, settlement of neighboring ground, or contamination migration. Discharge must comply with permits and water-quality requirements.

8. Tunneling methods

Tunneling provides underground transport and utility corridors while reducing surface disruption, but it requires advanced geological and risk management.

Conventional excavation

Drill-and-blast uses drilling jumbos, explosives, scaling equipment, loaders, muck trucks, rock bolts, shotcrete, and ventilation. It is adaptable to variable rock but creates vibration, fumes, overbreak, and cycle-time variability. Roadheaders use a mechanically mounted cutting head and are suited to selected rock and mixed ground, with continuous muck removal.

Tunnel boring machines

A tunnel boring machine (TBM) integrates cutting, spoil removal, ground support, and often lining installation. Main types include:

• Open or shielded hard-rock TBMs: Used where the face can remain stable or is supported by mechanical shields.
• Earth-pressure-balance TBMs: Conditioned excavated soil provides pressure support at the face, especially in soft ground.
• Slurry TBMs: Pressurized slurry supports the face and transports spoil to a separation plant.

The machine must balance face pressure, advance rate, cutter wear, lining installation, thrust, torque, settlement control, and logistics. Segmental linings are erected behind the shield and sealed at joints.

A simplified excavation-volume relation is:

where V is excavated volume, A is excavated cross-sectional area, and L is advance length. If a circular tunnel has diameter D, its area is:

These relations describe geometric volume only. Actual muck volume may be greater because of bulking, and production is reduced by setup, maintenance, lining, ventilation, and interruptions.

9. Trenchless construction

Trenchless methods install, replace, or rehabilitate utilities with limited surface excavation. They are valuable beneath roads, railways, rivers, and environmentally sensitive areas.

• Horizontal directional drilling (HDD): A pilot bore is drilled along a designed alignment, enlarged by reaming, and followed by product-pipe pullback. Drilling fluid transports cuttings and stabilizes the bore. Risks include inadvertent fluid returns, bore collapse, excessive pullback force, and inaccurate alignment.
• Microtunneling: A remotely controlled, pipe-jacking system excavates and supports the face while pipes are installed from a launch shaft to a reception shaft. It offers high alignment accuracy and good groundwater control.
• Pipe jacking: Hydraulic jacks push pipes through an excavated or shielded path. Intermediate jacking stations may reduce friction over long drives.
• Auger boring: A casing is jacked while an auger removes soil. It is relatively simple but offers less steering control than microtunneling.
• Cured-in-place pipe and sliplining: Existing pipelines are rehabilitated without full-length excavation. Condition assessment, bypass pumping, resin handling, and quality verification are essential.

Trenchless construction is not automatically low-risk. Poor utility records, unknown ground, drilling-fluid escape, and launch-shaft instability can create serious consequences.

10. Equipment productivity and selection

Production estimates should separate theoretical capacity from effective output. For cyclic excavation equipment, a basic hourly production estimate is:

where Q is production in loose or specified volume per hour, C is rated bucket capacity, F is the fill factor, E is job efficiency expressed as a decimal, and t_c is cycle time in minutes. The equation assumes a reasonably stable cycle and consistent material. Swell, shrinkage, haul distance, grade, operator performance, and site interference may require additional corrections.

Worked example. An excavator has a 1.20 m³ bucket, 0.85 fill factor, 0.75 efficiency, and 32-minute cycle time. Then:

Thus, estimated effective production is approximately 1.8 m³/h in the stated volume basis. If the project reports compacted volume, a specified conversion factor must be applied; it should not be assumed.

Selection should consider total installed cost, not only machine ownership. Relevant criteria include production rate, mobilization, fuel, labor, maintenance, standby risk, access, emissions, noise, vibration, operator skill, spare-parts support, and compatibility with other activities. A high-capacity machine can reduce productivity if trucks, concrete supply, spoil disposal, or working space cannot support it.

11. Applied case: illustrative urban basement excavation

Background

A six-story building requires a two-level basement beside an occupied masonry building and a busy road. The site contains fill over soft clay, with groundwater above the basement formation. Vibration and ground movement must be minimized.

Problem

Open-cut excavation would require excessive right-of-way and could undermine the neighboring building. A temporary wall must retain soil and water while allowing basement construction.

Analysis

A diaphragm wall offers high stiffness and continuity, while internal slabs can act as permanent lateral restraints. Drilled piles are preferable to driven piles for the building because impact vibration is undesirable. Groundwater inflow and settlement caused by drawdown are critical risks. The working platform must be designed for the diaphragm-wall crane and concrete deliveries.

Approach and solution

The team installs guide walls, excavates diaphragm-wall panels under polymer slurry, places reinforcement cages, and tremie-pours concrete. The wall is excavated in stages using internal slabs as struts. A low-vibration rotary rig installs bored piles. Monitoring points measure wall deflection, ground settlement, groundwater pressure, neighboring-building movement, and vibration. Trigger-action-response plans define normal, alert, and intervention levels.

Outcome

The method reduces surface disruption and vibration but requires specialist slurry management, careful panel-joint control, continuous monitoring, and reliable concrete logistics. The principal lesson is that a technically suitable method succeeds only when temporary works, monitoring, utilities, access, and emergency response are integrated.

12. Safety, environmental, and governance considerations

Foundation and underground works require documented risk assessments, engineered temporary works, competent supervision, exclusion zones, lifting plans, and emergency procedures. Major hazards include collapse, struck-by incidents, suspended loads, entrapment, rotating equipment, underground services, contaminated soil, noise, vibration, silica, cement exposure, drilling-fluid release, and confined atmospheres.

Designers and contractors must comply with applicable building, excavation, occupational-safety, environmental, transport, water-discharge, explosives, and public-right-of-way regulations. Local legal requirements govern excavation slopes, trench protection, crane operations, worker access, and professional sign-off; therefore, this chapter does not replace the governing code.

Environmental good practice includes low-emission equipment, electric or hybrid machinery where feasible, noise barriers, vibration monitoring, slurry recycling, spoil classification, controlled de-watering, sediment treatment, and prevention of inadvertent drilling-fluid returns. Ethical practice requires truthful reporting of ground conditions, test results, incidents, and deviations. Artificially improving records or concealing movements can endanger workers and the public.

Digital tools, machine guidance, BIM coordination, automated drilling logs, remote monitoring, and AI-assisted anomaly detection can improve planning. They do not replace competent engineering judgment. AI outputs require validation against site measurements, contractual requirements, and physical behavior.

13. Best-practice decision framework

A defensible method-selection process follows these steps:

1. Define performance requirements: load, settlement, watertightness, movement, alignment, and service life.
2. Build and continuously update the ground and groundwater model.
3. Identify constraints from neighbors, utilities, access, noise, vibration, and environmental permits.
4. Develop at least two feasible methods and compare risk, cost, schedule, carbon, and quality.
5. Confirm equipment availability, working-platform capacity, operator competence, and supply-chain support.
6. Plan trials, inspection, testing, instrumentation, and acceptance criteria before production.
7. Establish trigger-action-response plans for movement, pore pressure, groundwater, vibration, and production failure.
8. Record as-built data and use observations to refine subsequent work.

Chapter Summary

Foundation and underground construction is a ground-interaction problem as much as an equipment problem. Shallow foundations are efficient where near-surface soil provides adequate bearing and settlement performance. Driven piles provide rapid, factory-controlled elements but may create vibration and noise. Drilled shafts reduce vibration but demand strict control of bore stability, concrete placement, and integrity.

Diaphragm walls, secant piles, sheet piles, soldier piles, anchors, and internal bracing provide different balances of stiffness, water control, cost, and constructability. Ground improvement changes soil properties through densification, reinforcement, drainage, mixing, grouting, or preloading. Trenches require engineered protection against collapse and careful groundwater control. Tunneling and trenchless methods reduce surface disruption but introduce specialized risks involving face stability, alignment, spoil, fluids, and underground interfaces.

Successful projects integrate investigation, equipment selection, temporary works, logistics, monitoring, quality assurance, safety, environmental control, and ethical reporting. Effective production is measured by reliable installed work, not by machine size or theoretical capacity alone.

Key Terms

Bearing capacity: Soil resistance to foundation loading without unacceptable failure.

Settlement: Downward movement caused by soil compression or deformation.

Driven pile: Prefabricated pile installed by impact, vibration, or hydraulic pressing.

Drilled shaft: Large cast-in-place foundation element formed in a drilled excavation.

Tremie: Pipe system used to place concrete through water or slurry.

Diaphragm wall: Reinforced-concrete wall excavated in slurry-supported panels.

Secant pile wall: Overlapping bored-pile wall forming a stiff retaining barrier.

Working platform: Engineered temporary surface supporting construction equipment.

Ground improvement: Modification of soil properties to meet performance requirements.

Dynamic compaction: Densification by repeated dropping of a heavy tamper.

Deep soil mixing: Mechanical blending of soil with binder to create improved ground.

Trench box: Protective shield used to protect workers inside an excavation.

Dewatering: Removal or control of groundwater during construction.

Tunnel boring machine: Integrated machine for excavating, supporting, and often lining a tunnel.

Earth-pressure-balance TBM: TBM using conditioned soil pressure to support a soft-ground face.

Horizontal directional drilling: Steerable trenchless drilling followed by reaming and pipe pullback.

Pipe jacking: Installation of pipes by hydraulic thrust from a launch shaft.

Trigger-action-response plan: Predefined monitoring thresholds and required interventions.

Review Questions

Short Answer Questions

1. Why is a ground model essential before selecting foundation equipment?
2. State two advantages and two limitations of driven piles.
3. What is the purpose of slurry in diaphragm-wall or drilled-shaft construction?
4. Distinguish a trench box from a shoring system.
5. Name three variables that affect effective excavator production.

Long Answer Questions

1. Compare driven piles and drilled shafts for an urban project beside a vibration-sensitive building.
2. Explain the construction sequence, equipment, and quality-control requirements for a diaphragm wall.
3. Compare compaction, stone columns, deep soil mixing, and preloading as ground-improvement methods.
4. Describe the principal equipment and risk controls for a tunnel constructed by a TBM.
5. Explain how safety, environmental, and monitoring requirements influence method selection for deep excavation.

Analytical / Application Questions

1. A project has loose saturated sand, a nearby hospital, and strict vibration limits. Evaluate suitable foundation and ground-improvement methods and justify your recommendation.
2. An excavator has a 1.5 m³ bucket, 0.80 fill factor, 0.70 efficiency, and 28-minute cycle. Calculate effective production using the chapter equation and identify two reasons actual output may differ.
3. A proposed HDD alignment crosses a busy road and passes through uncertain ground. Develop a preconstruction investigation, equipment, monitoring, and contingency plan.

Multiple Choice Questions with Answers

Question 1

Which method generally produces the lowest installation vibration among the following pile-installation options?

A. Drop-hammer driving

B. Hydraulic press-in installation

C. Diesel impact hammering

D. Vibratory hammering

Correct Answer: B. Hydraulic press-in installation

Question 2

What is the primary function of slurry during excavation of an unstable drilled shaft?

A. Increase concrete strength

B. Support the excavation sides and control groundwater inflow

C. Accelerate reinforcement corrosion

D. Replace the reinforcement cage

Correct Answer: B. Support the excavation sides and control groundwater inflow

Question 3

A bucket has a rated capacity of 1.20 m³, fill factor 0.85, efficiency 0.75, and cycle time 32 minutes. What is the estimated production?

A. 0.94 m³/h

B. 1.20 m³/h

C. 1.79 m³/h

D. 2.88 m³/h

Correct Answer: C. 1.79 m³/h

Question 4

Which ground-improvement method uses a surcharge and drainage paths to accelerate consolidation?

A. Preloading with vertical drains

B. Jet grouting only

C. Dynamic compaction of rock

D. Sheet piling

Correct Answer: A. Preloading with vertical drains

Question 5

What is the main limitation of a trench box?

A. It cannot protect workers from falling objects

B. It shields workers but does not necessarily prevent soil movement outside the box

C. It always requires explosives

D. It is used only in rock tunnels

Correct Answer: B. It shields workers but does not necessarily prevent soil movement outside the box

Question 6

Which TBM type uses conditioned excavated soil to support a soft-ground face?

A. Earth-pressure-balance TBM

B. Open hard-rock TBM

C. Roadheader

D. Drill-and-blast system

Correct Answer: A. Earth-pressure-balance TBM

Question 7

In microtunneling, pipes are commonly installed by

A. Dropping them into an open trench

B. Hydraulic jacking from a launch shaft

C. Suspending them from a tower crane only

D. Compacting them into the soil with a roller

Correct Answer: B. Hydraulic jacking from a launch shaft

Question 8

Which test directly provides evidence of foundation performance under applied load?

A. Static load test

B. Paint-thickness test

C. Noise survey alone

D. Fuel-consumption test

Correct Answer: A. Static load test

Question 9

Why is spoil-pile placement near a trench edge dangerous?

A. It reduces soil density at the trench bottom

B. It adds surcharge and can increase lateral earth pressure

C. It eliminates groundwater

D. It prevents equipment access to the site only

Correct Answer: B. It adds surcharge and can increase lateral earth pressure

Question 10

Which statement best describes a diaphragm wall?

A. A temporary timber fence with no structural role

B. A reinforced-concrete wall constructed in slurry-supported panels

C. A compacted gravel mound

D. A prefabricated roof-support frame

Correct Answer: B. A reinforced-concrete wall constructed in slurry-supported panels