Deep and Special Foundations

Introduction

A foundation transfers structural actions safely to the ground while controlling settlement, rotation, sliding and overturning. Most low-rise buildings can use shallow strip, pad or raft foundations when competent soil occurs near ground level. A deep or special foundation becomes necessary when near-surface soil is weak, compressible, expansive, erodible, waterlogged, contaminated, subject to scour, or unable to provide acceptable bearing capacity. Deep foundations may also be selected where construction loads, environmental constraints or site geometry make shallow excavation unsuitable.

For a civil draftsman, the task is not to replace the geotechnical or structural engineer. It is to interpret design information accurately and prepare coordinated plans, sections, reinforcement details, schedules and construction documents. Drawings must clearly distinguish design assumptions from verified site information and must identify elements requiring specialist design.

Learning objectives

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

• explain the purpose and behaviour of piles, pile caps, piers, caissons and underpinning;
• distinguish end-bearing, friction and combined pile action;
• prepare basic foundation layouts, sections and typical details from engineering information;
• recognize risks associated with problematic ground and groundwater;
• describe practical construction and quality-control requirements;
• identify when geotechnical, structural, temporary-works or specialist foundation design is required.

1. Selection of deep and special foundations

Foundation selection begins with a ground model, not with a preferred foundation type. The ground model should summarize strata, groundwater, soil and rock properties, variability, existing foundations, contamination, seismic conditions, buried services and construction constraints. Investigation may include trial pits, boreholes, in-situ tests, laboratory testing, geophysical surveys and groundwater monitoring.

A deep foundation is generally appropriate when loads must be transferred through weak upper strata to stronger material, or when the foundation must develop resistance along a considerable embedded length. A special foundation may be required for unusual loading, restricted access, existing buildings, water crossings, expansive soil, collapsible soil, karst, mining voids or severe groundwater conditions.

Important selection criteria include:

A foundation system must satisfy both ultimate limit states, such as geotechnical failure or structural collapse, and serviceability limit states, such as excessive settlement, cracking, vibration or unacceptable movement. Values and combinations are determined using the governing national code and project design basis.

2. Piles

A pile is a slender structural member installed or formed in the ground to transfer load to soil or rock. Piles may be concrete, steel, timber or composite. They may be driven, jacked, screwed, vibrated or bored and cast in place.

2.1 Load-transfer mechanisms

An axially loaded pile commonly obtains resistance from two mechanisms:

1. End bearing: load is transferred through the pile toe to a strong soil layer or rock.
2. Shaft resistance: friction or adhesion develops between the pile surface and surrounding ground.

The ultimate axial resistance is commonly represented conceptually as:

where Qᵤ is ultimate axial resistance, Q_b is ultimate toe or base resistance, and Q_s is ultimate shaft resistance. This is a conceptual decomposition; the calculation method, partial factors, drainage condition, pile-group effects and code requirements must be established by the geotechnical engineer.

For a uniform cylindrical pile, a simplified shaft-area expression is:

where D is pile diameter, L is embedded length and f_s is average unit shaft resistance. Units are consistent when D and L are in metres, f_s is in kN/m² and Q_s is in kN. In layered soil, the shaft resistance should instead be summed by layer:

where Lᵢ and f_sᵢ refer to the thickness and average unit shaft resistance of layer i. These expressions do not by themselves provide a design capacity. They omit, among other matters, group action, negative skin friction, installation effects, groundwater, cyclic loading and code safety factors.

The design resistance must be compared with factored actions according to the governing design code. A simple allowable-load relationship, used only where the project method explicitly permits it, is:

where Qₐ is allowable resistance and F is the specified factor of safety. Modern limit-state design often uses partial factors rather than one global factor, so a draftsman must not substitute this equation for the engineer’s design method.

2.2 Types of piles

Driven piles are manufactured members installed by impact, vibration or hydraulic jacking. They provide rapid installation and can be inspected before driving, but may create noise, vibration, ground heave and handling stresses. Driving records commonly include pile identification, length, hammer information, penetration per blow and final set.

Bored piles are formed by drilling or augering a hole and placing reinforcement and concrete. They are suitable where vibration must be limited and can accommodate large diameters. Their risks include bore collapse, groundwater inflow, necking, soil contamination of concrete and defects caused by poor concreting technique.

Continuous-flight-auger piles are formed by drilling with a hollow-stem auger while pumping concrete through the stem as the auger is withdrawn. Reinforcement is inserted into fresh concrete. Accurate control of concrete pressure, volume and withdrawal rate is essential.

Steel piles may be H-sections, tubes or sheet piles. They have high strength and can be spliced, but require attention to corrosion, buckling, welding, driving damage and fire protection.

Screw or helical piles use one or more helical plates. Torque measurements can assist installation control, but the relationship between torque and capacity is project-specific and requires an approved method.

Micropiles are small-diameter, often drilled and grouted piles installed with compact equipment. They are useful for underpinning, restricted access and difficult ground, although their design and testing are specialist activities.

2.3 Pile actions beyond compression

Piles may resist tension or uplift through shaft adhesion, enlarged bases, helical plates or self-weight. Lateral actions arise from wind, seismic effects, earth pressure, berthing, vehicle impact or structural frame action. Lateral pile behaviour depends on soil stiffness, pile flexural stiffness, head restraint, group arrangement and load duration. It cannot be inferred from axial capacity alone.

Piles may also experience negative skin friction, sometimes called downdrag, when surrounding soil settles relative to the pile. This adds load to the pile and reduces available resistance. It is particularly relevant where fill or soft clay consolidates after installation, or where groundwater levels decline.

3. Pile groups and pile caps

A single pile may support a small column, but building columns commonly require a group of piles connected by a reinforced-concrete pile cap. The cap distributes column actions to the piles and provides structural continuity.

A pile layout should show:

• grid lines and setting-out dimensions;
• pile centres, numbers and identification marks;
• pile diameter or section, cut-off level and founding level where known;
• column or wall location relative to the group centroid;
• pile-cap outline and thickness;
• orientation of non-circular piles;
• construction joints, access constraints and tolerances where specified.

The pile group should be arranged so that the resultant column load is compatible with the group centroid, unless eccentricity has been designed. Piles placed too close together may have reduced group efficiency, overlapping stress zones or installation interference. Minimum spacing, edge distances and cap dimensions come from the design code, pile type and engineer’s design.

A pile cap is not simply a thick slab. It may behave as a deep beam or strut-and-tie system, especially when piles are widely spaced and the column is concentrated. Reinforcement must therefore be shown exactly as designed, including bottom ties, top reinforcement, bursting or suspension reinforcement, anchorage, laps, links, cover and starter bars.

Basic pile-cap drafting sequence

1. Establish the structural grid and column reactions from the coordinated model.
2. Insert the engineer’s pile schedule and set-out coordinates.
3. Draw the pile-cap boundary with pile tolerances and clearances.
4. Add plan reinforcement using consistent bar marks.
5. Produce sections through representative pile rows and the column.
6. Show pile cut-off, pile-cap levels, concrete cover, construction joints and connection details.
7. Cross-check pile IDs, diameters, quantities, levels and bar schedules.

A typical section should make clear whether pile reinforcement is lapped into the cap, whether pile heads are broken down to sound concrete, and whether a waterproofing or blinding layer is required. It should not imply that reinforcement or dimensions have been designed if they were merely drafted as a typical arrangement.

4. Piers and drilled shafts

The term pier is used differently across jurisdictions. It may refer to a short deep foundation, a large-diameter bored pile, a masonry or concrete support, or a group of piles. Project documentation must define the term.

A drilled shaft or large-diameter pier is constructed by excavating a circular shaft and placing reinforcement and concrete. It may transfer load through base resistance, side resistance or both. Large shafts can carry substantial axial and lateral forces and may be economical for bridges, towers and heavily loaded columns.

Construction methods include open-hole excavation, temporary casing, permanent casing and slurry-supported excavation. A shaft excavated below groundwater may require casing or drilling fluid to prevent collapse. The base must be cleaned and verified before concrete placement. Concrete is often placed by tremie when water cannot be displaced safely. The reinforcement cage must be stable, correctly oriented and provided with spacers that maintain cover.

Drafting must identify shaft diameter, founding level, casing extent, reinforcement cage, concrete placement level, cut-off, inspection requirements and any access tubes for integrity testing. The final design depends strongly on geotechnical investigation and construction method.

5. Caissons and foundations in water

A caisson is a substantial watertight or partly watertight structure used to construct a foundation below water or below difficult ground conditions. Types include open caissons, box caissons and pneumatic caissons. In modern practice, large-diameter drilled shafts, driven piles and immersed or prefabricated systems may perform similar functions.

An open caisson is sunk by removing soil from within its cutting edge. A box caisson is fabricated above water and sunk onto a prepared bed, often remaining as part of the permanent foundation. A pneumatic caisson uses compressed air to keep water out of the working chamber; this method presents serious occupational-health risks and requires specialized controls.

Caisson work involves temporary stability, buoyancy, scour, currents, dewatering, compressed-air exposure, concreting underwater and navigation or environmental constraints. These are not ordinary drafting details. Drawings must be based on approved temporary-works and permanent-works designs, with clear construction stages and datum references.

6. Underpinning

Underpinning strengthens, stabilizes or extends an existing foundation. It may be required because of settlement, changed loading, nearby excavation, deterioration, subsidence, tree-related shrinkage, leaking services or a change in building use.

Underpinning is high-risk because the existing structure is already carrying load and construction may temporarily reduce its support. Common systems include:

• Mass-concrete pins: short sections are excavated and concreted in a carefully sequenced pattern, often with alternate bays left untouched until the first bays gain strength.
• Needle beams and brackets: load is transferred from walls or columns to beams supported by new bases, piles or jacks.
• Jack piles or micropiles: small piles are installed beside or through the existing foundation and connected using needles, brackets or a reinforced cap.
• Resin injection or grouting: voids may be filled or soil improved, but the technique must be selected and verified for the actual ground and structure.
• Raft or beam strengthening: a new reinforced system may distribute loads over a larger area.

A safe underpinning sequence normally includes condition surveys, crack monitoring, temporary works, service surveys, staged excavation, controlled load transfer, inspection and documented hold points. Excavation beneath an existing wall must never be represented as a continuous open trench unless specifically designed and sequenced.

Drawings should show existing and proposed work with different line types or colours, bay numbers, sequence arrows, temporary propping, excavation limits, packing or grout, waterproofing and monitoring points. The drawing set should include a prominent note that no underpinning work starts without the engineer’s approved sequence and site verification.

7. Foundations for problematic ground

Expansive clay

Expansive clay changes volume as water content changes. Seasonal drying and wetting can cause heave, shrinkage and differential movement. Responses may include founding below the active zone, stiffened slabs, void formers, moisture-control measures, vegetation management and specially detailed services. A foundation depth alone does not solve the problem if water movement remains uncontrolled.

Collapsible soil and loose fill

Collapsible soils may appear strong when dry but lose structure when wetted. Uncontrolled fill may contain voids, organic material or variable density. Treatment can include removal and replacement, compaction, deep mixing, grouting, dynamic compaction or piles. Verification must demonstrate that the treated zone meets the design assumptions.

Soft normally consolidated clay

Soft clay may consolidate slowly under new load, creating long-term settlement and negative skin friction on piles. Options include staged construction, preload or surcharge, vertical drains, lightweight fill, ground improvement, raft foundations or piles extending to stronger strata.

Loose saturated sand and liquefaction risk

During strong shaking, loose saturated granular soil may lose effective stress and stiffness. Site-specific seismic assessment may require densification, drainage, soil mixing, stone columns, deep foundations or structural isolation. Liquefaction can also cause lateral spreading and settlement outside the building footprint.

Made ground, voids and karst

Made ground can vary sharply over short distances. Mining, dissolution or buried structures may create voids. Investigation should locate anomalous zones before foundation construction. Bridging slabs, grouting, piles or relocation may be considered, but the selected system must account for potential void migration and loss of support.

Flood-prone and scour-affected sites

Flooding can erode soil around foundations. Piles or shafts must extend below anticipated scour depth, and the design should consider debris impact, buoyancy and lateral flow. Ground-floor arrangements, drainage and erosion protection should be coordinated with civil works.

8. Quality assurance, safety and documentation

Deep foundation quality depends on construction records as much as on drawings. Typical records include pile location, diameter, depth, strata, drilling time, casing, groundwater, concrete volume, reinforcement cage, tremie continuity, installation energy, torque, refusal or set, integrity tests and deviations.

Common verification methods include static load tests, dynamic tests, low-strain integrity testing, cross-hole sonic logging and inspection of exposed or excavated elements. The selected test is not interchangeable with another; it must address the relevant failure mode and be interpreted by competent specialists.

Major hazards include suspended loads, piling rigs, high-pressure grout, open shafts, unstable excavations, underground services, noise, vibration, contaminated spoil, flooding and confined spaces. A drawing should support safe construction but cannot substitute for a risk assessment, method statement or temporary-works design.

Ethically, practitioners must not conceal uncertain ground conditions, omit adverse records or present typical details as certified designs. Revisions, assumptions and departures must be traceable. Regulatory requirements commonly cover building safety, excavation support, environmental discharge, noise, worker protection and inspection; the applicable local legislation and standards must govern the project.

9. Digital practice and contemporary developments

Building information modelling can coordinate pile centres, caps, columns, reinforcement, services and excavation zones. Coordinate systems should be controlled so that survey coordinates and model coordinates agree. Clash detection is particularly valuable where piles conflict with basements, drainage, retaining walls or existing utilities.

Digital pile logs, automated rig monitoring and geospatial records improve traceability. Machine-learning tools may help identify anomalous installation records, but they do not replace geotechnical interpretation, field inspection or professional approval. AI-generated drawings require independent checking of scale, levels, reinforcement, labels and design intent.

Contemporary practice increasingly emphasizes low-carbon concrete, optimized pile lengths, reuse of existing foundations, low-vibration installation, recyclable steel, reduced spoil and whole-life assessment. Sustainability decisions must still preserve capacity, durability, constructability and safety.

Applied case: illustrative underpinning beside a deep excavation

Background

An existing two-storey masonry building stands 1.5 m from a proposed basement excavation. The existing strip footing is shallow and rests on variable fill over soft clay. The basement excavation is 4.5 m deep.

Problem

Unbraced excavation could cause ground movement and settlement beneath the existing footing. A conventional underpinning trench would expose too much foundation length at one time and could destabilize the wall.

Analysis

The project team requires a ground investigation, condition survey, service survey, excavation-support design and monitoring plan. The engineer evaluates wall loads, allowable movement, groundwater and the interaction between underpinning and the temporary retaining system. The draftsman must not select pile size or underpinning spacing independently.

Approach

The approved scheme uses sequenced reinforced-concrete underpinning bays connected to a designed beam, with temporary propping and controlled packing. Alternate bays are excavated first; concrete is placed and reaches the specified strength before intervening bays proceed. Crack gauges and survey points are installed. The basement excavation uses a designed retaining wall and staged excavation sequence.

The drawings show existing foundations, bay numbers, excavation limits, sequence arrows, temporary works interfaces, monitoring points, levels and hold-point notes. A section indicates the required connection between the new underpinning beam and the existing wall without assuming that the existing masonry is sound.

Outcome

Monitoring confirms movement remains within the project trigger levels. When a trigger is approached, work pauses and the engineer reviews the condition. The case illustrates that successful underpinning is a coordinated process of investigation, design, sequencing, observation and documentation—not merely the addition of concrete beneath a wall.

Learning points

• Existing structures require a condition baseline before intervention.
• Temporary conditions may govern more strongly than the final condition.
• Sequencing and monitoring are design requirements.
• Drawings must communicate limits of work and decision hold points.
• Specialist structural and geotechnical approval is essential.

When specialist design is mandatory

A draftsman should refer the work to a qualified geotechnical or structural engineer when any of the following applies: uncertain or highly variable soil; groundwater below excavation level; deep excavation near existing structures; significant lateral, uplift or seismic loads; pile groups or pile caps; underpinning; caissons or work in water; contaminated ground; expansive or collapsible soil; liquefaction; settlement-sensitive equipment; unusual construction methods; or any deviation from an approved design.

The draftsman’s professional responsibility is to communicate accurately, preserve design intent, identify missing information and stop the drawing process when essential engineering data are absent.

Chapter Summary

Deep and special foundations are selected when shallow foundations cannot safely or economically control bearing pressure, settlement, lateral movement, uplift or construction risk. Piles transfer load through toe resistance, shaft resistance or both; their performance is affected by soil layering, installation, group action, groundwater and time-dependent effects. Pile caps distribute column actions and require carefully coordinated reinforcement and setting-out information.

Piers and drilled shafts provide large-capacity deep support but demand control of excavation stability, casing, slurry, base cleanliness, reinforcement and concrete placement. Caissons are specialized systems for construction below water or difficult ground and involve major temporary-works and health-and-safety risks. Underpinning must be staged and monitored because the existing structure remains vulnerable throughout construction.

Expansive clay, collapsible soils, soft clay, liquefiable sand, fill, voids and scour each require a ground-specific response. Drawings should show levels, dimensions, reinforcement, sequences, tolerances, existing conditions and interfaces, while avoiding unapproved assumptions. Geotechnical, structural and temporary-works specialists must control design decisions beyond drafting competence.

Key Terms

Pile: A deep foundation member that transfers load into soil or rock.

End bearing: Resistance developed at the pile toe.

Shaft resistance: Resistance developed along the pile-ground interface.

Pile group: Two or more piles acting together beneath a structural support.

Pile cap: Reinforced-concrete element distributing loads to a pile group.

Negative skin friction: Downward interface drag caused by settling soil around a pile.

Bored pile: A pile formed by drilling and concreting an excavation.

Driven pile: A manufactured pile installed by impact, vibration or jacking.

Micropile: A small-diameter drilled and grouted pile, often used in restricted access.

Drilled shaft: A large-diameter bored foundation element.

Caisson: A structure used to construct or support foundations below water or difficult ground.

Underpinning: Work that strengthens, stabilizes or extends an existing foundation.

Expansive soil: Soil that changes volume substantially with moisture variation.

Collapsible soil: Soil that undergoes significant settlement when wetted or loaded.

Liquefaction: Loss of granular soil strength and stiffness during cyclic shaking.

Scour: Erosion of soil caused by flowing water.

Temporary works: Construction-stage works needed for stability or safety.

Integrity test: A test used to identify defects or continuity problems in a foundation element.

Ground model: An interpreted description of ground strata, properties, groundwater and variability.

Review Questions

Short Answer Questions

1. What are the two principal mechanisms by which an axially loaded pile develops resistance?
2. Why is a pile cap required in many column foundation systems?
3. State two construction risks associated with bored piles.
4. What is negative skin friction and when might it occur?
5. Why must underpinning usually be performed in stages?

Long Answer Questions

1. Compare driven piles, bored piles and micropiles in terms of construction method, advantages, limitations and appropriate applications.
2. Explain the information that should appear on a basic pile-cap plan and section.
3. Describe the investigation and construction controls required for underpinning an existing masonry building.
4. Discuss foundation strategies for expansive clay, soft clay and loose saturated sand.
5. Explain the role of quality assurance records and integrity testing in deep foundation construction.

Analytical / Application Questions

1. A proposed basement is adjacent to an old shallow-founded building on variable fill. Develop a preliminary investigation, risk-control and drawing-coordination plan, identifying decisions that require specialist approval.
2. A pile installation record shows pile positions outside tolerance, lower-than-expected concrete volume and rapidly changing groundwater. Explain the possible implications and propose an appropriate engineering response.
3. For a structure exposed to large lateral wind loads and possible scour, compare a pile group, large-diameter drilled shaft and shallow foundation. State the information needed before recommending a system.

Multiple Choice Questions with Answers

Question 1

Which mechanism transfers load primarily through the toe of a pile?

A. End bearing

B. Negative skin friction

C. Capillary suction

D. Surface drainage

Correct Answer: A. End bearing

Question 2

A pile cap primarily performs which function?

A. Waterproofing the building basement

B. Distributing column actions to the piles

C. Replacing all geotechnical investigation

D. Preventing every type of pile settlement

Correct Answer: B. Distributing column actions to the piles

Question 3

Which pile type is generally formed by drilling and then placing concrete?

A. Driven timber pile

B. Bored pile

C. Sheet pile

D. Rolled steel beam

Correct Answer: B. Bored pile

Question 4

Negative skin friction is most likely when:

A. The pile rises relative to the soil

B. Surrounding soil settles relative to the pile

C. The pile is installed in sound rock only

D. The pile cap is painted

Correct Answer: B. Surrounding soil settles relative to the pile

Question 5

Which condition is especially associated with volume change caused by seasonal moisture variation?

A. Expansive clay

B. Clean dense gravel

C. Sound unweathered rock

D. Dry structural steel

Correct Answer: A. Expansive clay

Question 6

Why is concrete commonly placed by tremie in a water-filled drilled shaft?

A. To allow controlled underwater placement without uncontrolled water mixing

B. To increase pile vibration

C. To remove reinforcement from the cage

D. To eliminate the need for inspection

Correct Answer: A. To allow controlled underwater placement without uncontrolled water mixing

Question 7

Which document should control the sequence of underpinning work?

A. An unapproved typical detail

B. The approved structural and temporary-works design

C. A paint schedule

D. A landscape plan

Correct Answer: B. The approved structural and temporary-works design

Question 8

Which issue may result from driving piles near an existing sensitive building?

A. Noise and ground vibration

B. Guaranteed reduction in settlement

C. Automatic soil stabilization

D. Elimination of groundwater

Correct Answer: A. Noise and ground vibration

Question 9

A draftsman should refer pile-cap reinforcement design to a structural engineer because:

A. Pile caps may act as deep structural members requiring designed force transfer

B. All concrete is self-designing

C. Pile caps never carry bending or shear

D. Reinforcement is only a graphic convention

Correct Answer: A. Pile caps may act as deep structural members requiring designed force transfer

Question 10

Which ground condition may involve loss of granular soil strength during strong shaking?

A. Liquefaction-prone loose saturated sand

B. Dry dense concrete

C. Stable sound granite

D. Non-cyclic steel framing

Correct Answer: A. Liquefaction-prone loose saturated sand