Bridge Foundations and Substructures

Introduction and Context

A bridge substructure transfers the loads of the deck and traffic to the ground while resisting water action, earth pressure, impact, temperature effects, braking forces, and earthquake or wind forces. It normally consists of foundations, abutments, piers, wing walls, retaining elements, scour protection, and approach works. For a civil draftsman, understanding this system is essential because drawings must communicate not only shape and dimensions but also levels, materials, reinforcement, construction joints, drainage, founding conditions, and protection measures.

The substructure must be compatible with the superstructure, site geology, river hydraulics, construction equipment, and maintenance requirements. A beautifully drafted detail is unsafe if it shows an unsuitable foundation level, inadequate scour depth, unsupported excavation, or poor drainage.

Learning Objectives

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

1. Explain the function and selection of open, pile, well, and caisson foundations.
2. Prepare plans, elevations, longitudinal sections, and cross-sections of bridge substructures.
3. Identify the principal components of abutments, piers, wing walls, and retaining elements.
4. Apply basic bearing-pressure, stability, and pile-cap calculations.
5. Show scour protection, drainage, construction joints, and approach works correctly on drawings.
6. Recognize constructability, safety, environmental, and quality-control requirements.

1. Substructure Load Path and Terminology

The load path is:

deck and bearings → pier or abutment → footing, pile cap, well, or caisson → soil or rock.

The principal terms are:

• Foundation: The part that transfers structural load to soil or rock.
• Footing: A widened base that spreads load over a larger soil area.
• Abutment: An end support that carries the deck and retains the approach embankment.
• Pier: An intermediate support between spans.
• Pier cap: The horizontal member supporting bearings or a continuous deck.
• Stem: The vertical or inclined wall portion of an abutment or retaining wall.
• Heel: The portion of a footing extending behind the stem.
• Toe: The portion extending in front of the stem.
• Wing wall: A wall extending from an abutment to retain approach soil and guide water.
• Pile cap: A reinforced concrete block connecting a group of piles.
• Scour: Removal of bed or bank material by flowing water.
• Freeboard: Vertical clearance between the design flood level and the underside of the superstructure.
• Cut-off level: The level at which a pile or foundation element is trimmed and connected to the cap or footing.
• Founding level: The base elevation of a foundation.

A drawing set should establish a common datum and show existing ground level, finished road level, ordinary water level, design flood level, bed level, scour level, founding level, and top-of-concrete levels.

Figure: Simplified longitudinal section showing an abutment, spread footing, deck bearing, and approach fill.

2. Site Investigation and Foundation Selection

Foundation selection begins with investigation rather than drafting. Relevant information includes soil classification, allowable or ultimate strength, groundwater, rock depth and quality, settlement characteristics, liquefaction potential, river velocity, debris, bed material, seismicity, and construction access.

An open foundation is generally suitable where competent soil or rock occurs at a manageable depth and excavation can be safely supported or dewatered. Piles are preferred where surface soils are weak, settlements must be controlled, or loads must be transferred to deeper strata. Wells and caissons are useful for large river piers or foundations requiring substantial lateral resistance and deep founding.

The selected type must also reflect environmental restrictions. In-water construction can increase turbidity and disturb aquatic habitats. Pile driving may cause noise and vibration, while drilled shafts generate spoil and may require temporary casing or slurry.

3. Open or Spread Foundations

An open foundation consists of a footing constructed in an excavation. Common forms are rectangular footings for abutments and isolated or combined footings for piers. The excavation is supported by slopes, timbering, sheet piles, trench boxes, or other approved systems.

The footing must be founded below the anticipated scour level in watercourses and below unsuitable topsoil, loose fill, or expansive material. The founding surface should be level, clean, and protected from softening. A levelling or blinding layer is often shown beneath reinforced concrete.

3.1 Preliminary bearing check

For a centrally loaded rectangular footing, the average service bearing pressure is:

where q is average pressure in kPa, P is the service vertical load in kN, and A is footing area in m². Since 1 kN/m² = 1 kPa, units are consistent.

For a footing of width B and length L:

This simple expression assumes uniform contact pressure and concentric loading. It is not sufficient where moments, eccentricity, layered soils, groundwater, or seismic actions are significant.

For a rectangular footing carrying vertical load P and moments Mₓ and Mᵧ, an elastic preliminary pressure distribution may be written as:

The signs are selected according to the corner being checked, and the orientation of B, L, and moments must be kept consistent. The equation assumes a rigid footing, linear soil response, and full contact. If the calculated minimum pressure is negative, the assumed full-contact model is invalid and the design requires further analysis.

A draftsman should not alter footing dimensions to make a pressure check pass without design approval. Drawings must follow the engineer’s calculation and governing code.

3.2 Detailing requirements

Typical open-foundation drawings show footing length, width, thickness, blinding, reinforcement direction and spacing, concrete cover, pedestal or stem dimensions, construction joints, drainage, excavation limits, and founding elevation. Reinforcement should be shown in plan and sections with bar marks, diameter, spacing, bends, laps, and cover. Waterstops are required where a construction joint may admit water.

4. Pile Foundations

Piles are slender elements installed or constructed in the ground. They transfer load by shaft resistance, end bearing, or a combination of both. Driven piles may be precast concrete, steel, or timber where permitted. Bored piles or drilled shafts are formed by excavating and concreting in place, often with casing or drilling fluid.

A pile group is connected by a pile cap. The drawing must show pile diameter or section, number and spacing, cut-off level, embedment, inclination if applicable, reinforcement cage, pile-cap thickness, pile layout, and tolerances. Pile spacing must be taken from the design; close spacing can reduce group efficiency and complicate concreting.

The axial resistance of an individual pile is conceptually:

where Qᵤ is ultimate axial resistance, Q_b is ultimate base resistance, and Q_s is ultimate shaft resistance. A design resistance is obtained by applying the partial factors or safety factors required by the governing standard. The expression is a model, not a universal design equation; each term depends on soil type, installation method, groundwater, effective stress, pile geometry, and testing.

For a preliminary concentric load distribution among identical piles:

where Qᵢ is the average axial load per pile, P is the total vertical service or factored load, and n is the number of piles. This assumes equal piles, a rigid cap, concentric load, and negligible group interaction. With cap moments, pile loads are unequal and must be calculated by the designer.

Worked example: preliminary pile load

An illustrative pile cap carries a vertical service load of 2,400 kN through six identical vertical piles. Ignoring cap weight, moment, and group effects:

This is only an average service load. It does not demonstrate that the piles are safe; allowable resistance, settlement, uplift, lateral load, negative skin friction, and structural pile capacity must also be checked.

Pile drawings should distinguish permanent reinforcement from temporary casing and should show the sequence where it affects construction. For bored piles, the designer may require inspection of the bore, cleaning of the base, concrete tremie placement, and integrity testing.

5. Wells and Caissons

A well foundation is a large hollow unit sunk through soil, often used for river piers. Typical parts include cutting edge, curb, steining, well cap, bottom plug, sand filling, and reinforced concrete top plug or cap. The well may be sunk by controlled excavation inside the shaft. Its plan may be circular, twin-circular, rectangular, or oblong.

A caisson is a large foundation unit constructed or assembled to permit work below water or within difficult ground conditions. Open caissons are sunk while material is removed from inside. Pneumatic caissons use compressed air in a working chamber and require strict control of decompression and occupational health. Modern drilled shafts may replace caissons where equipment and ground conditions permit.

Well and caisson drawings require special attention to:

• cutting-edge geometry and steel protection;
• steining thickness and reinforcement;
• sinking stages and temporary stability;
• water-tightness and concrete plugs;
• working levels and access;
• scour and debris impact;
• differential settlement and tilt tolerances.

The final drawings must be coordinated with a method statement. A section that shows the finished well but ignores sinking stages is incomplete for construction planning.

6. Abutments, Piers, and Wing Walls

6.1 Abutments

An abutment supports the end of the bridge and retains approach fill. Common forms include gravity, cantilever reinforced-concrete, counterfort, spill-through, and mechanically stabilized earth-supported arrangements. Selection depends on height, soil pressure, space, foundation conditions, and appearance.

Abutment drawings should show bearing shelves, bearing centre lines, backwall, ballast wall where specified, expansion gap, deck drainage, approach slab, drainage layer, filter material, weep holes, and reinforcement. The backfill must be free-draining and compacted in controlled layers. Weep holes alone are not a substitute for a properly designed drainage system.

6.2 Piers

Piers may have solid stems, columns, hammer-head caps, wall forms, or multi-column arrangements. A pier must resist vertical load, transverse and longitudinal forces, water pressure, debris impact, braking, and accidental collision where relevant. The section should identify pier height, cap overhang, bearing arrangement, pedestal, chamfers, reinforcement, construction joints, and foundation connection.

Hydraulic piers in rivers commonly have rounded or streamlined noses to reduce obstruction and debris accumulation. The orientation must be shown relative to the direction of flow.

6.3 Wing walls and retaining elements

Wing walls retain approach embankments beside the abutment. They may be parallel, flared, return, or independent walls. Their plan geometry controls earth retention and transition between bridge and road embankment. Retaining walls require drainage, relief of hydrostatic pressure, stable foundations, and movement joints where appropriate.

The active earth pressure coefficient for level backfill in a simplified Rankine model is:

where Kₐ is the active earth-pressure coefficient and φ is the soil friction angle. This expression assumes a homogeneous, dry, cohesionless soil, level backfill, a smooth vertical wall, and active wall movement. It should not be used uncritically for sloping backfill, cohesive soils, surcharge, seismic loading, or restrained walls.

For a retained height H and soil unit weight γ, the triangular resultant active force per metre length is:

The resultant acts at H/3 above the base for the idealized triangular distribution. Surcharge, water pressure, wall friction, compaction pressure, and seismic effects require additional consideration.

7. Scour Protection and Hydraulic Safety

Scour is often the controlling factor in bridge foundation depth. General scour affects the whole watercourse, contraction scour occurs where flow is narrowed, and local scour forms around piers and abutments. The design scour level is not simply the observed bed level; it is established from hydrology, hydraulics, sediment characteristics, flood frequency, debris, and applicable standards.

Protection may include rock riprap, concrete blocks, gabions, mattresses, launching aprons, cutoff walls, guide banks, and properly graded filters. A filter prevents the underlying soil from being washed through the protective layer. The drawing should show the extent, thickness, toe treatment, side slopes, filter layers, and relationship to design scour level.

Protection that ends abruptly at the edge of a foundation can be undermined. Transitions and termination details are therefore important. River works should also consider fish passage, sediment continuity, water quality, and temporary diversion arrangements.

8. Approach Works and Drainage

Bridge approaches must provide a smooth, durable transition between flexible embankment and relatively stiff bridge structure. Typical elements include approach slab, compacted selected fill, geotextile or filter layer, kerb, guardrail, side drain, pavement layers, expansion joint, and erosion protection.

Differential settlement is reduced by suitable fill, staged compaction, improved ground, approach slabs, and careful drainage. Drawings should show compaction requirements by reference to the specification rather than inventing a value. The approach slab must be supported and detailed for movement at the abutment interface.

Drainage details should identify deck scuppers, downpipes, behind-wall drains, outlets, toe drains, and discharge points. Water must not be directed onto an unprotected slope or into a foundation excavation.

9. Drawing Production and Coordination

A professional substructure drawing package commonly includes:

1. General arrangement plan with grid, chainage, north arrow, flow direction, and coordinates.
2. Longitudinal elevation showing spans, bearings, deck levels, flood levels, and foundation levels.
3. Cross-sections through abutments, piers, wing walls, and approaches.
4. Foundation plans for footings, pile caps, wells, or caissons.
5. Reinforcement details, bar schedules, development and lap information.
6. Drainage, waterproofing, joints, scour, and construction details.
7. Notes stating materials, concrete class, reinforcement grade, cover, tolerances, design loads, and governing standards.

Use consistent line weights: heavy outlines for cut concrete, medium lines for visible edges, thin lines for dimensions and reinforcement, and chain lines for centre lines. Dimensions should be related to stable datums, not accumulated from uncertain edges. Avoid dimensioning to hatching. Every level should include a datum reference and unit.

Before issue, coordinate bearing locations with the superstructure, pile positions with reinforcement, wing walls with road geometry, drainage with utilities, and foundation levels with the geotechnical report. Revisions should be clouded or otherwise identified according to the project quality system.

10. Worked Application: Illustrative River Bridge Abutment

Background

Consider a fictional single-span rural bridge crossing a seasonal stream. The bridge carries a two-lane road. The stream has erodible sand and gravel bed material, and the geotechnical investigation identifies dense soil at moderate depth. The design flood level is 1.20 m above the existing bed, and the estimated design scour level is 1.50 m below the existing bed.

Problem

The preliminary concept used a shallow footing close to the existing bed. Review showed that the founding level would be above the design scour level and that uncontrolled water behind the abutment could saturate the approach fill.

Analysis and approach

The revised concept uses an open reinforced-concrete footing founded below the design scour level, a cantilever abutment stem, flared wing walls, granular drainage material behind the wall, and an approach slab. Riprap with a graded filter is provided around the abutment nose and wing-wall returns. The drawing set includes:

• plan locating the abutment relative to the stream centreline;
• elevation showing flood, bed, scour, and founding levels;
• section through the stem and footing;
• wing-wall plan and reinforcement;
• drainage and weep-hole detail;
• riprap section with filter and toe;
• approach slab and pavement transition.

For a preliminary check, suppose the service vertical load on a 2.0 m × 8.0 m footing is 4,800 kN, excluding no other effects. The average pressure is:

This value must be compared with the geotechnical allowable bearing pressure, after including footing self-weight and checking eccentricity, sliding, overturning, settlement, scour, and load combinations. It is not a final design.

Outcome and learning points

The revised arrangement protects the founding stratum, provides a controlled drainage path, and makes the hydraulic assumptions visible on the drawings. The case demonstrates that foundation drawings are design communication documents: levels, water management, and protection details are as important as concrete outlines.

11. Construction, Inspection, and Safety

Foundation construction involves hazards from excavation collapse, flooding, lifting, concrete placement, pile-driving vibration, confined spaces, compressed air, and contaminated spoil. Drawings should not prescribe unsafe temporary works unless designed and approved for that purpose. Temporary cofferdams, access platforms, diversion channels, and excavation support require separate engineering.

Inspection and testing may include soil and rock verification, pile integrity tests, static or dynamic pile tests, concrete slump and strength tests, reinforcement inspection, survey checks, water-tightness checks, and as-built level surveys. Critical hold points should be identified in the inspection and test plan.

Ethical practice requires truthful representation of ground conditions, clear revision control, respect for approved standards, and prompt reporting of discrepancies. A draftsman must never conceal a conflict between drawings and site conditions.

12. Digital Practice and Contemporary Developments

CAD and building-information modelling improve coordination by linking geometry, levels, quantities, and revisions. A model can help detect pile-cap clashes, incorrect bearing offsets, and inconsistent elevations. Survey data, GIS, photogrammetry, and laser scanning support as-built documentation and inspection.

AI-assisted drafting may identify missing labels, compare revisions, or suggest clash checks, but it cannot replace engineering judgement or approval. Generated details must be checked against the design basis, geotechnical report, hydraulic study, code, and project specification. Digital files require access control, revision history, backups, and protection against unauthorized alteration.

Climate change increases the importance of reassessing flood levels, debris loads, erosion, and durability exposure over the bridge service life. Adaptable designs should allow inspection, repair, and replacement of vulnerable protection elements.

Chapter Summary

Bridge substructures transfer superstructure actions safely into the ground while resisting earth, water, scour, impact, and environmental effects. Open foundations are appropriate where competent founding material is accessible; piles transfer load to deeper ground; wells and caissons are suited to large or deep water foundations. Abutments retain approaches, piers support intermediate spans, wing walls retain side slopes, and drainage and scour protection preserve long-term performance.

A sound drawing package communicates geometry, levels, materials, reinforcement, construction joints, drainage, foundation conditions, and protection. Preliminary equations for bearing pressure, eccentric loading, pile-load distribution, and earth pressure are useful only within their stated assumptions. Final design requires coordinated structural, geotechnical, hydraulic, construction, and regulatory review.

Key Terms

Foundation: Structural element that transfers load to soil or rock.

Substructure: Bridge components below bearings, including foundations, abutments, piers, and protection works.

Abutment: End support that carries the bridge and retains approach fill.

Pier: Intermediate vertical support between bridge spans.

Footing: Enlarged base that spreads load over the ground.

Pile: Deep foundation element transferring load through shaft resistance, end bearing, or both.

Pile cap: Concrete member connecting a group of piles.

Well foundation: Large hollow foundation unit sunk into river or soil deposits.

Caisson: Large foundation unit used for deep or underwater construction.

Wing wall: Wall retaining approach embankment beside an abutment.

Scour: Erosion or removal of bed and bank material by flowing water.

Bearing pressure: Contact pressure between foundation and supporting ground.

Eccentricity: Offset between the resultant load and the centroid of a foundation.

Freeboard: Clearance between design flood level and the bridge underside.

Founding level: Elevation of the foundation bearing surface.

Riprap: Graded rock used to resist erosion.

Filter layer: Material preventing migration of soil through scour protection.

Approach slab: Structural slab providing a transition from pavement to bridge.

Blinding concrete: Lean concrete layer providing a clean, level working surface.

Cut-off level: Level at which a pile is trimmed and connected to its cap.

Review Questions

Short Answer Questions

1. What is the principal difference between an abutment and a pier?
2. State two conditions that may favour the use of pile foundations.
3. Why must a bridge foundation be checked against scour?
4. List four items that should appear on an abutment section.
5. What is the function of a filter layer beneath riprap?

Long Answer Questions

1. Explain the load path through a typical bridge from deck to supporting ground.
2. Compare open, pile, well, and caisson foundations in terms of use, construction, and drafting requirements.
3. Describe the components and detailing requirements of a reinforced-concrete abutment.
4. Explain how drainage and approach works influence bridge durability and settlement.
5. Discuss the information required in a coordinated bridge substructure drawing package.

Analytical / Application Questions

1. A rectangular footing carries a service load of 3,600 kN over an area of 12 m². Calculate the average bearing pressure and state why this is not by itself a complete foundation design.
2. A pile cap supports 2,800 kN through seven identical vertical piles. Determine the preliminary average axial load per pile and identify three effects that could make actual pile loads unequal.
3. A proposed abutment footing is above the predicted design scour level. Analyse the risks and recommend changes to the foundation and drawing set.

Multiple Choice Questions with Answers

Question 1

Which component transfers load from a group of piles into the pier or abutment?

A. Pile cap

B. Weep hole

C. Approach slab

D. Filter layer

Correct Answer: A. Pile cap

Question 2

A 2 m × 5 m footing carries a concentric vertical load of 2,000 kN. What is the average bearing pressure?

A. 100 kPa

B. 200 kPa

C. 500 kPa

D. 2,000 kPa

Correct Answer: B. 200 kPa

Question 3

Which foundation is generally most suitable when competent bearing material is available at a shallow, accessible depth?

A. Open foundation

B. Pneumatic caisson

C. Deep pile group

D. Suspended footing

Correct Answer: A. Open foundation

Question 4

In the simplified Rankine expression Kₐ = tan²(45° − φ/2), φ represents the:

A. Soil unit weight

B. Wall height

C. Soil friction angle

D. Water pressure

Correct Answer: C. Soil friction angle

Question 5

For an ideal triangular active earth-pressure distribution, the resultant force acts at what height above the wall base?

A. H/4

B. H/3

C. H/2

D. 2H/3

Correct Answer: B. H/3

Question 6

What is the primary function of a wing wall?

A. Support bearings directly

B. Retain approach embankment beside the abutment

C. Measure pile capacity

D. Reduce concrete cover

Correct Answer: B. Retain approach embankment beside the abutment

Question 7

If a six-pile group carries a concentric load of 2,400 kN and all piles are assumed identical, the preliminary average load per pile is:

A. 240 kN

B. 300 kN

C. 400 kN

D. 600 kN

Correct Answer: C. 400 kN

Question 8

Which level is most directly associated with potential erosion of a river foundation?

A. Design scour level

B. Bearing centreline

C. Road crown level

D. Top-of-kerb level

Correct Answer: A. Design scour level

Question 9

Why is drainage provided behind an abutment or retaining wall?

A. To increase hydrostatic pressure

B. To prevent all soil movement by friction alone

C. To control water pressure and protect backfill performance

D. To replace foundation reinforcement

Correct Answer: C. To control water pressure and protect backfill performance

Question 10

Which drawing view most directly shows the relationship among deck, abutment, footing, bed level, and founding level?

A. Longitudinal section

B. Bar bending schedule only

C. Title block

D. Material test certificate

Correct Answer: A. Longitudinal section