Shallow Foundation Systems

Introduction

A foundation transfers building loads to the ground safely and with acceptable settlement. A shallow foundation is one in which the load is transmitted to soil close to the ground surface, commonly at a depth less than or comparable to its width. The exact boundary between shallow and deep foundations depends on soil conditions, geometry and design practice, but isolated footings, strip footings, combined footings, stepped footings, rafts and slabs-on-ground are normally classified as shallow systems.

For a civil draftsman, foundation work is more than drawing a rectangle beneath a wall or column. Drawings must communicate load paths, dimensions, levels, reinforcement, concrete quality, cover, joints, excavation requirements and connections to the superstructure. Structural design must be carried out or checked by a suitably qualified engineer in accordance with the applicable national code. The dimensions and reinforcement examples in this chapter are educational illustrations, not construction prescriptions.

Learning Objectives

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

1. Explain how shallow foundations transfer load to soil.
2. Select an appropriate shallow foundation system for common building situations.
3. Describe strip, pad, combined, stepped, raft and slab-on-ground foundations.
4. Prepare drawings showing dimensions, levels, blinding, reinforcement, cover, joints and connections.
5. Distinguish geotechnical requirements from structural detailing requirements.
6. Identify common detailing errors and coordinate foundation drawings with architectural and services information.

1. Foundation Behaviour and Design Basis

The principal load path is:

roof and floors → beams and columns or walls → foundation concrete → soil.

A satisfactory foundation must address:

• Bearing capacity: soil must support the applied pressure without shear failure.
• Settlement: total and differential settlement must remain acceptable.
• Sliding and overturning: relevant for retaining walls, eccentric columns and lateral loads.
• Structural strength: concrete and reinforcement must resist bending, shear, punching shear and local bearing.
• Durability: concrete, reinforcement and joints must withstand moisture, chemicals, freeze–thaw action and other exposure conditions.
• Constructability: excavation, dewatering, reinforcement placement and concrete access must be practical.

The average gross contact pressure may be expressed as:

where q is average contact pressure in kPa or kN/m², P is the design vertical load in kN, and A is the effective foundation area in m². This simple expression assumes the load is approximately concentric and the pressure distribution is sufficiently uniform. It is not adequate by itself where moments, eccentricity, layered soils or uplift occur.

For a rectangular footing carrying a vertical load P and moments Mₓ and Mᵧ, the idealised linear soil pressure is often written as:

Here Wₓ and Wᵧ are the section moduli of the footing contact area about the relevant axes. The signs give maximum and minimum pressures. The formula assumes a rigid footing, linear elastic contact behaviour and full contact. If the calculated minimum pressure is negative, part of the footing may lift and a different analysis is required.

The effective width or length for a load with eccentricity e is sometimes represented, for one-way eccentricity, by:

where B is the footing width and B′ is the effective width. This simplified approach is applicable only when the eccentricity and soil-contact assumptions are consistent with the governing code.

Terminology

• Footing: the enlarged structural element that spreads load into soil.
• Founding level: the underside elevation at which the foundation bears on soil.
• Blinding: a thin layer of low-strength concrete placed over prepared ground to provide a clean, level surface.
• Cover: the distance from the concrete surface to the nearest reinforcement.
• Starter bars: reinforcement projecting from a footing to connect a wall or column.
• Construction joint: a planned interruption in concrete placement.
• Movement joint: a joint intended to accommodate shrinkage, thermal movement or structural separation.
• Differential settlement: unequal movement of different foundation areas.

2. Site Information and Selection of System

Before selecting a foundation type, the project team should review the geotechnical report, topographic survey, groundwater conditions, adjacent structures, site access and building geometry. Important soil information includes allowable or ultimate bearing resistance, soil profile, groundwater level, compressibility, expansive or collapsible behaviour, chemical aggressiveness and frost or erosion conditions.

A shallow foundation is generally appropriate where competent soil occurs near the surface and expected settlement is acceptable. Deep foundations may be required where near-surface soil is weak, compressible or highly variable, or where scour, expansive movement or heavy concentrated loads cannot be controlled economically.

The foundation system must also suit the construction sequence. A system that is structurally adequate but cannot be excavated safely, reinforced accurately or protected from water is not a good design solution.

3. Strip Foundations

A strip foundation is a continuous footing beneath a load-bearing wall or a line of closely spaced columns. It distributes wall load over a larger soil area. In plan, it is usually a long rectangle; in section, it may be uniform in thickness or have a thickened base.

For a wall carrying a design line load w in kN/m and a selected footing width B in m, the approximate bearing pressure is:

This is a preliminary relationship for a concentric wall load. The final design must include footing self-weight, soil overburden where relevant, eccentricity, structural bending and shear.

Strip footing detailing

Typical drawings should show:

• wall thickness and centreline;
• footing width and thickness;
• founding level and surrounding ground level;
• blinding thickness, if specified;
• longitudinal and transverse reinforcement, including bar diameter and spacing;
• concrete grade and exposure class;
• nominal cover;
• wall starter bars, dowels or masonry connection;
• laps, bends and reinforcement continuity at corners and intersections.

For a simple wall footing, the wall load produces upward soil pressure. The footing projects on each side of the wall and acts as a continuous slab spanning from the wall centre region toward each edge. Main flexural reinforcement is therefore commonly placed longitudinally or transversely according to the structural model and code; the drawing must follow the engineer’s reinforcement schedule rather than relying on a generic arrangement.

At corners, reinforcement should continue or be adequately anchored. A common drafting error is to draw two crossing strips without showing how bars are developed through the corner. Wall reinforcement and damp-proofing must also be coordinated so that moisture does not bypass the intended barrier.

4. Pad or Isolated Footings

A pad footing supports an individual column or concentrated load. It may be square, rectangular or circular. The column should normally be centred so that the load resultant lies close to the footing centroid, unless eccentricity is intentionally designed.

For a concentric column load, a preliminary square footing side may be estimated from:

where B is the footing side in m, P is the service load in kN, and qₐ is allowable service bearing pressure in kN/m². The expression neglects or separately accounts for footing self-weight and is suitable only for preliminary sizing when the allowable-pressure method is permitted. Structural design still requires checks for one-way shear, punching shear, bending and reinforcement anchorage.

A pad footing drawing should identify the column grid intersection, column dimensions, pedestal if used, footing plan dimensions, thickness or thickness profile, founding level and reinforcement in both directions. Reinforcement is generally placed near the bottom of the footing because the soil reaction produces upward bending; however, local top reinforcement may be needed around columns, for moments, tying action or detailing requirements.

Column starter bars must be developed into the footing for the required anchorage or lap length. Their position must match the column cage. The drawing should not show bars passing through the column in a way that conflicts with longitudinal column reinforcement.

5. Combined and Strap Footings

A combined footing supports two or more columns on one base. It is useful when columns are close together or when a column lies near a property boundary and an isolated footing would project beyond the permitted limit. The footing may be rectangular or trapezoidal. Its geometry is selected so that the resultant soil reaction approximately aligns with the resultant column loads, reducing uneven pressure.

A strap footing uses separate pads connected by a stiff strap beam. The strap transfers moment and helps prevent rotation of an eccentric edge footing. The strap may be above, within or below the floor zone depending on the design. It should not automatically be assumed to bear on soil; its structural role must be clearly shown.

Drawings must show:

• column locations and designations;
• pad and strap dimensions;
• top and bottom reinforcement;
• beam-column or beam-pad anchorage;
• construction sequence where the strap affects excavation;
• clear distinction between soil-bearing concrete and suspended structural members.

The combined footing should be checked for overall bearing pressure, longitudinal bending, transverse bending, one-way shear, punching shear and torsion where relevant.

6. Stepped Foundations

A stepped foundation is used where a continuous footing follows sloping terrain or where a change in founding depth is necessary. It reduces excessive excavation and permits the foundation to remain on competent soil while maintaining manageable construction levels.

Steps should be arranged so that the footing bears entirely on undisturbed or properly compacted material. The horizontal overlap and vertical rise must comply with the governing code and engineering design. Abrupt steps can create stress concentrations or sliding planes; reinforcement and concrete continuity must therefore be detailed at each transition.

A stepped strip footing drawing should include a longitudinal section with:

• existing and proposed ground profiles;
• each founding level;
• horizontal run and vertical rise of every step;
• minimum overlap between adjacent levels;
• reinforcement continuation and anchorage;
• blinding and excavation limits;
• drainage or waterproofing provisions where needed.

The top of the wall should remain level only if the wall and steps are designed accordingly. Otherwise, the wall may step with the footing. The distinction must be communicated in plan, elevation and section.

7. Raft Foundations

A raft, or mat foundation, is a large reinforced-concrete slab supporting several columns, walls or the entire building footprint. It is considered when individual footings would occupy a large proportion of the plan area, when soil bearing pressure is low, or when reducing differential settlement is important.

Rafts may be flat slabs, thickened slabs, beam-and-slab systems, cellular rafts or piled rafts. They require two-way structural analysis because soil pressure and superstructure loads vary across the plan. Typical checks include flexure in both directions, punching shear at columns, one-way shear, crack control, settlement, uplift and robustness.

Raft drawings must show column and wall grids, slab thickness, drop panels or ribs, top and bottom reinforcement in both directions, local strengthening, openings, construction joints, waterstops and penetrations. Reinforcement congestion is a major coordination risk, especially at column heads, service penetrations and wall intersections.

A raft is not automatically a remedy for poor soil. It may reduce differential settlement by tying the structure together, but total settlement can remain unacceptable. Geotechnical analysis must justify the system.

8. Slab-on-Ground Foundations

A slab-on-ground is a concrete slab cast directly over prepared subgrade, usually separated by a granular sub-base, vapour barrier and sometimes blinding. It may be non-structural, lightly reinforced, or designed as a structural slab supporting walls and columns. The terminology and design assumptions must be stated clearly.

A typical build-up may include compacted subgrade, granular fill, blinding or sand, vapour barrier, insulation where required, reinforced concrete slab and floor finish. Edge thickening, ribs or integral beams may be provided beneath walls. The slab must be isolated from walls or columns when movement compatibility requires it.

Joints are essential because concrete shrinks as it dries and changes length with temperature. Common joints include:

• Isolation joints: separate the slab from columns, walls or fixed elements.
• Contraction joints: create a weakened plane to control cracking.
• Construction joints: separate concrete placements.
• Expansion or movement joints: accommodate larger relative movement between structural portions.

Joint locations should be coordinated with room geometry, column grids and finishes. Reinforcement should not unintentionally bridge an isolation joint. Dowel systems across movement or contraction joints require a deliberate design for load transfer and movement.

9. Concrete, Blinding, Cover and Reinforcement

Concrete grade is specified by the applicable code, commonly through characteristic compressive strength at a stated age, exposure class, maximum water–cement ratio, cement type and durability requirements. A notation such as C_30/37 has different meanings in different standards; the project specification must define it. Concrete strength alone does not guarantee durability.

Blinding concrete is generally a thin, relatively low-strength layer used to provide a clean working surface, prevent reinforcement contamination and establish accurate levels. It is not normally assumed to contribute to the structural capacity of the footing unless specifically designed to do so. The drawing should state thickness, concrete specification and whether it extends beyond the footing.

Nominal cover protects reinforcement from corrosion and fire and provides construction tolerance. Required cover depends on exposure, fire resistance, bar size, aggregate, placement tolerance and the governing code. Cover must be shown as a dimension or specification, not guessed from a typical detail. Chairs, spacers and concrete supports must be compatible with the specified cover and environment.

Reinforcement drawings should identify bar mark, diameter, spacing, shape, location, lap or anchorage requirements and steel grade. Bars must be supported so that they do not sink into blinding or soil. Laps should be staggered where required and located away from zones of maximum stress when specified by the engineer.

10. Dimensions, Levels and Connections

A professional foundation drawing normally includes:

1. gridlines and setting-out dimensions;
2. overall foundation dimensions and offsets;
3. wall and column sizes;
4. excavation and founding levels;
5. top-of-concrete and finished-floor levels;
6. blinding, sub-base and slab build-up;
7. reinforcement notation and schedules;
8. concrete grade, cover and joint requirements;
9. sections through representative conditions;
10. notes on soil preparation, compaction, inspection and concrete placement.

Connections require particular attention. A masonry wall may connect through a concrete footing and damp-proof course, while a reinforced-concrete wall uses starter bars and a construction joint. A column may be cast monolithically with a pad, connected through dowels, or supported on a pedestal. Each arrangement has different force transfer, water and reinforcement implications.

Construction joints should be located where shear and bending demands are acceptable and surfaces should be prepared according to the specification. Water-retaining or below-ground walls may need hydrophilic strips, PVC waterstops or other joint-sealing systems. These items should appear in enlarged details rather than being left to interpretation.

11. Applied Case: Illustrative Small Commercial Building

Background

Consider an illustrative single-storey commercial building on level ground. The building has reinforced-concrete columns on a regular grid, masonry perimeter walls and a ground floor. A geotechnical investigation reports competent granular soil at shallow depth, moderate allowable bearing pressure and no shallow groundwater.

Problem

Isolated pads beneath interior columns are feasible, but two perimeter columns are close to a property boundary. A separate pad for each boundary column would be eccentric and could extend beyond the site. The floor must also resist ordinary commercial traffic and control moisture transmission.

Analysis and approach

Interior columns are assigned preliminary square pad footings using the relationship B = √(P/qₐ), followed by structural checks for punching shear, one-way shear and bending. The boundary columns are supported on a combined footing designed so that the resultant soil reaction remains compatible with the column loads and site boundary. The perimeter masonry wall is supported on a continuous strip footing. The ground floor is a slab-on-ground over compacted sub-base with a vapour barrier and isolation joint at the walls.

The drawings show a common founding level, a thin blinding layer, specified concrete grade and cover, bottom reinforcement in each pad, column starter bars and a combined-footing reinforcement plan. At the slab perimeter, the isolation joint prevents floor movement from loading the masonry wall. Service penetrations are sleeved and coordinated before concrete placement.

Outcome

The arrangement separates the structural roles of the pads, combined footing, strip footing and floor slab. It avoids an encroachment at the boundary, provides a clear load path and allows the floor to move independently of the wall. Inspection points are established for subgrade approval, reinforcement and cover, joint installation and concrete testing.

Learning points

A foundation system should be selected from soil, geometry and construction constraints together. A neat plan is not sufficient: levels, joints, reinforcement continuity and connection details determine whether the design can be built as intended.

12. Quality, Safety and Professional Practice

Excavations must be assessed for collapse, water ingress, adjacent-foundation undermining and safe access. Workers should not enter unsupported excavations. Subgrade should be inspected before blinding; soft spots, uncontrolled fill and organic material must be removed or treated as directed by the engineer.

Concrete placement requires checking formwork, dimensions, reinforcement, cover, embedded items, cleanliness and weather conditions. Concrete should be placed, compacted, cured and tested according to the specification. Concrete must not be diluted with unapproved water on site.

Ethically, a draftsman must not conceal uncertainty, alter structural details without approval or copy typical details into conditions for which they are unsuitable. Revisions should be traceable, and discrepancies between architectural, structural, geotechnical and services drawings should be reported before construction.

Building information modelling can improve coordination by linking grids, levels, reinforcement zones and penetrations. Automated checking can identify clashes, but software does not replace engineering judgement. AI-assisted drafting may help classify drawing elements or detect inconsistencies, but all generated details require review against the project specification and applicable code.

Challenges and Limitations

Shallow foundation selection is limited by uncertainty in soil variability, seasonal moisture, construction quality and incomplete information about existing structures. Simplified bearing-pressure equations do not capture all soil–structure interaction. Rafts may reduce differential settlement but cannot eliminate total settlement. Slabs-on-ground can crack even when structurally adequate because of shrinkage, restraint and poor curing. Generic reinforcement details may be unsafe when exposure, seismic action, water pressure or unusual loading differs from the assumed case.

The correct professional response is to identify assumptions, obtain missing information, refer design questions to the responsible engineer and revise drawings through controlled documentation.

Chapter Summary

Shallow foundations transfer building loads to near-surface soil through strip, pad, combined, stepped, raft and slab-on-ground systems. Selection depends on load arrangement, soil bearing and settlement behaviour, site boundaries, levels, groundwater, construction access and durability. Preliminary pressure relationships such as q = P/A assist understanding but do not replace geotechnical and structural design.

Strip footings serve continuous walls; pads support individual columns; combined and strap systems manage close or eccentric columns; stepped footings follow sloping ground; rafts distribute many loads over a broad area; and slabs-on-ground provide ground-supported floors that require deliberate subgrade, moisture and joint design. Accurate drawings must communicate dimensions, levels, concrete grade, blinding, cover, reinforcement, joints and connections. Safety, code compliance, inspection and coordination are essential parts of professional foundation drafting.

Key Terms

Foundation: Structural system transferring building loads to soil.

Shallow foundation: Foundation bearing near the ground surface.

Strip footing: Continuous footing beneath a wall or line load.

Pad footing: Individual footing beneath a column or concentrated load.

Combined footing: One footing supporting two or more columns.

Strap footing: Separate pads linked by a stiff strap beam.

Stepped footing: Footing arranged in levels to follow sloping ground.

Raft foundation: Large slab or beam-and-slab foundation supporting several loads.

Slab-on-ground: Concrete floor supported directly by prepared ground.

Bearing pressure: Contact pressure transmitted from foundation to soil.

Bearing capacity: Soil resistance against foundation failure.

Settlement: Downward movement of soil and foundation.

Differential settlement: Unequal settlement between foundation areas.

Blinding: Thin concrete layer providing a clean working surface.

Nominal cover: Specified concrete thickness protecting reinforcement.

Starter bar: Reinforcement projecting from one concrete element into another.

Construction joint: Planned interruption between concrete placements.

Isolation joint: Joint separating elements to permit relative movement.

Punching shear: Local shear failure around a concentrated column load.

Founding level: Elevation of the underside of a foundation.

Review Questions

Short Answer Questions

1. What is the principal function of a shallow foundation?
2. State two situations in which a combined footing may be appropriate.
3. What purpose does blinding concrete serve?
4. Why is nominal cover provided to reinforcement?
5. Name two types of joints used in slabs-on-ground.

Long Answer Questions

1. Explain the selection process for strip, pad and raft foundations, including soil and structural considerations.
2. Describe the information that should appear on a reinforced-concrete pad footing drawing.
3. Explain the purpose and detailing requirements of stepped foundations.
4. Discuss the difference between construction, isolation, contraction and movement joints.
5. Describe how a slab-on-ground is constructed and how cracking and moisture are controlled.

Analytical / Application Questions

1. A column carries a concentric service load of 360 kN and the allowable bearing pressure is 180 kN/m². Using the preliminary square-footing relationship, estimate the footing side. State the assumptions and explain why further design checks are required.
2. Two boundary columns cannot have independent centred pads because of a property restriction. Compare a combined footing with a strap-footing arrangement and recommend what information the engineer must obtain before finalising the system.
3. Prepare a coordinated list of plan, section and reinforcement information required for a stepped strip footing on sloping ground. Explain how missing levels or reinforcement continuity could affect construction and performance.

Multiple Choice Questions with Answers

Question 1

Which foundation is normally used beneath a continuous load-bearing wall?

A. Strip footing

B. Pad footing

C. Pile cap

D. Isolated pedestal

Correct Answer: A. Strip footing

Question 2

For a concentric load, the preliminary average soil pressure is represented by which expression?

A. q = P/A

B. q = A/P

C. q = P × A

D. q = P − A

Correct Answer: A. q = P/A

Question 3

What is the primary purpose of blinding concrete?

A. To replace all structural reinforcement

B. To provide a clean and level working surface

C. To increase soil bearing capacity in every case

D. To act as a movement joint

Correct Answer: B. To provide a clean and level working surface

Question 4

Which foundation is especially useful when several columns occupy a large portion of the building footprint?

A. Raft foundation

B. Lintel footing

C. Kerb footing

D. Single strip footing only

Correct Answer: A. Raft foundation

Question 5

What does nominal concrete cover primarily protect against?

A. Column eccentricity

B. Reinforcement corrosion and fire exposure

C. Soil settlement calculations

D. Setting-out errors only

Correct Answer: B. Reinforcement corrosion and fire exposure

Question 6

A stepped foundation is mainly used to:

A. Follow sloping ground while maintaining suitable founding levels

B. Eliminate the need for soil investigation

C. Support only roof trusses

D. Replace all construction joints

Correct Answer: A. Follow sloping ground while maintaining suitable founding levels

Question 7

Which check is particularly important around a column supported by a pad or raft?

A. Punching shear

B. Paint adhesion

C. Window deflection

D. Roof drainage capacity

Correct Answer: A. Punching shear

Question 8

A strap beam in a strap-footing arrangement primarily helps to:

A. Transfer moment and control rotation of an eccentric footing

B. Serve as a vapour barrier

C. Replace all column reinforcement

D. Increase floor finish thickness

Correct Answer: A. Transfer moment and control rotation of an eccentric footing

Question 9

Which joint separates a slab-on-ground from a wall to permit relative movement?

A. Isolation joint

B. Lap joint

C. Rigid splice joint

D. Bearing joint

Correct Answer: A. Isolation joint

Question 10

For a concentric square footing with P = 360 kN and qₐ = 180 kN/m², the preliminary side length is approximately:

A. 0.50 m

B. 1.00 m

C. 1.41 m

D. 2.00 m

Correct Answer: B. 1.00 m