The foundation and abutment are where a bridge meets the ground, and they must carry the vertical load, resist any sideways thrust, and stay out of reach of scouring water. The ground fails more often than the span: a 175 kN per metre load on a 2 m wide base presses at 87.5 kPa, which sand can bear and soft clay at 24-48 kPa cannot. Choose the ground first, set the foundation below the lowest scour, and only then build the span.
Part of Bridge Types & Construction
The structures that pass a bridge’s load and thrust into the ground, and the checks that decide whether the ground can take them.
The Span Is Only As Good As What It Stands On
A bridge span needs support at both ends. The parent article says the abutments transfer the bridge load, and the arch thrust if there is an arch, into the ground, and that they must not shift. Three things can go wrong with the ground. It can settle, because the soil is too soft for the pressure. It can slide or tilt under sideways thrust. And it can be washed away, because moving water digs out the bed around the foundation, which the parent calls the primary cause of bridge failure.
The detailed designs are in Abutment Construction for dry stone, mortared stone and log cribs, and in Scour Protection for keeping water off the foundation. This page explains how to choose the ground and size the base.
What You Need
- A probe rod — a steel rod or hardwood stake, to feel for soft layers.
- A pit — dug to foundation depth, to see what is there.
- The load — from Dead Load vs Live Load.
- The soil type and its safe bearing pressure. See Soil Bearing Capacity.
Method: Choose And Size The Foundation
Step 1: Probe the bank Push the rod in at several points. Easy penetration by hand means soft soil, loose sand or saturated silt. Avoid banks that show curved cracks, tilted trees or fresh slumps.
Step 2: Dig to firm ground Dig through topsoil and soft material to rock, dense gravel or firm clay. Foundation stones must sit below the lowest scour level, the lowest water will ever dig, as the parent says.
Step 3: Find the bearing pressure Divide the vertical load per metre of wall by the base width.
Step 4: Compare with the soil Foundation Design gives soft clay 500-1,000 lb per sq ft and sand 2,000-4,000 lb per sq ft. At 0.0479 kPa per lb/ft², that is 24-48 kPa for soft clay and 96-192 kPa for sand.
Step 5: Widen the base or change the ground If the pressure is too high, widen the base, move to firmer ground, or switch to a lighter structure.
Worked Example: An Arch Abutment
Inputs, from Arch Principles: arch reaction V = 75 kN per metre of width, abutment weight 100 kN per metre, base 2 m wide.
- Total vertical load: 75 + 100 = 175 kN per metre.
- Bearing pressure: 175 / 2 = 87.5 kPa. This ignores the sideways thrust, which makes the pressure uneven under the base and raises the pressure at one edge.
- On sand (96-192 kPa) the ratio of allowable to applied is 96 / 87.5 = 1.1 to 192 / 87.5 = 2.2. Marginal at the low end.
- On soft clay (24-48 kPa), the width needed is 175 / 48 = 3.6 m to 175 / 24 = 7.3 m. A bridge on soft clay needs a much wider base, a lighter structure, or piles.
A weak test. Abutment Building says that if 500 kg on 0.25 m² causes no visible settlement in 24 hours, the soil is adequate for light bridges. That load is 500 × 9.81 / 0.25 = 19.6 kPa, below even soft clay’s 24-48 kPa, so almost any soil passes it. Do not use it to approve an arch abutment.
Common Mistakes
- Founding on loose bed — scour removes it. Go below.
- Assuming both banks are alike — one abutment on rock and one on clay settle differently and crack the span.
- Forgetting the thrust — a vertical bearing check alone does not cover sliding.
- Skipping the test pit — a probe finds soft layers that the surface hides.
Safety
Deep trenches and loose banks
A trench below 1 m can collapse on a worker. Shore or slope the sides, keep spoil back from the edge, and never work alone in a foundation pit. Do not dig on a slumping bank.
What’s Next
- Learn next: Abutment Construction — gravity walls and cribs
- Combine with: Scour Protection — keeping the foundation from being undermined