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Abutment Construction

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Parent
🧱
Foundations & Abutments
Where the bridge meets the ground
Current
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Abutment Construction
Dry stone, mortared stone, log crib

A dry-stone abutment 2 m high on a base half its height is marginal against sliding: with a 1:6 batter its sliding factor is about 1.2 with dry backfill and 0.6 once the backfill is saturated, so drainage matters more than extra stone. Set the foundation below the deepest scour, widen the base toward 0.7 of the height where you can, and keep water out of the backfill with weep holes. A log crib abutment works the same way as a gravity wall, filled with stone and pegged layer to layer.

Part of Bridge Types & Construction

Building the end supports of a bridge as gravity walls of dry stone, mortared stone or timber crib, and checking them against sliding, overturning and bearing.

An Abutment Is A Gravity Wall Holding Back The Approach Fill

The parent article says an abutment transfers the bridge load, and any arch thrust, into the ground and must not shift. It also holds back the earth of the approach, so it is a retaining wall. Earth pushes it forward and, if the fill is wet, pushes much harder. The wall resists by its weight and by friction on its base.

The parent describes three forms:

  • Dry stone: a massive wall of interlocking stones, battered (sloped back) at 1:6 to 1:4, with a base at least half the height, foundation stones set below the scour depth, and the interior filled with compacted rubble.
  • Log crib: logs stacked in alternating layers like a cabin, each layer pinned to the one below with hardwood pegs, filled with stones and gravel. Useful when stone is scarce and timber is plentiful.
  • Mortared stone — the same as dry stone with mortar in the joints; the parent mentions it in the node title only.

What You Need

  • Stone — large, flat-bedded, for the base and faces.
  • Compacted rubble for the core.
  • Weep holes — pipes or gaps through the wall, so groundwater can leave.
  • A level and a string line — to hold the batter.
  • A probe rod — to check the ground. See Soil Bearing Capacity.

Method: Build And Check A Gravity Abutment

Step 1: Dig below scour Dig the foundation trench below the lowest level water will ever scour. Stand on firm ground, not on loose bed.

Step 2: Set the base Lay the largest stones first, bedded flat. The base width should be at least half the wall height.

Step 3: Batter the face Step each course back so the front face slopes at 1:6 to 1:4.

Step 4: Fill the core and place weep holes Compact rubble in layers behind the facing. Leave drains through the wall near the base, as in Abutment Building.

Step 5: Backfill with free-draining gravel Clay backfill holds water and pushes harder. Use gravel against the wall.

Worked Example: A 2 m High Dry-Stone Wall

Assumptions, which are mine and not the parent’s: backfill unit weight 18 kN/m³ and friction angle 30 degrees (typical for compacted granular fill, from standard soil-mechanics texts such as B. M. Das, Principles of Foundation Engineering, recalled and not checked here); wall of dry stone at an effective 1,800 kg/m³ after allowing for voids; friction coefficient between base and ground 0.5; level backfill, nothing on top; results per metre length of wall. The parent’s base is half the height, so 1.0 m; the front face battered 1:6, so it leans back 2 / 6 = 0.33 m over the height; the back face is vertical.

Earth push. The Rankine active coefficient K = tan²(45° − 30°/2) = tan²(30°) = 1/3. Force P = ½ K γ H² = 0.5 × 0.333 × 18 × 2² = 12.0 kN/m, acting 2 / 3 = 0.67 m above the base. Overturning moment about the toe = 12.0 × 0.67 = 8.0 kN·m/m.

Wall. Base 1.0 m, top 1.0 − 0.33 = 0.67 m, area (1.0 + 0.67) / 2 × 2 = 1.67 m². Weight W = 1.67 × 1,800 × 9.81 / 1000 = 29.4 kN/m. Its centre of gravity is 0.58 m from the toe, so the resisting moment is 29.4 × 0.58 = 17.0 kN·m/m.

  • Overturning factor: 17.0 / 8.0 = 2.1.
  • Sliding factor: 0.5 × 29.4 / 12.0 = 1.23. With a friction coefficient of 0.4, 0.98, which is sliding.
  • Where the resultant lands: (17.0 − 8.0) / 29.4 = 0.31 m from the toe. The middle third of the base runs from 0.33 m to 0.67 m, so the resultant is outside it: the heel carries no load and the toe carries all of it, at about 2 × 29.4 / (3 × 0.31) = 64 kPa. Soft clay allows about 24-48 kPa (Foundation Design gives 500-1,000 lb per sq ft, 0.0479 kPa each), sand 96-192 kPa.

Saturated backfill. If the backfill fills with water, the soil weight under water falls to 18 − 9.81 = 8.2 kN/m³ but water pushes with full weight. P = 0.5 × 0.333 × 8.2 × 4 + 0.5 × 9.81 × 4 = 5.5 + 19.6 = 25.1 kN/m, 2.1 times the dry value.

  • Sliding factor: 0.5 × 29.4 / 25.1 = 0.59. The wall slides.
  • Overturning: 17.0 / (25.1 × 0.67) = 1.0.

Remedies, with their effect.

  • A rectangular wall 1.4 m wide (0.7 of the height) with the same stone: W = 49.4 kN/m, sliding factor 2.06 dry, 0.99 saturated.
  • Mortared stone at 2,200 kg/m³ on the battered shape: W = 36.0 kN/m, sliding factor 1.50 dry, 0.72 saturated.
  • Weep holes and gravel backfill, which stop the saturated case occurring.

Log crib. A crib filled with stone is a gravity wall of lower density. At an assumed 1,500 kg/m³ and a base of 1.0 m: W = 29.4 kN/m, sliding 1.23 dry. At a base of 1.4 m: W = 41.2 kN/m, sliding 1.72. A crib wall rots; Abutment Building gives 20-40 years for oak and similar durable species.

Result: the sliding factor is about 1.2 with dry backfill and 0.6 with saturated backfill. The parent’s half-height base is a minimum that works only with dry, drained backfill. Water behind the wall is the failure that the base width cannot cover.

Common Mistakes

  • Treating the batter as a stability gain — it improves the look and sheds water, but cutting the front face lowers the weight. The calculation gives lower sliding than a straight wall of the same base.
  • Backfilling with clay — it holds water. Use gravel.
  • No weep holes — see the saturated case.
  • Resting on loose bed — scour removes the support. See Scour Protection.
  • Taking the batter from one source — the parent gives 1:6 to 1:4 and Abutment Building gives about 1:10. The two disagree. Use the calculation for the wall you are building.

Safety

Trench and wall collapse

A trench deeper than 1 m with unsupported sides can bury a worker. Shore it or slope the sides. Stones weighing tens of kilograms must be placed with levers and rollers. Do not stand below a wall under construction, and do not load the span until the abutment has settled.

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