Structural Engineering & Statics
where the load goes
Concept

Lateral pressure — where it appears

Horizontal pressure from soil, water or stored material on a surface retaining it. Its magnitude depends on what is retaining it as well as on what is retained, which is why the same soil pushes differently on a wall that can move and one that cannot.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

Five loads behind one wall, and the water is the biggest. The horizontal pressure on a 6 m wall retaining soil at 18 kN/m³ with a friction angle of 30°, a surcharge of 10 kPa and the water table 2 m down, drawn once as the profile the wall feels and then once per term. The terms are surcharge 20.0 kN/m at 3.00 m, soil above water 12.0 kN/m at 4.67 m, soil at the water table 48.0 kN/m at 2.00 m, submerged soil 27.2 kN/m at 1.33 m, water 78.5 kN/m at 1.33 m, and they sum to 185.7 kN/m — matched to 7e-8 by integrating the drawn profile numerically. The largest single term is the water, at 78.5 kN/m: water has no shear strength, so its coefficient is exactly one where the soil's is 0.333, and it acts on top of the soil's effective stress rather than instead of it. The combined resultant sits at 1.90 m above the base, 0.317 of the height rather than the third point at 2.00 m that a pure triangle would give.

The load that depends on what carries it

Every other load in this collection is a number the structure is given. Retained soil is not — it pushes with a fraction of its own weight, and the fraction is decided by how far the wall moves. Six millimetres of retreat on a six-metre wall takes a third off the load, and being held still puts it back.

equilibrium · Lateral pressure The same concrete, held sideways. Two stress-strain curves for one concrete. The lower is a cylinder test: it peaks at 30 N/mm² near a strain of 0.002 and has nothing left by 0.0035, because it fails by splitting apart sideways. The upper is the same material inside a 12 mm hoop at 100 mm centres, which cannot stop it expanding but can make the expansion stretch steel: the lateral pressure of 2.48 N/mm² — 8% of the strength it is multiplying — takes the peak to 44.4 and the ultimate strain to 0.028. The strength gain is 1.48 times and the strain gain 8.0; the area under the curve, which is the toughness, goes up by 11. It is the third number the confinement is provided for.

Squeezed sideways into a different material

Concrete in a cylinder test fails by splitting apart sideways under a load pushing it down. Put a hoop round it and the splitting has to stretch steel — and a lateral pressure of a twelfth of the strength raises the strength by half and the ultimate strain by eight.

materials · Confinement Where a bending stem sheds its compaction pressure. The share of the rigid build-up's pressure shed at each depth by a cantilever stem 3 m high, bending stiffness 67,500 kN·m² a metre (a concrete stem about 300 mm thick), backfilled in 250 mm lifts of soil at 20 kN/m³ and 30°, each compacted by a roller of 60 kN a metre: nothing at the base, which does not move; little in the top lift, which nothing is placed after; most, 20 per cent, at 0.9 m down, where the pressure was locked early and the stem went on moving away from it.

The stem that bends away from the roller

A roller compacting backfill against a wall leaves a lateral pressure behind it that the soil's weight alone would never make, and for a wall that cannot move that residual is Ingold's envelope. A cantilever stem moves as the fill goes up, and a residual locked in against it relaxes as the stem bends away. But not everywhere: the base never moves, so it keeps its pressure, and the last lifts have nothing placed after them, so they keep theirs. A 300 mm stem sheds an eighth of the rigid wall's base moment and the softest stem a third — never more, because the two ends of the stem are where the relief cannot reach.

equilibrium · Lateral pressure

Named alongside it

The objects these essays reach for when they reach for this one.

Active pressureArchingBearing pressureCompactionConfinementDuctilityEarth pressureEnergy dissipationFrictionHoopLever armLoad path

All concepts