Concept

Consolidation — where it appears

The slow settlement of a saturated clay as water is squeezed out of it under a sustained load. It runs on a timescale set by how far the water has to travel, from weeks to decades, so a foundation can keep moving long after the structure on it has been built.

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

Weakest the day it is finished. The factor of safety against tipping of a 15,000 kN tower on a 10 m square base over soft clay (undrained strength 35 kPa, gaining 0.22 of the effective stress the tower puts into it, consolidating with a 90 per cent time of 7.1 years), through twelve years, for construction times of three months, one year and four years; its wind push grows as it rises. Built in three months, it bottoms out at 1.14 on the day it is finished; built in one year, it bottoms out at 1.33 on the day it is finished; built in four years, it bottoms out at 1.60 on the day it is finished. After that every curve climbs toward 1.93, the factor once the clay has consolidated. Had the finished weight arrived in an instant the factor would be 0.92; on ground that could not fail it would be 3.00.

The ground that arrives after the weight

A block's resistance to tipping on ground that can yield is a parabola in its weight, with its peak at half of what the ground can bear. On soft clay the ground's capacity is not fixed: it grows as the clay drains under the weight, and it grows years after the weight has arrived. A tower on such ground is at its weakest on the day it is finished — 1.33 against the 1.93 it will have once the clay has drained — and a silo filled in a week walks over the top of the parabola and down the far side, which is the other way a structure can be built.

equilibrium · Overturning

The gaps that open after the arch is closed. For the 60 m fixed concrete rib rising 6 m under 60 kN/m, closed at 28 days with its abutments jacked apart to take up the day's gap: the gap between the rib and its springings that each movement opens after closure, in millimetres, against the years since, on a log scale — the rib's creep under its thrust, its shrinkage, and the ground consolidating under the thrust by 1.0 times its 8 mm immediate spread with 90% done at 10 years. After a hundred years they are 24.8, 16.2 and 8.0 mm, 48.9 in all, against the 13.6 mm the thrust shortened the rib on the day. The rib resists them while it creeps itself, so the gap it acts as if fixed against is 20.9 mm (fine dashes).

The jacks the arch forgets

Jack a fixed concrete arch's abutments apart before the crown is closed, by exactly the gap its thrust and its footings open on the day, and the springings start with no moment at all. A century later they carry 173 kN·m — half as much again as the 112 the jacks were set to remove. The thrust's own moment never relaxes; the jacks' correction does, as the rib creeps; and shrinkage and the ground's consolidation arrive after the jacks have gone. No single setting cancels movements that run on different clocks.

deflection · Rib shortening

Named alongside it

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

Abutment Arch Axial shortening Ballast Bearing capacity Construction sequence Creep Factor of safety Imposed deformation Overturning Relaxation Shrinkage

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