Structural Lightweight Fill with EPS (Geofoam)
Geofoam is not insulation that ended up underground: it is a construction material chosen for what it lacks, namely weight. It is used when the ground cannot carry the embankment the design calls for, and the answer is not to strengthen the ground but to remove load. From this follows a complete inversion of the problems: the fear is no longer that the works will sink, but that they will float.
Overview
TAV. 00The process covers lightweight fills in expanded polystyrene blocks behind retaining structures, under road embankments and over compressible soils, from the uplift check through to settlement monitoring. The design logic is the opposite of a conventional embankment: the governing constraint is not bearing capacity but hydrostatic uplift, and sizing starts from the highest water table reasonably to be expected. Everything else is built around that — the drainage that stops water accumulating, the membrane that protects against the hydrocarbons that would dissolve the blocks, the distribution slab that turns a deformable material into a dependable bearing surface. The recurring defects all come from forgetting that the advantage of the material is also its weakness.
The site score
TAV. HThe detail of the craftWeight above against uplift below
Check uplift before anything else
Sizing a lightweight fill begins with a question that does not exist in conventional embankments: if the water table rises to the highest level reasonably expected, does the polystyrene volume stay down or float? The uplift on the submerged volume is calculated and compared with the sum of the weights above it — distribution slab, road build-up, any backfill. If the balance does not close there are three levers: ballast from above, reduce the submerged volume by raising the formation level, or permanently lower the water table with drainage.
Uplift check on the lightweight volume at the design water table, balancing hydrostatic thrust against the permanent loads above, and definition of the counter-measures adopted.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
PLATE 01
The problem inverts: it does not sink, it floats
In a conventional embankment the check is that the ground can carry the load. In a lightweight fill the check is the opposite: that the weight above is enough to hold the volume down when the water table rises.
The diagram illustrates the logic of the check. Coefficients, design water table levels and acceptance criteria are to be taken from the design and the applicable standards.
PLATE 02
What destroys a polystyrene block
Water does not damage it, but it lifts it. Hydrocarbons dissolve it on contact. Concentrated load marks it permanently. Three different problems with three distinct defences, all to be built on site.
The diagram illustrates the mechanisms. Density classes, allowable pressures and requirements for protective materials are to be taken from the design and the applicable standards.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QWhy not use expanded clay instead of geofoam?
Because they are weight reductions of a different order. Expanded clay lightens the fill by roughly half compared with an earth embankment, polystyrene by nearly two orders of magnitude. If the problem is marginal, lightweight aggregate is preferable, since it fears neither hydrocarbons nor flotation; if the ground genuinely cannot carry the load, only geofoam closes the balance.
How long does buried polystyrene last?
In the absence of ultraviolet light, hydrocarbons and above-threshold loads, expanded polystyrene is chemically very stable: it does not rot, is not attacked by soil micro-organisms and does not absorb water in significant quantities. The conditions that limit its service life are all external to the material, and those are exactly what the process addresses.
Does it really carry heavy traffic?
Not directly: no block ever receives a wheel. Between the traffic and the polystyrene there is always the reinforced concrete distribution slab, which turns a concentrated load into a much lower uniform pressure. It is the slab that decides whether the system works, not the block: if the specification treats it as a finish, the design already has a problem.
Is it a fire risk on site?
Yes, during assembly, and that is when the material is most exposed: blocks stacked in the open, with welding and open flames from other trades nearby. Once complete, the volume is sealed under membrane and slab and the question does not arise. The measures that matter are organisational and apply only to the weeks of placing.