Why does filled ground settle?
Fill settlement is when the surface of placed fill drops after filling or after construction is complete. Almost all causes come from two soil layers that must be considered separately, because the fixes are entirely different.
1. The fill layer itself is not compacted densely enough — If fill is placed without layering, or compaction does not reach the specified density, the soil particles remain loosely packed with many voids. Under their own weight or as water seeps through, the particles rearrange into a denser packing, so the surface drops. The thicker and faster the fill, the greater the settlement.
2. The original soft clay beneath the fill consolidates — This is the most often overlooked cause, especially on Bangkok soft clay. The added weight of fill presses on the soft clay below, slowly squeezing water out in a process called consolidation, and settlement follows. Water drains out of clay with difficulty, so settlement can continue for months to years, even when the fill layer above is already well compacted.
| Issue | Settlement from the fill layer | Settlement from the original soft clay |
|---|---|---|
| Cause | Poor compaction, thick fill placed in one lift | Fill weight makes the soft soil consolidate |
| Timing | Occurs quickly, ends quickly | Slow, continuing for months to years |
| How to check | Field density testing | Soil investigation to find the soft layer thickness |
| Remedy | Remove and re-compact in layers | Preloading, ground improvement or piles |
When these two layers settle by different amounts or at different times, problems follow, such as cracked slabs, slabs separating from the building, broken underground pipes, or road joints sinking into depressions.
How long after filling does settlement stop?
There is no fixed figure. Fill stabilizes once the fill layer has finished adjusting and the original soil has almost fully consolidated, which depends on three main factors.
- Type and thickness of the original soil — On sufficiently firm original soil, settlement is small and ends quickly. If there is thick soft clay below, settlement lasts much longer.
- Fill thickness and method — Thicker fill adds more load on the original soil, while compacting in layers greatly reduces settlement of the fill layer itself.
- Soil drainage — Sandy soils drain quickly, so settlement ends fast; clays drain slowly, so settlement takes a long time.
Projects that must be sure settlement is complete before construction usually install settlement monitoring instruments such as settlement plates and piezometers, to see from real data whether settlement has reached the design target, instead of guessing from the calendar.
Fill has already settled: how do you fix it?
Fixing fill settlement means dealing with the root cause, not just topping up the surface. Start by diagnosing whether the settlement comes from the fill layer or the original soft soil, using soil investigation and density testing, then choose the approach.
- Problem in the fill layer — Remove the loose fill, then re-compact it in layers with selected material, controlling the density of every layer with a Field Density Test
- Problem in the original soft soil below — Surface compaction does not help. Consider improving the lower soil, for example by preloading to accelerate consolidation, or other methods as designed by the engineer. See all options in the article 8 ground improvement methods
- Structures that must carry load reliably — In some cases the more cost-effective option is a pile foundation that transfers load to a firm layer, instead of relying on the fill.
Simply filling in sunken spots treats the symptom. If the real cause remains, it will settle again.
What should be done from the start to prevent fill settlement?
Preventing fill settlement means doing the filling correctly before work begins, which costs far less than fixing it later. There are 5 key principles.
- Soil boring before filling — Find out how thick any soft soil layer under the site is. This is part of our soil investigation and testing service.
- Use selected fill and compact in layers — Place fill one layer at a time at the specified thickness and compact every layer to the specified density. Do not dump a thick pile and compact it once.
- Test the density of every layer — Use a Field Density Test to confirm each layer meets the criterion before placing the next, and a Plate Bearing Test to check the bearing capacity of the completed fill.
- Deal with any soft soil below — If a thick soft clay layer is found, preloading or ground improvement may be needed first, as designed by the engineer.
- Provide drainage — Prevent ponding or erosion that makes the fill lose density.
Which signs mean you should call in an engineer?
Minor settlement in the early period after filling is normal, but if you see the following signs, have a geotechnical engineer diagnose the problem.
- The ground around the building has settled and separated from the building, leaving a clear gap
- Slabs or roads sink into local depressions, or settle faster and faster with no sign of stopping
- Cracks appear in slabs, walls or structural joints along with ground settlement
- Water pipes, drains or underground channels break or leak due to settlement
- Spots that were filled and repaired settle again in the same place
A common case is a house on piles that reach a firm layer: the house stays at its original level, but the fill around it settles. The parking slab, walkways and pipes laid on the ground settle with the soil, until gaps appear under the beams and pipes break at their joints with the house.
These signs must be told apart: is it only fill settlement, or has it extended to settlement of the structure itself? This can be diagnosed through soil investigation and inspection by an engineer.
Summary
Fill settlement comes from two layers that must be considered separately: a fill layer that was not compacted densely enough, and original soft soil below that consolidates. An effective fix requires finding the cause first and then dealing with it at the source, not topping up the surface. The best prevention is soil boring before filling, using selected fill, compacting in layers and testing the density of every layer. When abnormal settlement appears, have a geotechnical engineer diagnose it from actual soil data.
This article provides general knowledge only and is not design advice for any specific project. Diagnosing the cause of settlement and designing the actual remedy must be done by a licensed engineer, based on soil investigation data for that site.