Settled concrete slab foundation or floor? CJGeo’s experienced polyurethane grouting crews are slab foundation repair experts and can raise your settled floor for a fraction of the price and almost none of the disruption associated with slab piers or replacement. Unlike overlays such as self-leveling floor treatments, polyurethane grouting addresses the cause of floor settlement to help ensure the longest lifetime repair possible. CJGeo’s crews have repaired thousands of slab foundations, from single family homes to grocery stores and warehouses.
Cause of Slab Foundation Problems
The most common cause of slab foundation settlement problems is improper backfill placement. The general sequence of floating slab floor construction is to install the exterior walls first, then install any needed plumbing, electrical, HVAC or other utility lines under the slab. After the utilities are run, sand, stone or dirt are spread out to level the surface. If the fill material isn’t compacted or has potential to consolidate over time, it settles over time. This settlement causes voids (air spaces) to develop under the slab foundation. Very few slabs are designed to act long term as bridges over these voids, so the slab will eventually crack and start to settle.
After a structure is built, the rule of thumb is that it takes about 10 years for the soil placed during construction to settle, which removes support from the floor. Floors generally settle slower than the soil because of the friction against adjacent walls, floors and area of adequate support. Sometimes plumbing, particularly sewer lines, can develop bellies and misaligned joints which can cause further problems.
Drainage problems can also be the cause of slab foundation problems. A downspout which discharges next to a footing can cause the rainwater to soften the underlying soil, reducing its ability to bear weight. Over time, this allows the foundation to settle (think of the difference of stepping in deep mud instead of onto dry soil). Drainage problems can also cause soil to actually wash away or erode from under your slab foundation. This is quite rare.
Signs of Slab Foundation Problems
Differential settlement is when one slab sinks relative to another slab, a wall, or some other part of a structure. Typical signs of differential settlement are trip hazards between slabs, gaps below interior walls, or lips at exterior doors.
In large commercial buildings, common locations of differential settlement include column bases, where the floor is lower than the small diamond shaped separate slab around a column, or between slabs at construction or control joints.
Global settlement affects large areas. In commercial and industrial settings, global settlement may cause sloping of floors throughout a facility. For example, if you feel like you’re walking downhill and then back uphill when walking front to back in a facility, that’s global settlement.
Many structures are built with pile-supported foundations, but non-pile supported floors. It’s fairly common for the floors to settle globally–the floor within an entire building may be relatively flat, but just a few inches below where it was installed.
Slab Foundation Repair Methods
The most common method for addressing slab settlement is grouting immediately below the slab. Shallow grouting is the least disruptive option for reestablishing support of the slab. While the fastest and least expensive, shallow grouting does not address the stability of underlying soils. If the underlying soils continue to consolidate, resettlement can occur.
Shallow grouting can be used to correct settlement of slabs, but is unlikely to correct settlement of spread footings.
Deep grouting can be done in conjunction with shallow grouting to increase the stability of underlying soils and reduce chances of resettlement. Deep grouting can also be used lift shallow foundations, such as column bases and spread footings.
The need for deep grouting is generally determined by performing testing of the underlying soils. DCP testing is the fastest and least disruptive testing method; boring and sampling may also be done.
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