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What Causes Erosion Under Concrete?

We get a lot of calls for repairing erosion under concrete.  With our changing climate, the ground moves a lot in a given year.  Most of the calls are for homes with basements, and generally the suspected erosion is under stoops, carports and sidewalks near the foundations.  Voids (air spaces) under the concrete are the cause for concern.

In the vast majority of times, the cause of the voids under the concrete isn’t the result of erosion.  Soil consolidation is a natural process that occurs when soil is placed loose (no tamping).  After being placed, it can take up to 10 years or more for the soil to slowly consolidate.  As this happens, it causes a void fill form under the overhead concrete.

If the settlement is extensive enough, it can alter drainage patterns, which can cause some erosion.  So, settlement usually starts the process, which can then be worsened by erosion.  These voids can create water flow issues as it can give water a new place to fill and build pressure against your basement walls.

Sometimes, erosion is the only cause for voids under concrete.  This is usually a result of poor grading and landscaping which directs water to flow alongside or pond upstream of structures.  The flowing water washes out soil from under the concrete, resulting in voids.  If the voids are extensive enough, this can result in the concrete settling.

void filling below porch steps

Fixing Erosion Under Concrete

The best way to fix erosion under concrete is to fill voids with material that’s resistant to erosion.  This is usually done with the slab jacking process.  Scritchlow Concrete Lifting and Slab Jacking uses only crushed limestone and Portland cement which is not susceptible to erosion.  Alternatively, sand and dirt based grout can be installed, but is much like your surrounding soil and is susceptible to erosion which is what caused your settlement issues in the first place.

After slab jacking is used to fill voids from erosion under concrete, it is really important to correct drainage problems to direct water away from the affected area.  While the materials used for void filling are designed to not wash away, poor drainage can continue to wash out the underlying soils below the void filling material.

Ways to address drainage problems include extending or redirecting downspout discharges, backfilling alongside slabs, installing drains and other similar repairs.

We have well over 20 years of experience in correctly evaluating your water and drainage concerns and are able to recommend many different systems to tackle your drainage issues.

Pressure and Compaction Grouting 

Pressure and compaction grouting is the process of injecting our mud under the ground and into loose soils to solidify and strengthen the foundation soils. The mixture can be injected as deep as 80 feet or more underground. This process can be used to stabilize soil or in some cases to lift structures that have settled.


Compaction Grouting is a specialized technique for compacting soft or loose soils. 

A stiff grout is pumped into a soil mass to form an expanding bulb.  Maintaining a controllable bulb of grout is the essence of this technique. Critical to the success of the compaction grouting process is the injection of the grout in a globular form.  Compaction grouting work is nearly always performed in stages, with a few feet of the grout hole to be injected at one time.  The injection stages can be performed from the top down, commonly referred to as downstage, or from the bottom up, or upstage.

Any soil capable of being mechanically compacted can be done.  The extent and the intensity of soil densification depend on the injection point configurations, the depth of the injections, the grout mix, and the amount of grout injected.  Compaction grouting is effective in most man-made fill, organic soil, sand, silt, peat and most clays.

All types of foundation systems can be leveled and/or stabilized by compaction pressure grouting.  This method increases the load bearing capacity of problem soil and lifts structures that have settled due to poor soil conditions or erosion.  Compaction Grouting can be used effectively for the controlled lifting of foundations, slabs and other structures.

Trip Hazzard Before Slab Jacking


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