How To Compact Soil And Gravel

By Ken Hunter

When it comes to structural building projects, I don’t have many pet peeves. Most conventional wood-frame structures feature tried and true building practices that will stand up well to most environmental stresses short of severe quakes and tsunamis. Good materials well braced and supported are generally sufficient, even when used by less experienced builders. If there is an area of neglect, it is that many builders, professional and amateur alike, often forget that a substrate – the ground – is also a building material.

So this is my pet peeve in a nutshell: why go to the trouble of installing rigid, durable materials like concrete and wood when these rest on a material that if neglected or untreated are incapable of supporting their own weight, let alone a building above them. It’s equivalent to piling bricks on a sponge.

Compacting the Soil

The solution to this problem is easily remedied in a word: compaction. That first of structural materials – the substrate – common to every building project from skyscraper to woodshed can be turned to a sound building material by compressing (also called “tamping”) it until it is so tightly packed that it will properly support almost any building load. In typical highrise construction, piles are used. You may have heard these being pounded into the earth, often from a great distance away. These piles are aiming for bedrock, the outer surface of the earth’s mantle. Due to settling and other compressive forces, bedrock is an ideal substrate for building.

Most buildings, however, do not rest on bedrock. Although this would be ideal, it is far too expensive to drive piles deep into the earth, and in the case of most buildings, unnecessary. This is because properly compacted substrates will easily bear the building loads above them.

Some substrates require no compaction at all. Some clays, for example, have been so tightly compressed over time that if dry, are said to have greater compression strength that concrete. The same can be said about some stone. We’ve often built directly on both of these materials, pinning building footings to the substrate by embedding steel pins that tie substrate to concrete footing. In some cases a clay substrate can be so dense that a sledgehammer will do no more than lightly dent its surface when struck. A similar test for compressive strength can be used for stone substrates. A hammer striking most sandstone and some granites will turn these materials to dust, but as for supporting buildings, few stone substrates will be exposed to the pound-per-inch striking force of a steel hammer.

Most buildings sites don’t offer such substrates as these. Instead, what a little digging reveals is a composite of sand and gravel ranging from fines to boulders, these just beneath a layer of organic material usually called topsoil. As topsoil usually contains a high percentage of compostable materials, it does not usually make a good building base and should be excavated until a so called “undisturbed” base is reached.

This brings me directly to my pet peeve. In 20 plus years of building, I have watched as numerous builders simply scrape of the topsoil, then place their footing forms directly on an undisturbed base. I long ago gave up saying anything about it; the glassy stares alone demonstrate the futility of mentioning that they are simply, and returning to the analogy, placing a brick on a sponge. Let’s stop here for a moment to consider what will happen to a building so placed. Will it fall over? Not likely. As mentioned, conventional building practice is far too sound to allow this. Instead, what will happen is that the concrete footings will settle, usually irregularly, meaning the overall building may drop inches in one area and a fraction of that elsewhere. Builders (and even some inspectors) often over optimistically assume that at least the building will settle consistently, but the result will be those cracks or fissures visible in all improperly foundated concrete work from house foundations to sidewalks and driveways. Further, the building may have a disjointed, uneven look under extreme circumstances. This is not a doomsday scenario, of course, but again, why go to the trouble of using durable materials when these are so quickly and unnecessarily compromised?

How to Tamp

So now that we appreciate the importance of tamping, how is it done? The easiest method is a power compactor. These come in various sizes and models, and usually rely on vibrating substrate materials until they can settle no further. Weight is a factor here also, and compacters tend to be heavy. The beauty of this method is that it exposes the substrate to far more pounds-per-inch force that the building ever will. In the event a trench foundation is being used, a compactor often called a “jumping jack” can be dropped into a trench or pad hole and let to work its magic. Compactors will usually cause your hands to itch, and may even cause blistering, so use padded gloves when using them. When tamping a small area, I’ll attach handles to a six by six beam of wood and pound the earth with this low-tech device. It, too, subjects a substrate to powerful pounds-per-inches forces, but also subjects the body to a good deal of perhaps unwanted exercise.

How much compaction is required? I will usually use a length of 2″ x 4″ (5 cm x 10 cm) (or even a hammer depending on the substrate composite), pounding this against the substrate to test it. Like striking dense clay or stone with a hammer, there should not be much ‘give” in a properly tamped base. There are other ways to test for degree of compaction, but this simple method works well for most purposes.

Another important note on compaction is something a great many experienced builders do not know. Regardless of the type of compaction devise used, no compacters adequately compress more than a few inches of substrate at a time. This means that if you are adding fill to create a building base, it must be done successively. For example, if you’ve removed loose soil to the “undisturbed” level, then need to bring that level up (because you’re backfilling an excavation, perhaps), add a few inches of backfill, compact this, and repeat the process until the desired elevation of compacted substrate is reached. I once watched an excavator backfill a newly-formed swimming pool, adding approximately twelve feet (4 metres) of backfill to create a grade surrounding the finished pool, and all in one go. The crew then tamped this unstable mixture at grade, and placed a standard thickness concrete slab on it. Bearing in mind your know-how on prepping substrates for subsequent materials, you know the answer to the question of how well that newly-placed concrete slab held up.

Building on Bog

Sometimes there is no point in compacting a substrate at all. This scenario is encountered worldwide, usually in lowland areas where buildings are located on boggy soils. In such circumstances piles could be driven to bedrock, but because of the cost and other considerations, this is not usually done with smaller buildings. Theoretically, such efforts to compact here will continue until the tampers and their equipment reach bedrock far below days or weeks later. Typically, building in these cases anticipates a high degree of settling, and gives a building a correspondingly shorter lifespan. Sometimes under this less than desirable situation, a wood-frame foundation is constructed to ensure living areas are elevated above the grade and the building essentially floats on the bog. Occasionally, builders will “float” a concrete slab or some other similar building support, then build on this. Without exception though, gravity eventually has its way.

Recently on such a substrate, we constructed supports by driving many steel rods through the bog to bedrock six feet (2 metres) below. These steel rods were hand driven, and wrapped in concrete at and above grade, essentially becoming a piling. No settling has occurred, and the solution has worked well, although the complexity of this approach will certainly increase the further down a load-bearing base is found to soon become impractical. Any deeper, and we would have needed to resort to conventional pile driving as “floating” a base was not an option.

Except for those circumstances when a solid base is impractical such as on a bog, building is always better on the solid base that sound compaction practice creates. Buildings look better, work better, and last longer when builders take the time to tamp.

About the Author: Ken Hunter is chief structural designer, service director and owner of Hunterstruct Construction, a Vancouver based construction company offering design, building, structural repair and

home renovations

throughout Southwest British Columbia, including the BC interior and Sunshine Coast. This

deck builder

showcases its best sundecks on the Hunterstruct.com website. We love building, and it shows!

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