At one time or another, we’ve all most likely experienced cracks in interior walls, uneven floors, dips in roads, trip hazards in sidewalks, or ponding water issues, and have probably wondered “Why does this happen?” Frankly, there can be a lot of explanations, but the common denominator for each of these occurrences, as well as an array of others, is improper or poorly compacted soil. Soil compaction is one of the most fundamental aspects of soil engineering (which in practice is called “geotechnical” engineering) and also of basic construction itself. Let’s look at what soil compaction is, the mechanics behind compaction, and the various compaction techniques in order to answer that question.
What exactly is Soil Compaction?
Essentially, it’s the densification of loose soil, which increases its unit weight. Think of loose soil as a mass of irregularly shaped marbles. In-between the marbles are open spaces or voids. These voids are filled with either water, air or, most often, a combination of both. In compacted soil the size of the open space between the soil grains is decreased, thus creating a more dense soil mass.
Why is soil compaction a good thing?
In geotechnical engineering and construction, soil compaction is considered a good, albeit sometimes elusive, thing. This is because it turns loose soil into denser soil and increases its strength characteristics. An intuitive way of looking at this is that it makes the soil less compressible, thus lessening the amount the soil can physically settle or compress over time. In construction, it is desirable to avoid or at least minimize and control how much the soil settles. This is simply because anything built on top of the soil will also settle, typically leading to damage.
The increase in strength characteristics of compacted soil also increases the bearing capacity of the soil, which in turn allows for larger, heavier loads such as buildings, dams, or other such structures on top of the compacted soil. Other advantages of soil compaction include durability of the soil structure, minimized deterioration (i.e. erosion), and achieving a desirable soil permeability (water passage).
Why is soil compaction a bad thing?
Given all the accolades of proper soil compaction, how could it be a bad thing? In soil/geotechnical engineering and construction, it rarely is. However, in agricultural situations, it certainly can be. Compacted soil, because of the decreased size of the void spaces between individual soil grains, can prevent water infiltration into the soil mass. This can lead to increased water runoff and soil erosion. In addition, soil moisture is important for plants, and the compressed/densified soil grains can inhibit root growth.
So if your next case involves a failure such as structural cracking/shifting, uneven flooring, sinkholes, sidewalk and other surface hazards, or water infiltration/damage, our Failure Analysis team of experts can investigate and analyze the facts to determine causation.
KURT AHLICH is a Senior Engineer with ARCCA specializing in cause and origin failure analysis, civil and geotechnical analysis and condition assessments of residences, manufacturing facilities, commercial properties and office/government buildings. He is also experienced in construction methodologies and defects, as well as cost estimation.