Posted about 1 year ago

Why Soil Matters When Building – Remembering the Surfside Condo

Soil. The earth is full of it and everything we have rests upon it. We build on it, we grow grass on it, we play sports on it, and we build offices and homes on it. Yet it’s something we rarely think of, unless of course you happen to be a geotechnical engineer.

A geotechnical engineer is someone who studies soil and how it behaves under the pressure of a load sitting upon it. So for example, they could tell you whether or not a particular type of soil could support a high rise, a stadium, or even a modest house.

Common Issues

Some common issues include dealing with expansive soil, which as the name suggests expands and contracts as it mixes with water or dries out. This naturally makes for an unstable foundation. Similarly collapsible soil makes for a poor foundation as well. An example of collapsible soil would be if the wind over time swept up a pile of dirt. That dirt would not be well compacted, but rather it would be rather loose. You can imagine what it would look like if you suddenly placed something heavy on such a pile.

Landslides, sinkholes, and faults in the earth are other common concerns and issues that a geotechnical engineer must take into consideration. Any of these could easily could serious damage to property and loss of life.

Water can also be a significant hazard. Having a high groundwater table means that there is less compacted soil and less structure for a building to rest on. If that water is forced down by the pressure of the building it can actually be forced out and upwards elsewhere. As a metaphor, imagine putting a brick in the center of a water balloon. If the balloon pops the water has to go somewhere. In the case of groundwater, it could shoot upwards around the side of your building or affect neighboring properties.

Another water related issue is known as liquefaction. This occurs when water logged sediment loses its strength and the individual particles separate following shaking, such as after an earthquake. The soil, which is a solid, starts to act more like a liquid.

Mitigation

One of the simplest and most common ways to make soil more supportive of structures placed upon it is compaction. Loose soil will not hold, but if the soil is compacted it acts more like rock and less like sand. Sometimes soil is taken off site to be compacted and then replaced. Other times a machine literally pounds the ground like a person kneading dough in the kitchen.

Another technique is to use rammed aggregate piers. This essentially means sticking a pole into the ground and forcing rock into it. When this process is repeated throughout a site, it forces the soil around these underground pillars of rock to be more condensed and more tightly packed.

What Can Happen – Surfside, FL

Over the summer in 2021, the Champlain Towers in Surfside, Florida collapsed killing nearly 100 people. Many of the people who lost their lives were sleeping at the time. While the official investigation has not yet been completed as of the time of this writing, the investigators will likely consider the underlying soil condition.

I’m not suggesting anything other than to say that given the fact that Surfside is located on a barrier island, that is, it is surrounded by water on all sides, the potential impact of that water should be examined.

A scientist working for the South Florida Water Management District reportedly equated the underlying geology of the region to Swiss cheese. The article, written three years earlier, further explained that the area consists of a sizable amount of limestone and compressed reefs full of tiny holes. These holes allow water to literally rise up through the ground.

It’s not a far reach to consider that sort of rising water would have the potential to affect the stability of the ground under which a building stands.



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