This article originally appeared on Arup Connect as “Ask Arup: Silty Sand Solutions.”
Architect Juan Esteban Correa Elejalde’s client tasked him with designing an off-the-grid getaway for a rural site near Medellín, Colombia. After completing the initial design concept, Correa Elejalde ordered a soil study of the client’s land.
Unfortunately, the results showed the site to be “pretty much a pool,” he said; the high water table and thick layers of loose soil would provide little capacity to support heavy objects above.
After seeing Arup Connect’s call for engineering-related questions on ArchDaily, he reached out to see if we could offer any insight.
In Colombia, Correa Elejalde told us, the typical solution for this scenario would be to build a floating slab — a flat layer of concrete resting on top of the soil, supported by vertical concrete piles beneath the outer edges. However, as this would both defeat the client’s environmental objectives and exceed the available budget, he wanted to know if Arup could suggest any alternatives to investigate.
Risk factors
Arash Erfani, a geotechnical consultant in our San Francisco office, advised him to get a better understanding of several critical risk factors before taking the design any further.
First, because the initial geotechnical investigation showed the ground to be made of silty sand, the project could be at risk for seismically-induced settlement due to liquefaction. In other words, the ground could essentially turn into jelly during an earthquake, causing vertical settlement and lateral spreading.
Further, since fat clay was observed in some locations within the project site, he advised him to ascertain whether expansive clay was an issue at the site. If water enters the clayey soil during the rainy season and causes swelling, unwanted movement could occur.
Based solely on the initial soil study results, Erfani said that he could not recommend one solution over another. He cautioned, however, that if liquefaction was a significant issue on the site, the floating slab Correa Elejalde had been considering would not be advisable.
Potential alternatives
Instead, he suggested working with a local geotechnical engineer to investigate two alternatives. The first, known as deep foundations, involves driving long piles into the ground to reach the more stable layers of earth below.
The latter, known as ground improvement, mitigates liquefaction consequences and can take a variety of forms. One, the vibro-replacement stone column technique, uses a heavy vibrating weight to drill deep into the ground and create columns of compacted rock that support the earth around them. Another, deep soil mixing, uses heavy equipment to mechanically combine weak soils with cementitious slurry. Various types of grouting — compaction, permeation, deep mixing, chemical, or jet grouting — might also be options.
Before choosing any of these solutions, however, Erfani said it would be critical to attain a stronger knowledge of subsurface conditions and soil characteristics at the site.
Cost, sustainability, feasibility
Both deep foundations and ground improvement have potential downsides, however, he cautioned. The latter might not be feasible if the site is at high risk of liquefaction. The former would likely be very costly, which could be impractical given the project’s limited budget.
Framing these approaches in terms of the client’s other primary concern — sustainability — would require more information about the specifics of the site, Erfani said.
Structural considerations
Armin Masroor, a structural engineer also based in San Francisco, added that Correa Elejalde’s initial design concepts needed further coordination with a local structural expert, as the vertical posts in the existing scheme wouldn’t be sufficient to transfer lateral force from the upper floor to the ground.
Next steps
Since speaking with Arup, Correa Elejalde has been working with a local engineer to move the design forward.