Choosing a Coastal Residential Foundation Type?

[jgeng]

What are the general design considerations when deciding between a shallow foundation vs. piles for a coastal residential foundation design. A colleague is saying you have to go with piles because beach sand does not compact well? do all agree with this? can someone ellaborate?

In general I though sand typically has high bearing capacity and is ideal for shallow foundations and that you would only go to pile foundations when the allowable bearing capacity could not be met with a reasonable shallow foundation footprint? I know that piles might be better if flooding/erosion is of concern.

 

[msquared48]

Whenever a high degree of erosion is possible, which beach sand is especially susceptible to, deep pile foundations should be very seriously considered.

Depending on the location on the beach, you also might want to consider elevating the structure 10 to 12 feet or so to avoid the smaller storms and tsunamis. You'll have to kiss the structure goodbye with the larger ones.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SlideRuleEra]

A major factor will be storm surge and associated high wind loading from hurricanes. Piles will withstand the impact, inundation, potential scow from flowing water, etc., much better that a shallow foundation. Since a building typically must be elevated to be above this flooding piles lend themselves to this type design. IMHO, the properties of sand (good or bad) are not a factor in this decision.

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[jgeng]

thanks for the responses. I understand that storm surge and erosion make piles a good choice for coastal construction and they maybe required to elevate anyways to get above flood elevations. but forgetting about the coastal part of this... my colleagues statement about not being able to compact the beach sand and therefore "you have to go to piles" doesn't make any sense does it? how about in general away from the coast what are the main factors for going to a pile foundation system (which I assume is typically more expensive) than a shallow foundation?

assuming the house is on the beach but the grade is elevated high enough above storm surge and flood levels is there any reason to not go with shallow foundations?

thank you!

 

[msquared48]

Sand is also very prone to liquefaction, which may or may not be a problem with your site, depending on the water table and seismicity.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jgeng]

location is south east coast low/no seismicity but high wind. none of of the neighboring houses are elevated so i assuming the elevation is sufficiently above storm surge levels and since it it sand near the beach i am assuming the water table runs quickly down to the ocean. If my water table is sufficiently low so liquefaction is not a concern is there still there any other reason not to go with a shallow foundation? And in general, forgetting about this site being coastal, what are the contributing factors when
you have to go to piles (which I assume is typically more expensive) over shallow foundations? thanks again!

 

[SlideRuleEra]

Since you say that the ground elevation is high enough to avoid storm surge, then using a shallow foundation should be reasonable. Beach sand usually has suitable properties for structure foundations if, as stated, seismic and erosion are not concerns; suggest getting a Geotech's opinion on you site's materials, however.

Piling are most useful with "poor" soil. They either transfer the load "down" to a "stronger" soil layer (point bearing piling), or

Distribute the loading (vertically, along a portion of their length) - friction piling. Then the loading distribution is low enough for the "poor" soil to support to support the load.

A second use for piling is to support very heavy loads that cover a small area - such as massive industrial equipment.

A third piling use is to resist uplift. A shallow foundation typically resists uplift with its weight alone. A properly selected pile type can mobilize the strength and weight of the soil along its length to resist uplift.

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[TDAA]

A clean sand can be difficult to compact, at least at the surface. This type of sand can move around as you apply effort, since there is little to no cohesion.

Now, this get into how much you want to compact the sand. If you are looking to base it on a Proctor, you may not even be able to get a good curve to go off of. If you go to a relative density, then you will likely be shooting for a lower percentage. Where these values are set, how the contractor works with the material, whether or not there are moisture constraints, and how good the techs are with testing it (surface tests or down in the material) will all impact the perceived difficulty of working with the material.

So, I agree and disagree.

 

[Ron]

Most beach sands are poorly graded and lack fines for cohesion. Fluffing at the surface is common during compaction as noted by TDAA. It generally has a high bearing capacity if protected from erosion/scour. TDAA also makes good points about the testing.

There are many structures along the coast sitting on shallow foundations. The reasons not to do this have been well covered by others in this thread.