sand foundations

[nicam]

Hi, I've desigened an extension to a house (a balcony structure) and we've ask to confirm the foundation type. We asked the builder to excavate against the existing house footings. the inspection revealed the house is bearing on sand. The house is very old two storeys and showed no signs of settlement or cracking.

now my question is, do I rely on the sand for my bearing material (the load is 80kN) for the pad footings supporting the balcony or do I introduce piles (say timber or formtube instu concrete)There's no guarantee we will find rock or stiffer material. I guess it will be more for settlement.

my experience with sand bearing cap is >300kPa for compacted sand and 100 to 300kPa for medium dense sand.

your thoughts please.

 

[civilperson]

Nothing is better than sand if the bearing limit is followed. Instantaneous settlement occurs during construction preventing future cracking. Hire a good geotech to write a report with recommendations.

 

[BAretired]

A load of 80kN is trivial. A footing of 900 x 900 should suffice under the most conservative of assumptions. The fact that the existing building has behaved well over the years should give you some confidence, but if in doubt, it is always a good idea to obtain a soil report from a qualified geotechnical firm.




Best regards,

BA

 

[Ron]

I live in an area that, if we couldn't build on sand, we couldn't build! Civilperson and BAretired nailed it. A key to foundation performance is historical performance in the area. In this case, the "area" is adjacent. Not a guarantee (there could be muck pockets for instance), but you won't know until you have a few near surface auger borings or standard penetration tests done by a qualified geotechnical firm.

 

[nicam]

Thanks for the comments, We asked for a geotech but the client wasn't willing to pay for one despite our insistance. I don't understand why the client is not short of money living in a very well to do area and his two mercedes, but to spend a bit of money on geotech..too much!

 

[jeffandmike]

One word . . . Florida. Most foundations in that state are bearing on sand. This can be fine, depending on site conditions, but there is also a reason why the Bible warns against this (Matthew 7:24-27).

I've seen the resulting problems firsthand. Water/stormwater management is key. Ensure the code-prescribed site grading (slope away from the foundations) and minimum foundation embedment requirements are followed. Ensure gutters/downspouts are installed and discharge water a sufficient distance from the foundations. Ensure the sand is sufficiently compacted prior to placing the foundations. Ensure that all inspections by the jurisdiction's building department actually occur and that the contractor is licensed/experienced in this type of construction.

civilperson stated "Instantaneous settlement occurs during construction preventing future cracking". This may be true in other places, but not in Florida. Long-term settlement of the typically loose, sandy surficial soils is very common, particularly for residential structures. Over 95% of the problems we run into can be directly attributed to shoddy construction practices, including not following my (actually the building code's) recommendations above.

 

[Ron]

jeffandmike...long term settlement, assuming reasonable compaction of the sands in the upper 5 feet or so, is usually related to something other than sands. One of the problems we have in Florida is not the sand, but the organic and/or clayey materials that often underlie the sands near the surface. These materials can cause long term settlement.

All the more reason to get a good Geotechnical engineer involved.

 

[civilperson]

Using the Bible to recommend foundation design criteria is poor engineering.

 

[apsix]

I believe the Bible is implying that sand is not recommended where there is the potential for scour. I wouldn't argue with that.

 

[jeffandmike]

Ron . . . you mentioned that "long term settlement, assuming reasonable compaction of the sands in the upper 5 feet or so, is usually related to something other than sands."

I agree with you. One common culprit in the state is sinkhole activity. You'll notice my post referred to "the typically loose, sandy surficial soils" . . . "particularly for residential structures". My key assumption was that the sandy surficial (including the top five feet) soils are typically loose (not reasonably compacted). It is my experience that the residential contractors "move" the soil around to level the site, then fail to properly compact it (due to cost, laziness, ignorance and/or lack of proper oversight).

Even if they are reasonably compacted, scour can still present an issue, particularly if the building code requirements described above are not followed.

apsix is correct. The Bible is specifically addressing the failure of a sand foundation due to "rain", "flooding" and "wind". These, including grade slopes, work to form the mechanism for scour or erosion.

The passage reads as follows:

24Therefore everyone who hears these words of Mine and acts on them, may be compared to a wise man who built his house on the rock.
25And the rain fell, and the floods came, and the winds blew and slammed against that house; and yet it did not fall, for it had been founded on the rock.
26Everyone who hears these words of Mine and does not act on them, will be like a foolish man who built his house on the sand.
27The rain fell, and the floods came, and the winds blew and slammed against that house; and it fell--and great was its fall.

Feel free to ignore the Bible's recommendations in this instance if you think they're wrong.

 

[Ron]

While the literal interpretation has physical validity, this popular simile from the bible is an obvious reference to building on faith in God for a proper foundation of life. Even in the middle east, many structures must be built on sands, with the warning that your building, as life, is subject to outside influences if not built on a firm foundation.

jeffandmike...I agree that sinkholes and poor compaction for residential foundations create lots of problems. More residences are affected by poor construction practices than are affected by sinkholes.