Coastal site with artesian conditions and mixture of cobbles/boulders and soil 1

[Rakra]

Hi Everyone,
My site is a coastal site with the following prevalent geotech conditions (established by drilling and test pitting):
1. Sandy silt and clay, peat alluvial sequences (up to 5m depth)overlying unconsolidated sands and gravel dominate soils and also mixture of cobbles and gravels. Dominate subsurface layers are 'mixture of soil and cobbles/boulders' and Sandy gravel and gravelly Sand layers. These are intersected by discrete pockets of silty mixtures. Unconsolidated sediments persist to at least 30 to 50m below surface (limit of excavation).
2. Artesian conditions were encountered in several boreholes. Though the pressure above hydrostatic has not been measured.
3. Site is partially on-land and partially in the sea
4. Marine subsurface is sand dominate profile, SPT N values > 18

I am thinking to request for additional geophysical testing to be performed to better define the subsurface.
My question is, what are my foundation options in such cases? Some examples in literature I have come across suggest that under artesian conditions, locate foundations above artesian conditions and use spread footing on upper layer. In marine areas, drill and install open steel pipe piles.

 

[MTNClimber]

What are the foundations going to support?

 

[oldestguy]

My experience with geophysical methods, they do work, but the results are quite general, only depicting major changes and even then approximately. I'd not be optimistic about the results adding anything. This especially below the water table.

 

[SlideRuleEra]

My question is, what are my foundation options in such cases?

Driven piling... friction piling, not point bearing.

Could the artesian pressure be the result of tidal action? That is, boreholes located below mean high tide elevation being the ones that have artesian pressure (when observed at low tide).

http://www.SlideRuleEra.net

 

[fattdad]

In the absence of data, I would not design anything for a boundary condition that may not exist.

Spend the money, install a few wells, get data, know your actual boundary conditions and then design.

Or drive piles and carry on. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[Rakra]

Hi everyone, thanks for the prompt responses.
@MTCN, foundations are going to support a wharf and jetty.
@oldestguy, thanks for the tip. Having not used geophysical data prior, I am just going off what i've come across in the literature
@SlideRule, thanks for that feedbac. Actually 4 piezoes were installed with 2 at locations where artesian like conditions were observed. Initial standpipe level observations are that they fluctuate with tidal flows! So looks like you're correct.
@fattdad, thanks for highlighting that need.

From these feedback, going with driven, friction piles, my options are driven steel piers. Any additional comments?

 

[SlideRuleEra]

Rakra - Corrosion issues with steel piles in seawater, especially if the piles will be exposed to air in the intertidal zone, and higher. For steel, allow a generous corrosion allowance. Also, while steel piles will work as friction piling, displacement piling, such as prestressed concrete or appropriately preserved wood make better friction piles. Persevered wood only if it will be continuously underground.
On the other hand, steel piles are likely better for driving through the "cobbles & boulders" than either concrete or wood.

As so often the case, no "easy" answer... always a compromise. IMHO, prestressed concrete piles (designed for salt water use) would be the leading candidate... until evidence proves otherwise.

http://www.SlideRuleEra.net

 

[Mad Mike]

Make sure you have trial borings at each pier to know exactly what the limitations are to piling - at each pier!

It sounds like you have a diverse profile extending from terrigenous gravel and cobbles out into the marine sands- if you don't provide the piling guys enough information up front, you could be in for some heavy claims if they drive into boulder pockets.

May need to pre-drill the holes prior to driving, depending on the density profile and scour depths.

All the best,
Mike

 

[BUGGAR]

Except for the Peat and Wells (sounds like a music group) your situation may be similar to the Ocean Beach Pier in San Diego - dense sand and silt conglomerate with cobbles. They predrilled holes and grouted the piles in. They used tall skinny prestressed piles for our earthquakes. I may have details if interested.

 

[BUGGAR]

Attached is OB Pier records.

Whoops, it wouldn't load. This seems like a poor site to share data. Is there another site besides Eng Tips?