Recommended Excavation Methods for an urban project

[GeoNik]

Hello every one,

I am dealing with an excavation of about 11' (~ 3.3 m) depth, and dimensions of 190' x 130', this will be an underground parking for a 4 story building. The soil is mostly sand with little silt and clay, not very cohesive. The project site is adjacent to two major streets from north and south, and the ground water level is almost 5' (1.5 m) below ground surface. The excavation will start in May and rain fall is expected.

Considering the above conditions, what method of excavation and equipment would you recommend specially for the two sides adjacent to the streets? What would you recommend to control the ground water in these sides? Sheet piles, slurry trenchs, point wells?

Thanks for your valuable suggestions.

 

[civilperson]

Sheet piles (water tight) or slurry walls are preferred. Dewatering may cause distress to adjacent building with subsequent lawsuits and damages. I would modify the design to keep the lowest slab elevation above the existing water table elevation, (foundations can be under water).

 

[BigH]

Get a specialist who designs shoring systems for excavations in city on board your team. They are worth their weight in platinum.

 

[PEinc]

I agree with BigH. Get a shoring contractor who is local or who has experience in that city. Also, you need to consider the dewatering method when choosing the shoring system. If the site can be dewatered to below subgrade inside the site and behind the site, then you could use the cheapest shoring method - driven, cantilevered, steel soldier beams and wood lagging. If you can't dewater behind the wall, then you probably need steel sheet piling (almost water tight). A slurry wall will be extremely expensive, probably impractical, and you may not get anyone to bid such a small job.

 

[rchanke]

In special conditions such as yours Jet Grouting has been used for water controlled excavations. This is not the cheapest means of excavation support but works well in conditions that other methodologies do not.

 

[Patgeotech]

Are the foundations going to be of a conventional type or piled?

If conventional then sheet piles would be my preferred option as mentioned in previous threads.

Should piles be undertaken, then a soldier pile system would be my option (perhaps could be incorporated into the design?). In terms of water control, I would be wary of well points that could draw the water table down and effect surrounding foundations (check what the other foundations comprise of). That could lead to unnecessary claims and problems. The use of a subsurface drainage system (we call it a geodrain in South Africa, essentially a french drain system) installed in the sand along the perimeter and centre of the excavation would collect/control the groundwater and surface runoff. The geodrain could then be connected to a sump collection point and discharged. A cheaper solution to other dewatering methods.

How thick is the sand beneath the founding level? and what does the underlying geology comprise of?

Hope this is of help.