Heavy Section vs. Bracing
Heavy Section vs. Bracing
(OP)
Is there an advantage to using a standard sheetpile (e.g. AZ-18) with a moderate section modulus and have to use interior bracing, rather than specifying a pile with a section modulus of 75-80 in^3 in order to achieve a cantilevered dig? I see a lot of designs that could be modified to get away with a very heavy cantilevered section but chose to use a lighter pile with tiebacks or cross-bracing.
Thanks in Advance
Adam
Thanks in Advance
Adam





RE: Heavy Section vs. Bracing
On the other hand, smaller, shorter, lighter sheets with tiebacks or bracing are cheaper and easier to install but you have to add the price of the braces or tiebacks and wales. If the job is small, you may have trouble getting a tieback contractor inrterested enough to give you a price.
Remember, performance is more important than economics. However, you also need to decide what level of performance is needed.
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing
Thanks in Advance
Adam
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing
The current design shows a braced wall dig, but a peer review has recomended lowering the water table behind the perimeter sheeting to relieve the hydrostatic pressure in order to use a beefy canilevered section (AZ-36) instead of AZ-18 sheets with corner bracing and struts. The cantilevered section will work if the water table is lowered 10' with well points, but is showing deflections of 7" in the model at the surface. There is also the safety concern of lowering the water table to relieve pressure. (i.e. failure of the well point system and increase in hydrostatic pressure).
RE: Heavy Section vs. Bracing
I agree that this may well happen. I usually design the cofferdam sheeting for water at subgrade in front of the sheeting. This is a worst case and gives the lowest passive resistance.
20' is a very high cantilever. 7" or more deflection is too much. How big is the excavation area? Is the excavation to be sheeted completely around the perimeter? Can you dewater behind the sheeting? Will extra dewatering require extra, expensive water treatment? Is there an impervious layer that would prevent water drawdown behind the sheeting? If so, you may need to design the sheting for water pressure also. Internal bracing or tieback anchors drilled from above the water table may be the better way to go.
There are many things to consider when designing sheeting. Your posts really do not give enough job information for me to give you more specific guidance or opinion.
RE: Heavy Section vs. Bracing
I believe the typical settlement/deflection ratio is closer to 1:1 depending on the soils shear strength and relative density. This post sounds like an accident waiting to happen.
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing
The other problem with the cantilever system is that for a cantilever of that height, espically for a box dam, you will probably have to assemble a driving frame anyway to ensure proper alignment of the sheets.
If it were more econmical to use stiffer sheets, more contractors would. In general, however once you get over 12-14 feet, you are probably looking at a frame.
RE: Heavy Section vs. Bracing
RE: Heavy Section vs. Bracing