Tall sheet pile bulkhead w/ surcharge

[atomicman]

I've gone through a design for a sheet pile bulkhead for a barge docking facility using the Navy DM 7.02 manual and checking hand calcs with the Corps' CWALSHT program and have arrived at a proposed solution, but before finalizing things, I've got a few questions I'm hoping someone can provide some insight into.

Wall is 38' exposed height. Existing soil is a medium dense sand. I've been given the following geotechnical data for design: Unit weight = 125 pcf, Buoyant unit weight = 63 pcf, friction (phi) = 35 deg, soil/wall friction (delta) = 20 deg (considerend on passive side only), FS applied to passive side = 2.0

Design criteria: 1000 psf surcharge or wheel load from loader (130 k/wheel spaced 15' apart - ignore rear wheel since load << front wheel load). Top of wall at +18.0, Water on back side at +2.0, water on front side at 0.0.

Results: With an anchor at +7, using the free earth method, an assumed corrosion reduction to section modulus of .0354 in (0.9mm) in fresh water, fb = 30 ksi (for Grade 50 sheets), E = and 30,000 ksi, I get an AZ36 (Sreduced = 63.65 in**3) to work after reducing the maximum moment using Rowe's method (reduction between .75 for loose soil and 0.46 for dense soil). Min. penetration depth = 19 feet. Surcharge load controls over vehicle load.

Questions:
1. Axial loading: There may be common occurrence of loads directly to the top of the wall (assume steel cap). Navy DM 7.02 doesn't expressly say anything about axial loading of the sheet piles (that I could find). I went to Corps EM 1110-2-2504. The equations for combined axial and bending (eq 6-11) do not use a reduced moment. What is the reason for not allowing Rowe's reduction?

2. Tie rod design: I have seen on another project, a recommendation by the geotech to increase the working loads on the rods by 20 or 25%, to use mild steel rods to limit elastic elongation and to reduce corrosion effects seen in high-strength steel. I can get the tie-backs to work with 150 ksi HS bars. Do you see any reason not to use high-strength bars if I provide good corrosion protection? Is the load increase a rule of thumb or is that in a code somewhere (I couldn't find it in DM 7.02). PTI recommends a design load no greater than 0.6*spec. min. tensile strength (=0.6*150=90ksi) for prestressed anchors. Yield is 0.8*SMTS (=120ksi). Corps EM 1110-2-2504 says use ft=0.4fy, but I'm fairly sure this is just for mild steel. I've used the PTI recommendation before. Is there any reason or code that differs on this?

Thanks.
tmojo

 

[wmta]

Be careful with tieback loading over long tern. One factor that must be considered is the effects of stress corrosion on highly stressed reinforcing bars.

William Tucker

 

[PEinc]

Properly corrosion protected grade 150 threadbars and Grade 270 strand tendons are routinely used for tiedback sheet pile bulkheads. I haven't seen in many years a bulkhead built with mild steel tieback or tie rod tendons. Get a copy of PTI's Recommendations for Prestressed Rock and Soil Anchors. PTI calls for a maximum tenton working load of 0.6 x GUTS and a maximum testing load of 0.8 x GUTS. I don't believe adding 20 to 25% more tieback force is standard, but it does add cost to the project.

I would guess that moment reduction was not used in the combined stress equation because the writer never even thought about it. The geotech probably wrote the section on Rowes moment reduction while a structural guy probably wrote the combined stress section. They probably never even consulted with each other and the question never arose. I would use the moment I thought was correct.

The FHWA has several good manuals on anchored walls. There is much good information on corrosion and protection of anchors.

 

[jheidt2543]

tmojo:

Just a comment as I'm passing through to say very well written questions! Lots of background infomation leading up to the questions to help responders.