Cantilever sheetpile wall with sloped fill both sides 2

[PSF]

I am designing a cantilever sheetpile wall with sloped fill on both sides. There is sloped backfill of 20 degrees up away from the wall and in front down away from the wall at 39 degrees. The wall is only "retaining" 7', but the front slope does not seem stable. How can I take this into account in my analysis? How does this affect the design? The passive pressure?

thanks

 

[VoyageofDiscovery]

Hi PSF,

Upward sloping fill behind the wall must be accounted for with a Ka formula including this angle.

The sloping fill in front of the wall cannot be utilized as the slope has no resistance. The deeper your sheetpile gets the less significant this slope issue becomes.

HTH

VOD

 

[PEinc]

Sloping fill in front of the sheet pile wall can also be handled with the proper equation for the passive earth pressure coefficient. However, if the slope is steeper than the soil friction angle, the equation will not work. 39 degrees is more than most people would use as the friction angle in sheeting design.

Sometimes, in a case such as yours, I will ignore much of the upper portion of the sloped dirt in front of the wall and then design the wall for a greater height. This essentially assumes that the wall is higher and the slope in front is flatter.

 

[Touma08]

I concur with PEinc on his advice. However, if memory serves me right, the angle of the slope in front of the pile (passive side) can (theoretically) be greater than the soil's friction angle only as far as Kp is concerned because it is an 'added' quantity while for Ka it is a 'deducted' value, thus if included the equation fails.

Having said that, and theory asside, using a friction angle of 39 degrees will yield weaker springs at the pile's dredge line which will force eventually to design for a deeper cut anyway.

Good luck.

 

[PEinc]

Using MathCAD, I just ran the Coulomb passive coefficient equation for a 30 degree soil with 15 degree wall friction and a downward slope in front of the wall. The answer was an imaginary number for angles greater than 30 degrees.

 

[fattdad]

DM 7.1 or 7.2 provides earth pressures for slopeing backfill. Use this for your conditions (active chart and passive chart). Also consider the interface friction between the soil and the sheeting, which should be on the order of 2/3 phi. I think you will get what you need for your analysis using these charts.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

NAVFAC DM 7.2, Page 67, Figure 6 is a chart with passive earth pressure coefficients for sloping ground surfaces. The curves are for no steeper than Beta/Phi = -1. Therefore, the chart doesn't handle a slope steeper than the soil friction angle, same as the Coulomb equation.

 

[GeoPerson]

Your active forces should include the upward sloped “surcharge” behind your sheeting (the equation below). Similarly, calculate the Passive pressure forces taking the negative angle. I would use a minimum of 1.5 factor of safety for driving depth.

Ka = Sin^2 (?+?)/(sin^2? sin(?+?) [1+[(sin(?+?) sin(?-?)/ (sin(?-?) sin (?+?))]^0.5]^2)
Where:
?: face inclination from horizontal = 90 for vertical
?: Surcharge slope angle.

FT = 1/2 Ka *? *h^2

FV = FT sin? and FH = FT cos?

Skyline Steel have a free prosheet design softwear that u could check your calculations.

Good luck.