stability evaluation of existing waterfront sheet pile wall

[yfmessi]

Dear Fellow Engineers,

Regarding the subject above, I was assigned to evaluate the stability of an existing waterfront sheet pile wall.

I've done some research on previous cases and notice that it is a common practice to recommend for the following parameters used in the stability calculation :

1.) "Passive thrust in front of the sheet pile" value used for analysis is REDUCED by a factor of ~ 1.5

2.) "Penetration depth of the sheet pile below dredged level" used in the analysis is REDUCED with a a factor of ~ 1.2 from the actual depth

I will appreciate if fellow engineers could share their opinion on the rationale behind the above recommendations.

Thank you in advance for your valuable opinions.

 

[cvg]

scour anticipated to remove the soil in front of the sheet

 

[PEinc]

You should not need to both divide the passive resistance by 1.5 and multiply the calculated embedment by 1.2 or more. This is a double application of the safety factor. If you divide the passive earth pressure coefficient by 1.5, you will get a more conservative design than if you used the full coefficient and then increased the calculated embedment depth. Passive resistance is a function of the embedment depth squared. Therefore, if you multiply the embedment depth by 1.2, you shoud be multiplying the passive resistance by 1.44 (= 1.2 x 1.2) which is close enough to 1.5.

If you divide the passive coefficient by 1.5, you will increase the bending moment, brace load, and sheet pile length. Most references use the full passive earth pressure coefficient and then multiply the calculated embedment depth by at least 1.2. This way, you increase the passive resistance without increasing the wall's bending moment or brace load. Both design approaches work; one is more expensive and conservative.

http://www.PeirceEngineering.com

 

[cvg]

except that with the second method (increasing the depth of sheet pile) ignores the likely probability that the material in front of the wall will be scoured away at some point in the future and not available to apply passive pressure. This will in fact increase the moment and bracing load as you indicate.

A better method might be to estimate the scour depth and reduce the passive thrust to account for a calculated depth. Then apply a safety factor.

 

[PEinc]

Usually, the scour depth is added to the design height when the wall is first designed or when it is being reanalyzed. While it is important to consider scour, the question really had nothing to do with scour. Why design a wall for 20 feet if you expect the maximum height to increase to 25 feet with scour? A greater bulkhead height due to scour will affect more than just the passive resistance.

http://www.PeirceEngineering.com