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TIMBER LAGGING BELOW EXCAVATION

TIMBER LAGGING BELOW EXCAVATION

(OP)
Drilled soldier piles with tiebacks have been proposed to stabilize a slope. The piles are WF sections, in drilled holes, 30" in diameter, backfilled with regular concrete and spaced 6 ft on centers. Installing the tiebacks requires excavating a bench in front of the piles. The excavated surface exposed above the bench requires timber lagging for support. However, the designer requires installing timber lagging also below the bench, down to the slip surface of the slide. What would be the justification to use deep timber lagging, considering that the soil is stiff clay with unconfined compressive strength of 2 kip/sqft? Would this use of deep timber lagging be considered normal practice? Will appreciate your insight.

RE: TIMBER LAGGING BELOW EXCAVATION

Not normal practice, and just asking for trouble. How does the designer propose installing this lagging?

RE: TIMBER LAGGING BELOW EXCAVATION

Makes no sense to me. Seems like this would generate the need for larger pile too.

Mike McCann, PE, SE (WA)


RE: TIMBER LAGGING BELOW EXCAVATION

(OP)
To hokie66 and msquared48,

Thanks for your response. No explanation given. However, it is obvious that they will have to excavate down to the slip surface (about 15 ft) to install the lagging. And that is what prompts my question. It looks to me the designer is guided by some sort of circular reasoning: "Excavation is needed to install the timber lagging, and timber lagging is needed because he intend to excavate".

Continuing with this issue. When I asked if consideration of the arching capacity of the soil was not sufficient reason to eliminate deep timber lagging, the response was that "the soil mass above the slip surface was remolded when it failed and that its arching capacity was considerably diminished". I am still not convinced. Please comment.

Thanks.

RE: TIMBER LAGGING BELOW EXCAVATION

Timber lagging isn't really worth much, and will eventually rot away. If the designer really wants something more permanent wood probably isn't the answer.

To me a 2.5' shaft at 6' centers will do more to hold back soil than some 3x lagging ever could.

RE: TIMBER LAGGING BELOW EXCAVATION

Unless the geotech specs CDF backfill behind the lagging.

Mike McCann, PE, SE (WA)


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