Return Wall or Full Braced Wall...should the wall be designed for active or static pressure 1

[bpiermat]

Hi,

I have a return wall in a retaining wall, the wall is steel soldier pile (single brace in strong axis) with timber lagging. If the soldier pile is fully braced by the lagging for the full height of the excavation, I assume the correct soil load diagram is 0.65*gamma*H*Ka or is it 0.65*gamma*H*ko? Something else?

Thoughts?

Thanks,

Ben

 

[oldestguy]

Do you have soil report for the site? A sketch of the detail, from the side will help, particuarly what is below the excavation also.

 

[DaveAtkins]

I am not sure where the 0.65 comes from, but you should be using Ka (active coefficient) for this type of wall.

DaveAtkins

 

[bpiermat]

Assume it is Sandy soils. The soldier pile is braced in the strong axis and has an embedment length. The question here is NOT with regards to the strong axis bending of the soldier pile.

It is with regards to the weak axis. In the weak axis, the lagging shown on the top of the attached screen shot presses on the weak axis of the pile. The lagging on the opposite side of the pile resists this force as well as some soil pressure against the pile.

My questions has a couple of parts, but to make it simple, if the lagging supporting the pile in the weak axis runs the full height of the pile...the pressure distribution assumed to act on the pile should be rectangular in the weak axis? and if so, static or active? The lagging will be installed "on the way down". The 2nd portion to this question, can passive pressure be used to reduce the load on the lagging resisting this force? if not, how about static pressure? I think the answer is no. What are your thoughts?

 

[oldestguy]

Since the lagging is installed with its lengths less than the clear web to web distance between piles, it would not be expected that it can transfer any significant load to adjacent piles via sxisl compresson onto its ends. In the height from bottom of excavation to top it is questionable to be able to make use of any passive pressure from the earth retained around the corner so to speak. However, no failure expected in the corner pile weak axis of significance due to those loose lagging sections that can still provide resistance to some extent with lagging compression axial (horizontally). There might also be enough support pressure contact of those lags within the piles to help in this regard. Id use the term "continuous bracing" also as an explanation. Don't forget the soil to wall shear resistance of the combination of lagging and piles in the horizontal direction also taking some of that horizontal load on the weak axes of all piles.

 

[fattdad]

temporary excavation bracing design using Terzaghi and Peck's method uses the formula that includes the "0.65." If you consider the stress envelope from that equation it equates to a 30 percent increase over active earth pressure. T&P's method also has you reconfigure the envelope to either a rectangle or trapezoid, to, "Force" more stress to occur near the top.

The actual corner of the braced excavation is further restrained, which I think is the point of the OP. I'm not accustom to seeing the earth pressure at these, "Returns" modified as you may expect for at-rest conditions.

We do require slope inclinometers and survey monitors to quantify vertical and horizontal movements as the excavation progresses.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

Let's suppose you didn't have any wall and dug without any such support, as when digging a grave. Where is the earth more likely to cave in? Not at the corner, but at the long wall. The reason is the horizontal pressures are directed to the non-dug area, as in arching of the vertical pressures of a tunnel. The meaning is then the horizontal loads on the corner steel pile are less that at the piles along he main wall and main return at some distance away from the corner. In some circumstances with tight ends against steel (due to the horizontal loads on the lagging causing high friction) you would not get much of any loads on that corner pile, due to horizontal soil arching around the corner. In summary, don't worry about it.

 

[fattdad]

The loads at the corner are greater than the loads along the long face! The long face allows for the development of plane strain conditions and likely will allow for more movement to engage the soil friction. The corners will not move as there is more structural rigidity. As a consequence, there is less engagement of the soil friction and the loads will be closer to at-rest. Fortunately, the corners have more structural support.

In summary, worry about the first principals and look into all details.

f-d

ípapß gordo ainÆt no madre flaca!