Soldier pile lagging top down installation 1

[sgsibob]

Can someone please enlighten me about the constructibiity pitfalls of top-down lagging of soldier pile walls? If, for example, a lift is excavated 5 ft with lagging in place, what keeps the lagging there -- earth pressure on the back side of the lagging? Suppose the wall is 50+ ft high -- is it reasonable to expect that the earth pressures would be adequate to pinch all that lagging so none of slips? Also, suppose the vertical height of drilled shaft hosting the lagged portion of the piles is backfilled with weak cementitious material, such as soil cement or a 1/2 sack slurry...how is this material removed from between the flanges (and from the front flange, for that matter) so the lagging can be installed? Finally, if the wall is tied back, how does the lagging accommodate the space between the flanges lost by the tieback and web stiffeners? Be as verbose and comprehensive as you like -- I am interested in all the "war stories" also. Thanks.

 

[PEinc]

Timber lagging is installed as you described. Dig about 5' and install the lagging boards from bottom to top. Make sure that the lagging has good, firm contact with the dirt behind. You should try to not dig behind the rear face of the lagging. If you do, you need to pack the gap with soil as you install each board. The lowest lagging board in a 5' cut can also be packed with additional, nailed lagging board cut-offs which can act as a shelf to supoport the packed-in soil behind higher lagging boards. This prevents the upper soil from running out when the next lower cut is made.

Next. Make the next lower 5' cut and again install the laagging bords from the bottom up to the lowest, previously installed board. The soil is sufficient to keep the timber lagging boards in place as each new 5' cut is made. Timber lagging is not supposed to be installed using pre-made panels that are slid in and down from the top of the soldier beams as the excavation is made.

THIS DOES NOT APPLY TO OR WORK WITH PRECAST CONCRETE LAGGING .

Precast lagging should be used only in walls that can be open cut for the full height with a safe slope behind the wall. Then, the precast lagging is stacked from the bottom up with backfill being placed as the concrete lagging is set. This makes it difficult and expensive to install tieback anchors. Therefore, precast lagging panels are not a good system for tiedback walls.

The lean concrete or flowable fill can be removed using a small (about 35#) pneumatic breaker or chipping gun. Often, the timber lagging is installed onto the front face of the front flange using welded, threaded studs, plates and nuts. If you do this, you do not need to dig behind the front of the front flange.

After the lagging is installed down to just below the tieback grade, the tieback anchor(s) can be drilled through the lagging or you can cut a small hole with a chain saw. If you are using precast lagging, you cannot install the tiebacks through the lagging. In this case, the tiebacks usually are installed through the fabricated, paired soldier beams.

In more than 30 years of doing this work, I have never seen stiffeners needed for soldier beams.

 

[sgsibob]

As usual, PEinc, you have answered the issue with clarity and detail.

I am not trying to belabor this issue, in case you were wondering...My interest is permanent, long-term applications of top-down retaining walls for what would in LRFD terms be considered critical applications. In that context the risks associated with the construction process need to be thoroughly articulated.

So as I understand it, each piece of lagging has to be cut shorter than the c-c spacing of the soldier pile webs but longer than the in-in distance between the adjoining flanges. Each piece is angled in behind one flange or the other, swung into place, and then slid laterally so the lagging is centered between the flanges, catching equally on each, right? How much "catch" is adequate? How does one slide the lagging laterally if the earth is pressing the lagging against the front flange? Should there be some blocking pounded in between the lagging ends and the webs to prevent shifting due to friction from adjusting the next piece?

OK, just checking on issues here....the lowest (first) lagging member in each lift then rests on the ground surface, and earth pressures on the back sides of the lagging installed above, in the preceding lifts, keep it in place with no mechanical assistance? Is there a practical limit here? I was envisioning that in real SC-SM-GC type soil applications the soil may not all develop a lot of friction against the lagging (due to its consistency or overexcavation), and in real tall walls (>40-50 ft), the weight of the lagging may not be fully offset by frictional forces. Is this a risk and if so, what do contractors do to offset it?

If the builder doesn't guess exactly right when building up each lift from the bottom, do you end up with a gap at the top?

I was wondering why web stiffeners are not used for tieback applications. I have seen designs with web stiffeners paralleling the tieback depression angle, symmetric about the tieback perforation in the flange. I guess these designers think it is conceivable that the tieback loads (for tiebacks installed through the steel) could deflect the flanges.

Also, I was wondering about the holes for tiebacks installed through the steel. Presumably the holes have to be a diameter that will admit the drill steel, couplings, and bits. Fairly large hole, may weaken the flange. If the lagging is put in first, I would not think the tieback holes would be torch cut in place, but would have to be prefabricated before the soldier piles are stood in the hole? If so, does this present issues with the vertical positioning of the soldier piles so that the tiebacks are properly aligned?

I was also wondering why precast lagging members could not be stacked up from the bottom in the same manner as wood lagging members. What is the issue there? Weight? Uncertain spacing of the soldier piles (invalidating the widths of precast members)? It would seem that the

 

[PEinc]

How much "catch" is adequate? Several inches (about 2 to 3 inches) is usually enough. Check compression perpendicular to the grain of the wood.

How does one slide the lagging laterally if the earth is pressing the lagging against the front flange? It is very unlikely that the earth is pressing that tightly to the back of the lagging. Although you try to not over-excavate, some always happens. The key is to make sure the gap, if any, is filled.

Should there be some blocking pounded in between the lagging ends and the webs to prevent shifting due to friction from adjusting the next piece? Most installers will install one or two large nails at each end of the lagging board and curl the nails over the front of the flanges. This keeps the boards from shifting.

...earth pressures on the back sides of the lagging installed above, in the preceding lifts, keep it in place with no mechanical assistance? Is there a practical limit here? Correct, except for the nails. For deeper holes, the wall will be braced or tiedback. This puts more normal pressure on the boards. In fact, if you do not have good contact with the earth behind the soldier beams and lagging, you can damage the beams and/or the lagging. This method of lagging has been used routinely for very deep excavations. I am not aware of any practical limit to depth.

If you know the width of the lagging boards, you can adjust the lift height to match pretty closely. Occasionally, the contractor may rip a board or install some softwood dimension lumber to fill the gap. It is standard, prudent practice to install the lagging boards with louver spaces between the boards. This louver space is most often made by nailing 2 small blocks to the top edge of each lagging board befor placing the next higher board. The louver blocks are cut from cheap, softwood 2x4's. This 1.5" gap allows you to see if there is sufficient earth behind the lagging, to add some fill if there is a void, and to stuff some hay, straw, or filter fabric if needed to control water seepage from behind the lagging. Contrary to popular reviewer comments (accusations?), louver blocks are not installed in order to save on buying lagging boards.

I was wondering why web stiffeners are not used for tieback applications. In most cases, the soldier beam webs do not need stiffeners because the web is laterally supported by the earth , lean concrete fill, lagging boards, etc. If someone designed a wall with extremely high tieback loads, possibly stiffeners could be a concern, but in that case, the soldier beams are usually much stronger also. The stiffeners you have seen may be flange stiffeners for walls with thru-beam tieback connections. In this case, there is no tieback wale and the flanges (and some times the web) are cut to allow drilling and placement of the tieback anchors through the soldier beams. This is most often used for wall-line sheeting and permanent tiedback wall with attached facings.

Installing a hole in the soldier beam usually weaken the beam. That's OK if the resulting beam is still strong enough to function. Usually, when a hole is cut, the beam is reinforced to make the beam strong enough. Reinforcement can include adding flange plates and/or adding web plates. If you are designing a wall like this but are not the tieback contractor, you can run into problems because you probably do not know the size hole the contractor will need. One way to get around this is to use double WF or channel soldier beams where the tiebacks pass between the double members. Again, the designed should know how much room the driller needs. If you pre-fab the holes, you need to be sure that the soldier beam can be installed to the proper elevation to assure the tieback anchor is where you want it. With drilled-in soldier beams this is not a proble. With driven soldier beams, you may not known how deep the beams can be driven and you may damage the thru-beam connection while driving. Even if you pre-fab the thru-beam connections, you will probably need to fabricate some extras in the field due to the need to install supplemental tiebacks for those tiebackes that do not hold their full test load.

As I said above, precast lagging should be used only in walls that can be open cut for the full height with a safe slope behind the wall. Then, the precast lagging is stacked from the bottom up with backfill being placed as the concrete lagging is set. This makes it difficult and expensive to install tieback anchors. Therefore, precast lagging panels are not a good system for tiedback walls. Weight, spacing, slope stability, tieback installation and testing, and final aesthetics are all reasons for not using precast lagging on tiedback walls or walls that you cannot build from the bottom up using a sloped open cut.

 

[DRC1]

I pretty much agree with what PEInc has said. We simply tuck the lagging with out louvers and do not have a problem. Soldier pile walls are primarily temporary structures and most contractors have their own preferences on how to build them. Options other than tucking include proprietary clips that attach to the flange and attach to the lagging or welding theaded studs to the face of the pile. Both of these options put the lagging in front of the pile. I have done through beam tiebacks, but they do require attention to the details. Stiffeners are not used as they prevent tucking the lagging and make driving difficult. If possible piles should be vibrated in. As the lagging progresses. vertical pieces of 1x are sometimes nailed to the lagging to prvent the lower boards from shifting.
I agree that precast for a tied back wall would be difficult. A better option may be to consider placing studs on the soldier piles and pouring a one sided formed cast in place wall. This has been done many times and can be very econmical.

 

[PEinc]

Without the louver spaces between the lagging boards, you can't see if you have lost soil from behind the lagging boards and, if you have lost soil, you can't replace it without removing boards.

 

[hokie66]

Good explanations, PEinc and DRC1. But sgsibob said he was talking about "permanent, long term---critical applications". Timber lagging for that?

 

[DRC1]

PEInc.-
That is the way we and other local contractors have done it for as long as I have been around. Every now and then one of these large national firms comes in and does one with lovers and straw, but in general, the boards are just tucked, and butted next to the previous board.
Hokie66-
There are permenant soldier pile and timber lagging walls. Shnabel's literature shows a few examples. The lagging is generally3-4 inches thick and can be pressure treated, so you would get some life out of it. However, these are predominatly built as tempoaray walls, so if you are looking for long term performance, I would use a cast in place face bonded to the soldier piles, or interally braced by a floor or roof system. If you dou bond to the soldier piles, use double corrosion protected permanent anchors.

 

[PEinc]

DRC1,
I worked for Schnabel for 11 years, ran their PA ofice, and did projects in your area. Permanent soldier beam walls with exposed, treated lagging were few in number and were not recommended. Straw would be used only when water seepage was a problem (not often). Also, we recommended that the building structure not be attached to a permanent soldier beam retaining wall. The wall would be cantilevered if low or would be tied back if higher than about 12 to 14 feet.

 

[sgsibob]

DRC1,

I am intrigued about the possibility of installing a cast-in-place lagging between piles. My question is how tall a lift is realistic, how long between successive lifts (in other words, what cure times have been used -- 3 days?), and whether the use of internal reinforcement is required, such as rebar walers or welded wire fabric.

Does the feasibility depend in any way on access? Since we are talking about top-down construction the wall is obviously accessed from the top. If the wall is tall and there is no vehucular acess to the bottom, all materials have to be lowered into place, and this would include fresh concrete.

All,

I was not presuming that in a permanent application the lagging would be left exposed. It would be covered by some kind of final facing. However, the lagging does have to maintain its structural integrity over the long term because the facing would not be designed to resist earth pressures, just protect the lagging.

I was also presuming that, these being permanent tied back walls, one would not be driving the soldier piles, but would instead position them in pre-drilled holes (some kind of centralizers used?) socketed below grade with concrete.

 

[PEinc]

The facing is always designed to support the earth and surcharge pressures. I have never seen a facing designed or built that is just for protecting the lagging.

Permanent walls can have soldier beams that are driven or drilled in place. I have neve seen nor used soldier beam centralizers. Remember, the hole you drill may not be straight. Therefore, if crooked, the centralized soldier beam will be crooked also. Without soldier beam centralizers, you can plumb up or move the soldier beam around to get the proper alignment.

 

[DRC1]

sgsibob,

I think I may have not been clear. What we have done in the past is to install a soldier pile wall in a traditional manner, and use the wall as a back form and a onesided form on the front to cast a conventional wall. This saves time and forming costs. Depending on location and engineer a waterproof membrane was placed against the lagging to act as a moisture barrier. The toe footing is eliminated. There is no backfill. You also maximize your foot print. I was not talking about acst in place lagging and do not see any advantage in it. If driving cionditions are good and vibration is not a problem, I would vibrate piles in. The fact that they are permanent does not require the use of sockets.

 

[PEinc]

DRC!,

I disagree that a one-sided wall facing attached to a soldier beam wall saves time and forming cost. My experience is that a one-sided form costs at least as much as a two-sided form. With a one-sided form (wall-line sheeting), you may save some excavation and backfill, but the forming and waterproofing become more expensive. You will use more concrete than a two-sided wall. Also, you may need to install additional tiebacks depending on the "sheeting" design.

 

[HouseBoy]

I am currently evaluating options including a soldier pile wall wih wood lagging onto the face of which a basement foundation wall would be cast.
What techmiques are good for water-proofing these walls?
Also, if the house is going to be built directly on top of the pile wall, how is the top of wall constructed to make a straight and uniform condition to bear a sill plate?
Has anyone seen such installations and are there any pics available?
Links?

 

[PEinc]

IF you were going to build a house basement against a soldier beam andlagging wall, you would cover the lagging and soldier beams with a waterproofing membrane or bentonite panels. Then you would pour against and attach a concrete foundation wall to the soldier beam wall. The house and sill plate would be built on top of the attached concrete wall. However, if you are expecting a lot of water, you probably should not be trying to install lagging. Also, your idea sounds expensive. Why not open cut and build a regular house foundation wall? Or, sheet about 3' off the proposed foundation wall, build and waterproof the foundation wall in the usual manner, and then backfill.

 

[sgsibob]

DRC1 and PEInc,

I thought the one-sided forming idea was to cast concrete in place against the lagging and soldier piles.

1. PEInc, why would this save any excavation and backfill, and why would it use more concrete?
2. DRC1, how would the formwork be braced in a tall wall application? Also, in a tied back wall where bedrock is close to the wall bottom, would it not be preferable to drill the wall in place so that a rock socket can be constructed?

Thanks both of you.

 

[PEinc]

One-sided is pouring a concrete wall against another wall, such as a sheeting wall.

One-sided forming saves excavation and wall backfill compared to a situation where the sheeting wall is installed about 3 or 4 feet off the face of the proposed foundation wall. In this case, you would need to dig out the extra few feet, double-side form the new wall, waterproof the new concrete wall, and then backfill between the wall and the sheeting. The extra digging and the backfill cost more but the concrete wall construction is usually as cheap as and usually cheaper than a one-sided wall. With a one-sided wall, the waterproofing is usually more expensive. In addition, the one-sided wall usually needs more concrete because it is common to install the sheeting wall a few inches back from the planned outside face of foundation wall. This is done in case the soldier beams and lagging are not perfectly plumb. The couple extra inches hopefully will compensate for a slightly croocked sheeting wall without intruding into the planned thickness of the concrete wall.

One-sided formwork can be braced back to the ground or to a slab using inclined raker braces. Or, there are a-frame forming systems available from the forming companies. Or, the forms are tied to the steel soldier beams are reasonable vertical spacings so that the form ties do not concentrate too much load to any location along the soldier beam. Or, sometimes a lower wall pour is made and then subsequently higher wall pours are made with forms cantilevering up off the lower poured wall section.

If bedrock is at or slightly below subgrade, you can either drill a rock socket for the soldier beams or you can drive or drill the soldier beams to rock and install enough tieback anchors to support the entire lateral earth pressure. Of course, you need to make sure that the soldier beam does not plunge downward due to the vertical load from the tiebacks and the weight of the wall and any vertical surcharges on the finished wall. If rock is above subgrade, you should drill the soldier beams at least to subgrade, using a rock socket or the extra tiebacks as described above. There are other ways of handling high rock with wall-line sheeting but these methods are better left to those specialy contractors who have a lot of experience with tiedback walls.

 

[PEinc]

Sorry, I should have typed:

...the double-sided concrete wall construction is usually cheaper than a one-sided wall.

Or, the forms are tied to the steel soldier beams at reasonable vertical spacings

The couple extra inches hopefully will compensate for a slightly crooked sheeting wall...