"NEW" way to install soldier piles??? 1

[oldestguy]

Here is a question that may be a dumb one, but I have recently heard of a "contractor" installing soldier piles for retaining the sides of an excavation using this method. Comments??.

Before excavating for a basement, a soldier pile system with lagging was installed surrounding a proposed excavation site.

At each soldier pile location a casing is advanced into the ground with a continuous flight auger inside. Not sure if any force is applied downward, but unlikely. Also not sure if the auger was working ahead of casing.

After the casing and auger are at required depth, the auger comes out and an H pile is set inside. Then pea gravel is gradually dumped in the cavity, as the casing is withdrawn, leaving the H pile surrounded by gravel.

This site work is below the water table, in saturated fine sands with thin clay layers (lacustrine area).

Upon excavating inside this "wall" of soldier piles and lagging, the piles all have tilted inward and earth behind has settled.

Would not this procedure have been suspect from the start?? Has anyone heard of it before?

Subsequent nearby test borings show very loose soil near these soldier piles.

 

[yakpol]

Oldestguy, you have not mentioned placing concrete in the shaft below the bottom of excavation. If this is the case than H-piles don't have much of the support but the flange width.

 

[BigH]

Certainly have heard and seen low strength concrete to encase the H-piles, but not pea gravel.

 

[msquared48]

You previously mentioned:

"Before excavating for a basement, a soldier pile system with lagging was installed surrounding a proposed excavation site."

I do not see in your construction explanation that the lagging was installed anywhere prior to the excavation as you mentioned. Seems impossible.

Since the H piling are bending, they probably are not big enough, and/or do not have enough embedment below the bottom llevel of excavation.

The pea gravel should have just run out upon excavating, leaving an empty hole around the pile, if not loose, sluffed sand too.

The soldier pile will normally support a soil diameter of two to three times the diameter of the augered hole, if filled with concrete, not pea gravel.

This seems really hoaky. Where was your geotech?

Mike McCann
McCann Engineering

 

[MSEMan]

Your question is anything but dumb - a really interesting problem actually.

The only reason I can think of introducing gravel to the cased pile rather than concrete is to allow greater deflection - perhaps reducing earth pressure acting on the wall from Ko to Ka (or maybe even less if the soil becomes loose).

I could also guess that the counter forces required for stability were only generated once Kp resistance was generated and this may have reqired a whole lot more underground movement.

I'd really want to see the design assumptions, particularly looking at the assumed gross pile area assumed for resistance. I might start to worry if the steel section and gravel pile were considered as some sort of composite, particularly considering the limited confining pressures that would be generated by your saturated sand/clay.

 

[VoyageofDiscovery]

What is the benefit of this system? From what you say there is deflection and settlement, so earth forces are not being resisted in a controlled fashion. Granted earth forces become reduced after the movement, but what's that worth if the soldier pile has already deflected?

VOD

 

[msquared48]

Exactly VOD.

I think this contractor has tried this in other jobsites though, and it has probably worked - but with stiffer soils, no sand, and probably not as high a cut, or as much water content. Bad choce for this site. Were there any adjacent structures that were affected by the settlement?

Will the wall be integrated with the structure eventually to stabalize the cut or not?

Mike McCann
McCann Engineering

 

[oldestguy]

Hi all

To clarify, I was not on the job, but have learned of this via a fellow engineer. I would believe that lagging was insterted in the flanges as the excavation was dug.

The piles have leaned, not necessarily bent. There is no mention of soil sliding past the "wall" of soldier piles.

There was a geotech designing the job, but I would think (knowing the contractor's past not so good reputation) that the contractor "did the installation his way".

Nearby N values for borings in the excavation near this "wall" were very low, varying from a few blows to "weight of hammmer". There were no borings ahead of this work.

However, there is an answer to my question so far. It appears this is not a common procedure, especially in this kind of ground. It looks like a situation going into court some day. As I hear of the site situation I shudder to think it got that far with an experienced engineer designing it.

 

[PEinc]

The correct answer is that the contractor did what was cheap. he also had no idea of what he was doing besides saving money.

There are a few problems with what he did.
1. He is using lagging below the ground water level. This will not hold back the water and may cause him to lose soil from behind the lagging.
2. Many people try to backfill drilled soldier beam holes with gravel because it is easy and cheaper than clean concrete of flowable fill. However, the gravel will run out when he excavates in lifts, from the top down, to install the lagging.
3. The gravel may not be completely around the soldier beam below the excavation subgrade. If so, the soldier beam will deflect forward, possibly excessively. For driven and drilled-in soldier beams, it is pretty common to use 2 or 3 times the flange width or srill hole width for passive resistance as long as the material around the soldier beam toe is competent. Loosely dumped gravel may not be competent.
4. Cantilevered soldier beams should be used only with relatively competent soils and for excavations less than about 12' to 14' (unless very large beams are used, such as secant pile walls). You said that the borings showed very loose soils near the soildier beam. Therefore, that should have been a warning sign that cantilevered sheeting may not have been an appropriate shoring method.
5. Anyone can pick up a soils book and design a sheeting wall. However, the more important qualification for designing these walls is to know what you can or can't, should or shouldn't do. There is much more to designing a safe and economical retaining wall than just crunching the numbers. It's more important to know what not to do than to think you know what to do.

 

[facca]

We are a contractor just getting into this work for Highway departments.We always use engineering to do our shorings and usually go with the more expensive sheet pile designs.We have seen many old timers build chaep systems especially on private jobs as in this case there days are numbered.

 

[DRC1]

I have seen lean concrete below the mud line w/ sand above. Pea gravel is one of those things that might work in theory but would doubt in practice. As PEInc noted work below the water table in sands is problematic. If it can readily be controled with pumping it is still a tricky process. I would expect to see lateral displcement of the piles if the pea gravel failed. If the failure was only perpendicular to the trench, the embedment depth may be an issue. The fine sands (phi = 27?) would control emdement, not the pwae gravel. Along this same line, OSHA requires All Soldier beam angd lagging and all sheeting to be designed by a PE if someone is going to work below it.

 

[PEinc]

Even above the water table, I would not use sand or gravel to backfill the soldier beam drill holes. I use lean mix, low strength concrete or flowable fill.

 

[aeoliantexan]

Pea gravel is used so the soldier piles can be pulled after the excavation is backfiled. I have heard of the practice in stiff clay and bedrock. In sand, I would expect that the pea gravel would not provide as much support as concrete, but the difference may not be great.

I suspect the piles tilted because the embedment below the excavation was insufficient. The passive resistance of sand at shallow depth (below the excavation) is not large.

The loose condition of the sand behind the wall, if it didn't exist before, may have developed when the active wedge failed, or it may have developed when the holes were drilled. I have found 10-foot diameter columns of very loose sand around augercast piles when the sand removal during pile installation was excessive. Apparently, the sand flows to the auger tip, undermining the sand above, which flows like grain in a silo. The same could occur even when augering through a casing unless the procedure is carefully controlled.

The use of soldier piles and lagging below the water table has been done for years, but care must be taken to limit the loss of ground through the lagging by packing the joints with straw, etc. Since the wall is leaky, it doesn't have to support water pressure, just soil pressure.

Cantilever shoring can defelect excessively even when it is strong enough to suuport the ground. This can lead to damaging settlement ofstructures and pavements near the excavtion.

 

[oldestguy]

Good comments all:

Per the last comment, it is my suspicion that since the work below the level of the proposed excavation depth was in saturated, somewhat loose fine sand and silt layers, that more volume was excavated than the volume of the casing.

In any case, the lateral resistance to the loading from these soldier piles probably is very low to begin with.

I have seen this excessive excavation at hydraulic elevator casing installations with the subsequent settlement of the elevator lower floor.

A likewise situation was seen at a well house where the well driller removed much more soil than the volume of his casing.

Moral to the story: Know your soil conditions before doing the job and use a system suitable for the site. It is apparent that this system, though apparently not new, was not suitable there.

 

[PEinc]

On a recent project, my client removed soldier beams that had been backfilled with low strength flowable fill. He used a vibratory pile hammer and had no problems.