Bored Pile Wall Drainage

[TDAA]

I am working on Geotech recommendations for a contiguous bored pile wall. The wall will be used to retain 10 feet (15 feet temp to install a drainage system). Up to 15 feet of fill will be placed above the wall to form the approach to a bridge.

The subsurface conditions include sands (~20 BPF)and non-cemented sandstone (~50/3"). Groundwater is at about 5 feet below exiting grades.

I was looking at spacing the piles and using a drain board between them, when I was looking at a shotcrete facing. The structurals are thinking of precast panels to tie in the below grade work with the new MSE fill above. The road that is being lowered will need a drainage system under it. Large utilities that need to pass through the walls will likely require different methods to retain the soil, and would not allow to seal off the excavated area from the groundwater (pert of the decision to try and drain the entire wall system).

A couple of questions:
Would drain board still be most appropriate for drainage behind the panels?
Should the reinforced fill be modeled as a uniform load on the existing ground?
Any other issues you may have comment for?

 

[jdonville]

TDAA,

If MSE fill is being placed *above* the top of your retaining wall, why not just make it MSE/RSS too? You could then make it one big MSE wall or a series of superimposed walls. It should also help your unit cost on MSE work.

Secondly, get those utilities out of there! If they are indeed large, like you indicated, they should be routed between zones of MSE or around the approach (either under the bridge or near the take-off of the approach berm) altogether. Otherwise they will be impossible to repair/upgrade and may become damaged if the embankment as a whole undergoes settlements.

Third, yes, the reinforced zone (MSE) is treated like a block. See FHWA NHI-00-043 for external analysis of MSE structures available for download from

http://www.fhwa.dot.gov/bridge/geopub.htm

Hope this is helpful,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[TDAA]

The utils are being run to the north of the bridge structure. The are trying to push them as far fom the bridge as possible, with the space allowed. I think at the location we can get them, we will switch to MSE for the walls, all together, as the cuts and fills will be less.

As far as the MSE for all walls, we have limited area to work. We have to accomodate several lanes of traffic in each direction (two, 6-lane roads with turn lanes), and have areas with limited ROW (business on all sides, some to be removed). This has moved us in the direstion of top down construction, with limited room for tiebacks and reinforcing. Also, this would lead to needing to dewater more of the site.

 

[VAD]

TDAA

Re your original post, there may be a couple of ways to look at the problem. The information provided is not very clear to me. However, I will try to give some opinions/comments:

It appears that your bored pile will be embedded in sandstone. I presume the idea of the contiguous wall to avoid the problem of sand eroding between the piles. If you space piles is your drain board and shotcrete going to avoid this.

Before we get there, how do you plan to install piles. I presume that you will be using casing for pile installation. There could be issues in pile installation unless you have determined that this would proceed smoothly.

Re the use of precast panels by structurals. How is this envisaged - behind pile wall, between piles, if the latter is H-pile intended for use as reinforcement in bored piles.

It would seem to me that you may have to tieback the bored pile wall anchoring it in the sandstone unless your system would be rigid enough to minimize lateral deflection which could have some impact on MSE wall performance.

What type of MSE wall is envisaged to be used - Reinforced Earth Type or geogrid reinfoced wall type. Will the MSE wall facing sit on the bored pile wall.

Drainage- Can this be intercepted away from the face of the bore pile wall. It would be the best scenario to avoid trenching next to the pile wall. Presumably the drainage system will be within sand material. It may be difficult to have an effective drainage system implemented and hence wall may have to be designed to accommodate water pressure.
This is, however, not often the choice.

Other thoughts re drainage

There could be some thought about laying drainage pipes horizontally (perpendicular to bored pile wall) behind wall and draining parallel to wall to exit outside of wall area perpendicular wise to wall. I am guessing here as this depends on topography and whether the sand material can be accommodate a stable trench. Is material clean sand etc. Horizonally bored drains. Can this work

Unfortunately I may not be interpreting the site conditions clearly from the info provided. You do have an interesting problem which I think needs to be looked at carefully. A few more details or a sketch would help for additional, refinement or dicarding of comments made.

 

[TDAA]

VAD,

The site is actually more interesting than my original post describes (after rereading it). The bridge site is directly on top of a change in geologic formations (changes along the abutments). On one side the beardock is at the surface, and it slopes down to about 30 feet in depth. The groundwater approximates this change as well, from 5 to 30 feet. The sand is the surficial material over the sandtone. The sand is silty. The sandstone is coarse-grained, and clean. The 4-inch diameter boreholes were satble. The groundwater was at 5 feet based on follow up measurements in a piezometer (on the one side of the proposed bridge). There was not standing water in the holes until about 30, however, the soils were saturated by at least 9 feet below the surface (I was not the one drilling, but was onsite arround that time).

Topography is not a huge concern for being able to drain. There will be an approximately 6'X8' box culverrt installed below the lowered road to handle surface and subsurface drainage.

The spacing between the piles would be small, avoiding large expances. The facing is intended to be in front of the piles to form a continuous wall with the fill above.I was thinking the MSE portion would be constructed as a geogrid reinforced wall.

Hopefully that helps describe the situation.

Thanks for your help.

 

[VAD]

TDAA:

Thanks for additional info. The situation is a bit clearer. I am assuming that there is at least 15 ft clearance from the soffit of proposed bridge to ground level below. The groundwater is following the topography which is expected in undulating terrain. The sandstone is likely fissured. As such you need to ensure that your piles are socketed below the weathered depth in the sandstone profile. I presume that the sandstone being to surface on one side that you are contemplating a footing on one side. and the same type of foundation resting on the MSE wall on the other side.

The saturated condition of the sand above the sandstone could have result from capilliary action. I would venture to say that the action of water is from the sandstone layer which could be charged from the side of the bridge where it is exposed to the surface, hence fractures in the sandstone may be present. I would want to look at any fracture pattern orientation as well - dipping downslope etc. However you mentioned before that the sandstone is non cemented and not sure if your drilling included coring.

I would want to look at the MSE wall design and bored pile wall combination a bit carefully. Perhaps you may want to consider the abutment resting on the bored piles while still using the geogrid reinforced wall system with the panel facing as a cosmetic feature i.e not connected to the MSE wall.

There are a few things that you may have to sort out.

Regarding drainage, my feeling that the free water is associated with the sandstone layer with a small head in the lower sandstone layer which would assist in the saturation of the overlying silty sand as well. Frost heaving issues would have to be looked at if you are in a seasonally frost susceptible environment.

So much for now. Interesting project.