Piling New Bridge Structure Across Paleo-landslides

[Mad Mike]

Have any geotech. guys got experience with systems aimed at protecting piles from slope creep in a deep, unstable soil profile?

I'm investigating a proposed new highway alignment in which one of the slopes is very dodgy, approximately 1 000' in length, 15degree angle and underlain by 40 to 50' of fat clay over shale bedrock. The trees on the slope are all buckled and I am confident the slope has undergone a number of cycles of deep failure, as well as continual surficial creep. Stability analyses give factors of safety around 1.0 for deep failures in the saturated condition. The slope movement would be driven by saturation cycles- current investigations show it to be relatively dry.

The slope occurs along a bridge section of the proposed highway- my concern is that conventional piles will shear in the long-term. I've installed an inclinometer and will be monitoring slope movement- we're currently at the preliminary design stage so further investigations and monitoring would follow.

Point is, I don't expect major movement of the slope to be on-going; rather, I expect spikes in movement periodically (every 5 to 10 years say). The monthly inclinometer monitoring may therefore be misleading.

On a similar slope in our area, an historic bridge was constructed across a major landslide, in which instance they sunk large-diameter elastic concrete shafts (caissons) down to bedrock and installed piles within these caissons, which were sized to allow 5' of movement before coming into contact with the piers. Anyone ever used similar systems? Can they still be economic and if not, are there any structural alternatives? My soil volumes are far too great to consider stabilizing the entire slope.

Last note- realignment of the route would be very difficult, probably impossible due to the topography and land-use constraints.

Cheers,
Mike

 

[BridgeSmith]

For a similar situation, we're considering a group of drilled shafts, with a massive cap, in an arrow configuration pointing uphill on the upslope side of the bridge piers, to divert the sloughing soils around the pier, sort of like a huge bollard. We're having the design done by a consultant with experience in landslide mitigation.

 

[aeoliantexan]

A large landslide attacking a bridge across Lake Oahe about 45 miles north of Pierre South Dakota was stabilized in the 90s. There is an article by Hunt, Miller and Bump in Third International Conference on Case Histories in Geotechnical Engineering, 1993. There may be other publications. If you can't find those guys, try Dick Davidson.

This is a big problem and probably doesn't have any small solutions.

 

[Mad Mike]

Thanks Aeoliantexan,

I found your reference easily with a search of "hunt miller bump the forest city landslide". It's an interesting case; a little more drastic than what I'm up against but demonstrates your point very well- there are no small solutions with landslides.

HotRod- thanks very much for your response; that is exactly the sort of thing I wanted to hear. I managed to obtain an old cost estimate on a similar system, which showed the cost of the stabilizing system to be 2 - 3x the cost of the bridge piles (all additional cost)- do you think we're in the same ballpark?

All the best,
Mike

 

[BridgeSmith]

I'm not sure how the costs would compare. The main reason for considering a separate 'shielding' system, rather than massive piers, is to isolate the bridge piers from the deflection of the shield. If you're 40-50 feet to bedrock, the deflection of even a fairly massive concrete system would likely be too much for the connection to the superstructure to accommodate.

 

[Mad Mike]

Thanks HotRod,

Your last point is one of my big concerns- I think at this point time I will pass it on over to the detailed design stage; there's currently not enough field data for me to justify overthinking it. I've thrown in an alternative of a deep slope drainage system (stone buttress drains), which will definitely work but at who knows what cost.

Cheers,
Mike

 

[BridgeSmith]

If the slope of the underlying bedrock is steep, a tied-back system (post tensioned rock anchors, etc.) could be an option to support a pier shielding system, also.

 

[Mad Mike]

That's a good suggestion HotRod- I will keep it in my back pocket. I know the budget is going to determine what is ultimately used...the cost of 1000kN anchorage points very quickly becomes scary, and to make matters worse my slope is relatively gentle. I will need to run a very detailed slope model at the next stage of design.

Thanks for everything,
Mike

 

[aeoliantexan]

Before the Forest City slide was stabilized, the bridge abutment moved with the big slide about 10 inches towards the bridge and closed the expansion joint. The state added a short span as a way to install another expansion joint. I don't recall whether the abutment was pile supported.

Perhaps you could let the bridge girders and deck slide across the abutment and just cut away some of the approach fill occasionally

 

[Mad Mike]

Thanks Aeolian- I'm not sure I've reached that level of testicular fortitude yet- I want a bridge that does not move. More importantly, I want to know the industry standard (if such a thing exists) for dealing with these things...I think an anchored shielding system or large elastic caisson foundations would be the cutting edge.

All the best,
Mike

 

[BridgeSmith]

I don't think that situations of this type are common enough for there to be any kind of industry standard solution or approach. That said, there are several strategies and innovations that can be applied. Unfortunately, all of them are going to be on the expensive side compared to typical bridge, or even a typical landslide mitigation.

 

[Mad Mike]

Agreed; I'm happy with the outcome of this post- thanks guys.

 

[aeoliantexan]

Mike,
I googled "bridge expansion joints" and came up with a firm called USL that has expansion joints that will accommodate up to 5 feet of movement. If you can put your abutment on a footing or short piles that will travel with the slide, this might be an economical solution. Of course, if the slide has a sizeable vertical component at that location, it gets more complicated.

 

[BridgeSmith]

That's a nice thought aeoliantexan, but presumably the slide is perpendicular to the bridge. The expansion joint would have to accommodate longitudinal movement from thermal expansion and large lateral movements from the slide. You also have to somehow correct the misalignments between the approach roadway and the bridge that will develop periodically.

 

[Mad Mike]

Guys, thanks again- articulating the bridge could come into play; as Hotrod notes though, accommodating (or even fully quantifying) movement in 3-dimensions is where it becomes tricky; there will certainly be lateral, oblique and vertical movements.

It's good to know there are those kind of joints out there though- I'm going to dig into some research on the subject right now!

All the best,
Mike