Short H-Piles to Bedrock 1

[3Fan]

We are in the type study phase of a small replacement structure. More than likely the bridge will be a single span prestressed box beam on integral abutments (single row of piling). From the geotech report, rock will be about 9' from the bottom of footing. The abutment from bottom of footing to the roadway profile grade will be around 10.5' tall.

Are there any ROT or guidance on when an end bearing pile should not be used with a short embedment depth?

We have run some preliminary numbers in Lpile and the results show about 1/2" of lateral movement in the pile at the top of rock (bottom of pile.)

Thanks,
Greg

 

[crossframe]

You will likely need to socket the piles and provide an oversized hole filled with pea-stone so the pile can bend like you want it to. Many states use these types of details for short pile lengths.

Another alternative is a semi-integral abutment, where the stem is integral with the superstructure and has a "pinned" connection at the top of a spread footing bearing on the bedrock. This resembles a three-sided rigid frame and was common in certain parts of the United States thirty or so years ago.

The depth of existing ground to bedrock will be an important factor in deciding which option is more appropriate.

 

[Qshake]

Piles are required by AASHTO to be a minimum of 10'. So in cases where piles are used and depths shorter than 10' are encountered, and where feasible, rock sockets or pre-bored holes are used to achieve the required length.

Many times, I have used a detail to concrete the piling in place in a pre-bored hole. This is very helpful and is not too expensive but there are other means such as loose sand placement.

crossframe - all integral endbents/abutments are assumed pinned for the purpose of analysis. The pin is taken at the abrupt change in stiffness between the pile and pile cap. As such, I don't know that a semi-integral abutment would be beneficial in the OP situation over a conventional integral abutment. With the use of neoprene pads the bridge will have to overcome shear stiffness to provide movement that is not alledgedly transferred to the piling. For low neoprene pads, the shear stiffness can be large.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BigH]

For 3 m, why not simply dig out to rock and backfill with sand and gravel and use spread footings rather than piles/pile cap? This may be cheaper than bringing in a pile driver for such a small number of piles.

 

[Qshake]

BigH - not sure what the original poster's application is but many of the areas we've dealt with this are in locations of pinnacle rock and or karst topography. So in some cases we can drive a pile 30' next to one that may be 8'.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BigH]

Qshake - I understand your point and it is quite valid - the only thing I could go on was that bedrock was "9 ft" below the footing. In this case, I would think that excavating to rock, backfilling with crushed stone and compacted up to the depth of frost penetration, then put on the spread footing would be quite reasonable rather than bringing in a piling contractor.
As an aside to your point, one of our Senior Engineers when I started up had his first drill job up in Iroquois Falls, Ontario. He was working for the Ministry of Highways. He did the first borehole and hit rock at 10 ft. (old wash boring days). He called his boss and said he'd be home in two days after he finished the second (and last) planned borehole. He ended up going home 2 weeks later - the borehole to bedrock was in excess of 100 ft!!

 

[3Fan]

I guess I should have given a little more detail. Sorry.

The structure will be crossing a stream, so scour is a concern with spread footings. Also digging out down to the rock and then backfilling with crushed stone would require quit a bit of dewatering or temporary stream relocation which I'm sure the co. engineer isn't going to look favorbaly on.

The rock is a clayshale.

Thanks for the insight folks!
Greg

 

[DRC1]

We have done a feww of these short piles. The unit price is exeedingly high as there is lot of time mobing, welding tips, cutting offf, etc to actual driven lengths. Batter piles have a tenency to walk over time, esp if there is a slight slope to the rock. This can be enhanced by sockets, but that gets very expensive.
Generally the best bet is to put in a cofferdamand put the foundation on rock, with bars grouted into rock if needed.

 

[graybeach]

If scour is a concern, Short piles without sockets will not alleviate it. DRC1's suggestion of putting your footing directly on the rock is probably the way to go.

 

[cvg]

depending on size of stream, flow rate and velocity; and soil types etc. - 9 feet of scour at a pile is not unheard of. In fact, the majority of bridge failures over the years are due to problems caused by excessive scour. Whether you install piles or spread footing, either one will probably need to be protected from scour. This may mean excavation and placement of riprap or concrete or other measures in the river to control it. So it sounds like stream diversion and the associated environmental impacts may be unavoidable.