Drilled Shafts socketed into bedrock

[mfio]

Anyone has any experience with resistances to uplift forces of drilled shafts socketed into bedrock. How much critical is to consider rock mass uplift failure for a 3 feet diameter shaft with 8 to 10 feet of overburden soils? Typically I analyze for both rock/concrete socket length bond, and rock mass uplift failure (i.e. like analyzing for a rock anchor). Any comment? Input?

Thanks for any help on this.

 

[FixedEarth]

If it is expansive rock you need to consider uplift forces. See F.H Chen's "Foundations on Expansive Soils". He has a method of determining uplift load and with-holding force based on your loading and swelling pressure of the material.

 

[dik]

I've designed socketted caissons in rock using both the end bearing and the friction due to adhesion to rock. Don't recall the friction value, but thought it was quite high at the time. socket was approximately 6" smaller diametre than the main shaft. Was used for compression only and socket was in excess of 3' long... was a long time ago and I don't recall much of it... reinforcing did not extend into the actual socket.

Dik

 

[molerat2210]

I am not sure from your post where the uplift force is derived from?
Is the uplift force generated by expansive forces in the rock?
Uplift from expansive soils in the overburden?
Or, an uplift force applied at the top of the shaft?

 

[hokie66]

I've used a lot of rock socketed piles/bored piers. Consideration was given to uplift from the material above the rock, and from any applied load at the top, but have never thought about uplift from the rock itself. Maybe different type rock than I have experienced.

 

[mfio]

Sorry, probably my post was not clear. The problem is:
- 3 feet diameter drilled shaft socketed 5 to 10 feet into bedrock
- shaft has to resist an uplift force of approximately 200 kips
- subsurface conditions consist of approximately 8 to 13 feet overburden loose soil over bedrock. Groundwater very shallow.
- I analyze the concrete/rock bond strength for the lateral surface of the rock socket.

The question is: should I also consider a possible failure within the rock surface (designated as Rock Mass Uplift Failure), like happen when analyzing a rock anchor. Typically for rock anchors I assume an failure surface within the rock in the shape of an upward cone (60 to 90 degree angle at the cone tip). I was wondering if I should consider also this failure when designing drilled shafts socketed into rock.

Thank you all for your comments.

 

[dik]

If length is not great, can you use a couple of rock bolts... Williams, or something similar and get some 'real' anchorage...

Dik

 

[Qshake]

We use rock sockets quite abit in the central Midwest (USA) and typically can get about 75 psi on good solid rock. That said, the difficult is finding good solid rock. Though solid rock is definitely found all over many drillers/contractors don't want to drill 40' of socket to find it. The upper layers of rock are usually fractured and have seams of varying material (clay, shale, etc). I believe there are some references that provide guidance to the effect of subtracting out the seam of unwanted material to attain a 3' or 5' socket.

Regards,
Qshake

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

 

[Qshake]

And as Dik notes, combining drilled shafts with rock anchors (rock bolts) is not uncommon but one needs to do a cost analysis. The rock anchor will require a long development length that will likely surpass any loose "shelf" rock where as the rock socket will be shallow. We've had applications in the past where we've used drilled shafts to rock and then used rock anchors to deal with uplift.

Regards,
Qshake

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

 

[hokie66]

mfio,
I don't have any advice about the cone type failure you are contemplating, other than to say that intuitively it wouldn't happen. The confining effect of the overburden would extend the cone well above the rock. With conservative assumptions about side wall friction, withdrawal of the pile by failure at the concrete/soil interface would be the controlling case, I think.

The nature of the load would determine whether I would depend on tension piles or instead use rock anchors as others have suggested. If the load is transient, then the tension pile would be appropriate. If the load is constant, especially if with a vibratory component due to some type of equipment, then I would lean toward rock anchors.

 

[VAD]

mfio:

First I see nothing incorrect with your approach using a cone of rock to evaluate uplift resistance. In design, this ia plausible. The use of rock anchors below the drilled pile is also geasible and can perhaps be cost effective. You may wish to review the texts "Foundations in Tension" by Hanna and "Pile Design and Foundation Practice" by Tomlinson. I have noted that with embeddment depths greater than 1.5 m the failure due to pullout is governed by the bond between the pile and rock. One requires also to assess the characteristics of the rock re the various indicators of quality and use judgment to make a final decision especially where uplift resistance will govern the design. The references provided in these texts would be worthwhile to review.

 

[mfio]

Thank you to all for your replies.

 

[dik]

QShake... I've often used Williams rock anchors with about 6' embedment... they are mechanical and we use a slightly expanding grout to fill the remaining voids... am presently using them on 2 projects...

Dik

 

[Qshake]

Dik - Thanks for the note, my limited experience in the subject is with 10-20' anchors and 20' development length grouted. In that case, the 10-20' is to pass the fractured rock.

Regards,
Qshake

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

 

[molerat2210]

mfio,

I do not think you need any rock anchors as suggested above by others. I am sure they would work fine. You can use the allowable side friction in the socket to resist the uplift. There is not requirement for analyzing uplift resistance of a drilled shaft by rock mass uplift failure like a rock anchor.

The available side friction calculated for compression is reduced for uplift resistance for lateral shortening. I worked on a similar project recently for case where for 270' diameter tanks may be submerged and buoyant when empty. You can add some additional uplift resistance due to weight of the shaft and side friction in the overburden.

 

[reyv]

Uplift resistance is simply the weight of the concrete of the shaft plus the contribution of the skin friction times a reduction factor then apply a factor of safety (or resistance factor for LRFD). For rough surfaces (i.e. rock to concrete), FHWA recommends that generally 80% of the compression skin friction value can be used to resist uplift loads. For smooth surface (i.e. casing to concrete), a much lower factor then 80% is required.

Rey Villa, MS, PE

http://geotech-apps.com

 

[PEinc]

With a 3' diameter rock socket and a 200 kip tension load, bond of concrete to rock is not much of a problem. The bigger consideration is the cone mass for uplift. If the rock is very fractured, you will still have bond but the rock could behave as a pile of gravel. Therefore, I would check the cone of rock and soil above for uplift and use an appropriate safety factor. Also, consider buoyant rock and soil weights if there is any possiblility that the groung water could be located within the length of the drilled shaft. I also do not think you should be using rock anchors to resist the uplift. Doing so just adds cost for more pieces of drilling and grouting equipment. If drilling 3' diameter rock sockets is too expensive, consider using smaller diameter micropiles that could be designed for both compression and tension.

http://www.PeirceEngineering.com

 

[mfio]

Thank you very much for your response and advise.