Rock anchors for a bridge pier 2

[VFF]

Reading previous posts, I have a situation where the anchor to rock interface is going to be OK, but may have a problem with the weight of the rock pullout cone providing enough stability.

I am drilling into basalt bedrock that is fairly solid. I am new to this field, so two stupid questions: i)Is the shear strength of the rock normally taken into account or just the dead weight? and ii) Given there will be a river flowing over these bridge piers, would it be typical to use a bouyant weight for the bedrock? What sort of fissures would be required to allow the water pressure to get in there?

Many thanks.

 

[Panars]

There are two publications of which I am aware that are available on the internet and address rock anchors as tiedowns. The first is Ground Anchors and Anchored Systems (Geotechnical Engineering Circular No. 4) from FHWA. It can be found at

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

The second is the US Army Corps of Engineers manual on Rock Foundations (EM 1110-1-2908). It can be found at

http://www.usace.army.mil/inet/usace-docs/eng-manuals/cecw.htm

I do not typically include the shear strength of the rock along the surface of the cone. However, I am also usually dealing with fractured rock. The Corps indicates that for intact rock, you could consider the shear strength of the rock. For resistance calculations based on the unit weight of the rock, the Corps uses a factor of safety of 1.5. For all other resistance calculations, the Corps uses a factor of safety of 4.0, unless you have specific in-situ testing to justify a lower safety factor.

The FHWA manual says that for anchors in strong rock, the failure typically occurs at the tendon/grout interface.

Regarding your question about the bouyant weight, I would say use the bouyant weight of the rock.

 

[VFF]

Thanks for that Panars. I got hold of the US Army paper OK, but had trouble downloading the FHWA copy.

I realise I have probably used some incorrect terminology, as my reinforced concrete pile (800mm diameter) is actually a rock socket, and in tension.

My basalt is substantially stronger in compression than the concrete, and should also have a much higher shear strength. Presumably this is related to compressive strength in a similar manner to concrete.

Given what has been said and what is in the papers, does this mean that if my rock has a greater shear strength than concrete (after factors are applied, being more conservative for rock than concrete), that a pullout cone could never actually occur as the surface area of the cone (which would also use the higher shear strength of the rock) is always greater than the concrete/rock interface surface area?

ie the cohesive 'glue' holding the rock to rock is always greater than the rock to concrete 'glue' so the pile must fail at the concrete/rock interface?

Thanks again.

 

[sqrt]

Hey great reply Panars. I've considered rock anchors and have opted out, so I haven't actually specified them, but I have worked with rock on several occasions (with consultation of a geotech).

Don't get rock and concrete confused. Concrete is an engineered material that has much more predictable properties. Also remember that with hardness comes brittleness and that means crack propogation.

How much can really be determined by a couple cores?

 

[Panars]

VFF-

I can't say for sure in your specific case, but I would say generally you are correct that if your rock shear strength is greater than the concrete strength AND the rock is generally not fractured, then the pullout cone would not develop. You will probably get failure at the concrete/rock interface.

It sounds like you are actually designing a drilled shaft for uplift, not a rock anchor. Although the principles are generally the same, the details are different. I have no experience drilling in basalt, but wouldn't drilling a 0.8 meter hole in basalt for a significant length be extremely difficult? I wouldn't want to have to go more than, say, 10 feet into hard competent limestone we have in Ohio, USA, with a diameter like that.

 

[BigH]

VFF - in any rock socket/rock anchor design, you need to check the following cases:
1. structural strength of anchor/bolt or steel shell
2. grout/tendon (for anchors/bolts)
3. grout to rock (or concrete to rock)
4. cone pullout (usually dead weight with no allowance for shear strength/tensile strength on the cone surface - although, in special cases some allowance is permitted).
You check all cases to determine which is critical and design for the critical one. Panars makes good point about advancing the socket. I churned drilled, once, for many hours for token advancement in on Ontario limestone.

 

[VFF]

Thanks for pointing me in the right direction Panars. I finally downloaded the FHWA drilled shaft guidelines, which is the exact scenario I have. Interestingly, I only found indication to apply a factor of 0.7 to the shaft resistance capacity when in tension rather than compression, to take into account the stretching of the pile reducing the contact area. Not a mention of pullout cones to be seen!

Not being from the US, I take it that the FHWA carries some authoritative weight. Do its guidelines override State road authorities and/or codes?

Hope everyone had a great festive season.

 

[Panars]

FHWA guidelines do NOT override anything. The State DOT's usually rely and modify the AASHTO specifications. Where AASHTO is silent on a subject, the state will usually look to FHWA for guidance in developing their own specifications. The FHWA has some oversight of the State DOT's in that they control the federal money that the states get. Other than that, FHWA's role is that of an advisor to the state DOT.

 

[vulcanhammer]

If you still need the FHWA publication, it can be found at

http://www.vulcanhammer.net/download/retaining.php

as can the Corps one as well.