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Foundations in boulders, cobbles and large gravels

Foundations in boulders, cobbles and large gravels

Foundations in boulders, cobbles and large gravels


I am getting involved in a project which consists of boulders, cobbles and large gravels. After reaching refusal in the top 6 inches below surface, the drilling firm proceeded with rotational drilling up 30 feet. The same type of materials were recovered throughout the depth of the boring: boulders, cobbles and large gravels.

According to the geology, the site was the bottom of a river millions of years ago, which makes sense.

I have no SPT (just the very first spoon), thereafter I have boulder, cobble and large gravel samples. To the boulders, due to the size of the core (HQ) and length (L > 2D), I could perform UCS tests, even though I know they will have high strength values.

About logging the samples, at least for rock cores, I am used to follow the guidelines provided by "EM 1110-1-2908 - Engineering and design for Rock foundations" (Table 4-1, Summary of rock descriptors) BUT this this is not a rock scenario, so I was wondering how to log these conglomerates. Of course I could include size, color, shape, but was wondering I am missing anything else.

I was wondering if you know any reference that could provide guidance on how to perform engineering analyses for this type of scenario.


RE: Foundations in boulders, cobbles and large gravels

I was involved in a project where the geotech report indicated 'refusal'. The foundation I designed for cantilevered sheet piling went into bedrock. A foundation contractor suffered nearly $20,000 damage to his drilling rig because it wasn't true refusal, but glacial moraine boulders. They can cause damage.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates


RE: Foundations in boulders, cobbles and large gravels

I had a site like that. We used 400 kPa bearing capacity. There were some references used to determine that, but I forget what they were. It was a large shopping centre, and has stood the test of time.

RE: Foundations in boulders, cobbles and large gravels


That is correct. Boulders damage the rigs. Drilling contractors in my area avoid projects involving boulders otherwise they increment exponentially the price per foot of drilling.


The questions is, if you have no data (just boxes with pieces of boulders and large gravels), how did you use engineering judgement to recommend a bearing capacity?. I know they are boulders / large gravels and SPT would be over 100 blows per foot, and based on that could come with a recommended BC, however I am not sure if is the best approach.

RE: Foundations in boulders, cobbles and large gravels

Occasionally you’ll run into a project with a difficult issue to address. When there’s a big question mark that leaves a lot of risk to be assumed by someone, you have two approaches.

The first would be to do more explorations at the owner's expense. Make sure they understand that it’s an incredibly difficult site to drill and the costs of drilling isn’t fully known but give a conservative estimate to understand the potential costs. Drill the site even if it you get 15’ of progress per day. Drill until you feel comfortable with providing recommendations that you can rest your hat on at the end of the day. This approach must be taken if it’s a critical structure (hospital, Highway bridge, ect.)

The second option would be to shed all risk onto the owner. If they don’t want to pay for an expensive subsurface exploration program for you to fully understand the on-site geology so you can provide legitimate recommendations, they have to assume all risk of potential movement and distress of their structure(s). Get it in writing. I’ve done it this way when I perceived the risk to be low by considering the proposed structure and the size of the unknown. The owner was happy with taking the cost savings as the other option would have killed the project.

RE: Foundations in boulders, cobbles and large gravels

Regarding logging, log exactly what you see. That is all you can do.

If you drill through a boulder then log it as rock accordance with BS5930 or equiv ASTM.

You can just add a note that you believe any rock encountered is likely boulders/cobbles and not a continuous bed or rock.

Can you upload core pictures?

And what type of structure is being built here? this greatly dictates what level of judgement you need to apply

RE: Foundations in boulders, cobbles and large gravels

I had a surprisingly similar site on the side of a volcano recently (but on a fan-type deposit believed to be from the collapse of an edifice and then perhaps glacially / fluvially modified). The core was like that except volcanic rocks, and all of our SPTs had very coarse sand / fine gravel recovered but none inbetween (believe the water was washing this portion out or it was collapsing into the bottom). I just stated I was pretty sure that based on the geology and inference of what we saw in the boreholes that they are boulders / cobbles in a sand / fine gravel matrix and not bedrock and that the SPTs are mostly meaningless. Although a different consultant for a different part of the building did a scala, refused at 0.3m and said 'see! bedrock!'.

We used sort of moderately conservative gravel / rock parameters (ours was angular - I guess yours may be more rounded particles?). I'm not sure what else could really be done. You could also try maybe geophysics (MASW), or try for some full scale or surface testing - e.g. plate load testing (although obviously that is shallow) or like a shallow footing bearing test or a trial load of soil.

Has anything else been built in the area? I used to work in mountainous regions in Canada in the Rockeis with deep alluvial / glaciofluvial filled channels and often we had to sort of frame ourselves against, well, there are a hundred other 3 storey apartment buildings here on footings of roughly the same size designed with 300 kPa ULS bearing. Could never get anyone to pay for shallow footing testing or MASW but we did manage to get some test piling done and the results implied reasonable strength.

RE: Foundations in boulders, cobbles and large gravels

Man you have some great ground. I wouldnt be worried at all. My only issue is I presume you have more than one BH to prove similar ground conditions.

You could easily assume 300kPa allowable, not ultimate IMO. You are putting an electrical substation down, whats your bearing pressure. I imagine you are not more than 150-200kPa.

If you are worried you could do a 3m x 3m zone load test, but thats gonna be expensive.

RE: Foundations in boulders, cobbles and large gravels

alluvial / fluvial material can be highly variable. you could just as easily hit a layer of silty sediment along the edge of your paleo-channel. you definitely need more than one boring. maybe a test pit would be a better method

RE: Foundations in boulders, cobbles and large gravels

My teeth hurt just looking at that box.

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