Soil Stiffness

Dear Contributor,

I do respect your Father. For his sake you should follow his advices. Your question is too vague.

You should indicate what it is you intend to do with the soil and no less importantly what is the state of the soil, what it is doing or not doing. Am sure others as well as myself ( for sure I will do it freely, I can not speak for the others ! ) will be able to help.

Labor

To the unidentified , curious guest: The question is clear & concise, However, to satisfy your curiosity, our drilling rigs are portable & work on all soil types, from soft muskeg , to mountain rock. Structural analysis of these rigs has always assumed they work on solid ground. I would like to run analysis on varying conditions so as to determine rig limitations in soft conditions.

As a mechanical my best resource is Marks Handbook, but the only soil information there is safe bearing capacity of soil. I need a table of stiffness data.

I can hear it all now; there's a thousand stiffness gauges available on the web. Well I don't want to have to go out to collect a bunch of data that I'm sure already exists.

Soil Stiffness classes are defined differently for cohesive and non-cohesive soils. The stiffness is related to properties of soil strength and density. Soil strength is based on the non-cohesive stength, angle of friction, and the cohesive strength

what question are you trying to answer? Will the rig be stable? will it sink? will it tip over?

There is no accurate general answer for any soil type or types--because it is so highly variable depending on specific site conditions.

General stability concerns would be related to surface bearing capacity under the 'drill' footprint/supports. Tipping resistance is more difficult because sometimes the soil will support a distributed load adequately, but will shear on additional load exerted by an overturning moment and allow rapid and large subsidence under the 'overloaded' support.

Sorry I can't provide specific rules of thumb.

The soil bearing capacity will vary widely. Your rig will not be supported in some soils such as disturbed ("unconfined") clays, organic material ("peat bogs"), or saturated pemeable soils (sand) with high ground- water table ("quick sand"). You will need railroad ties or some such devices for the rig or you will lose the rig.

Glacial till, on the other hand presents the opposite problem. Harder than a rock.

The question is quite vague so the answers are many.

eriew

With rig design we have always treated reaction supports under the substructure as fixed. I have new structural analysis software, and the software developers make provision for spring reactions. They recommend treating ground reactions under the substructure as springs, because true reactions aren't fixed. "Give" at reaction points will certainly reduce the static limit of various structural members in the Rig. Our rigs are always set on timber & steel matting for flotation. If one corner sinks we jack & level the rig. No, tipping is rarely a concern. But, flexure in the matting will permit deviation at the reaction points, not normally taken into account.

When stuck in the hole, or for whatever other reason, drillers like to pull to the maximum static rating of the rig. If I could utiilize the spring feature of the software, I would have a better understanding of rig limitations due to ground conditions. In the past, drilling on muskeg was only done during winter drilling programs. Now muskeg leases are fully matted and we drill on them year round, with water barely under the surface of the ground. Every lease will be different, but if the driller asks how much he can pull, adding lease conditions to the criteria would be of added benefit.

The software requires soil deformation in force/length units. Force/Length³ units times the applied cross sectional area would give the rq'd force/length. However, I see that the Published Modulus of Soil Reaction are given in psi. I'm still working on the units, but I believe the numbers I'm looking for are Modulus of Soil Reaction and as the Jman said, They're scarce.

I appreciate your help. We've been matting rigs for year. The problem isn't how do we support them. Rather, what are the limitations of a supported rig.

Dear Silver Ghost,

Am very happy for you that you are receiving highly technical and well thought out advices. Without a " I told you so " to give you a specific answer is almost impossible, there being so many imponderables.

From my past experience in drilling in mining and in piling for foundations (bridges, wharves and heavy foundations) and unlike many other Engineers I have never abandonned the safety and security of : pile test and pile driving when you are uncertain of how the soil will react. More so when the type of soil is likely to be lubricated and made unstable by :rains and / or floods. Whilst I realise that cost factors may materially affect your operations, may I suggest two systems that you may have overlooked. Both are relatively cheap:

1.The Menard system works very well is areas that are soft , and those that have a high level of water. It is made up of a flat steel drop hammer ( You can make one up yourself by using a 20 foot steel flat ex-container yard. Removing the uprights, welding four heavy duty lifting lugs.Setting two beds of heavy duty welded mesh welded on the inside of the steel frame and pouring high density concrete so as to reach an all up weight of not less than 8 to 10 tons. Once this is ready a suitable crawler crane can simply lift the rammer and drop it from the highest point . It is pretty messy if mud is available, but the method is absolutely fool proof. There will be little give in the soil after a good pounding by this method.

2.The second method I would suggest is to use: steel screw piles . These are widely used in Australia and are very effective. If you have a 30 tonnes excavator ( a smaller one can also do ) I will send you separately details of Australian manufacturers who produce the hydraulic motors and quick hitch fittings that can be connected to a standard machine. The design of the crew piles is very simple and all you need are good welders to fabricate them. This could offer you the opportunity of doing foundation work as well !

These are not " software " methods but perhaps you may be attracted by their simplicity and might I add : reliability.

Labor

Can you confirm if 12.5MN/m is correct or should it be 1.25MN/m. 25,000N/m2 x 50 (i.e. 1000/20) = 1,250,000N/m or 1.25MN/m.