PM with your email address and I will send you a copy of ours. In addition to this document, when we are going to be hand-digging, we give an hourly rate and we are very upfront with the client that we have no idea what the soil conditions are. We give them an idea of how long it should take and an option for a NTE - not to exceed. Good clients will understand the situation - bad clients won't, and so it's a great indicator of the caliber of client they will be and whether or not you want to cater to them.
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Doesn't exist.... at least not one which would protect you fully. A SIMPLE disclosure/statement noting potential for "unknown" ob structions and additional costs would work just fine. The key here is to manage your customers expectations UP FRONT.... not after the fact.