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If the original engineer did a bad job, you need to put in a formal complaint to his superiors. When ever some one comes on your property (and it is up to you whether they gain entry or not) make sure you get their business card and name and job title so if they harm you in any way, financial or otherwise you know who to hold accountable. Also ask for their indemnity insurance details and this puts them on notice that you will be claiming for damages if anything that they are responsible for goes wrong and they will most likely make sure they do a job to your satisfaction. Film what they are doing so you have evidence. Tell them you are filming for your protection (you have every right to do this on your own property.