dear sureshduraikannu, welcome to hcm.
I would strongly recommend you to switch over to spectacles. Since u are a civil engineer by profession, I believe u'll hav to hav a site visit frequently. this always carries risk of dust and minute particulate to come in contact with the contact lenses and always a higher chance of developing a corneal ulcer
., which is very difficult to treat once developed.
the power variation is because of the increased work load and strain on the focussing mechanism of ur eyes. prefer to give breaks every 10mins in between working in front of the computer. stop staring at the monitor every 10mins and focus on a distant object for 20-30secs to let ur eyes shift the focus and reduce the strain.
I strongly recommend u to reduce the brightness of the monitor to the least possible limits everytime.
Use a glass which is big enough to cover ur FIELD OF VISION to almost without any hurdles by the edge of the glasses. U can use Powered Sunglasses for ur site visit.