Well you have 2 issues:

first off the heat pad can't heat through that much substrate (needs to be 1/2" or less)

Second you do need a thermostat either get one or unplug the heat pad there is documentation of them melting plastic and getting to 130 + degrees (if you want proof of this put the thermometer probe on the bottom of the cage directly over the heat pad.

Simply having all that substrate won't protect you. BP's will sometimes dig down into their substrate then they are on the bottom of the cage where it is too hot. they get burned and you have to go to the vet or worse your snake dies.

Here are 2 good thermostats don't waste your money on the thermostats sold in pet stores as they are junk.

The hydrofarm thermostat, sold on Amazon for $30 these will work but lack accuracy and safety features. these should be replaced with a better thermostat when possible.

http://www.amazon.com/Hydrofarm-MTPR..._bxgy_ol_img_b

The herpstat. These cost more but are extremely accurate and have multiple safety features built in. (starting at $99)

http://spyderrobotics.com/