I did something similar to this to the gravel on Nikko, It's real easy and is in the tdf file. But you need to edit it manually with notepad. Be aware though, each time you export the track from BTB it will overwrite the file if you have it set to clear folder, so make sure to back up any editing you have done to it.
What you are after is the Sink value.
Have a look at the comparison of these two entries in the tdf file, one is normal hard gravel (grvl) the other is a deep loose gravel trap (gtrp). I have set bold the values of interest.
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Here is the original grvl (gravel)
// Gravel
[FEEDBACK]
Dry=0.75 Wet=0.60 Resistance=5000.00 BumpAmp=0.022 BumpWavelen=10.0 Legal=false Spring=0.0 Damper=0.0 CollFrict=0.8 Sparks=0 Scraping=0 Sink=0.017 Sound=gravel
Reaction=dust Tex=Dustroad.tga Max=128 Scale=(1.0,1.0,1.0) Growth=(4.0,3.0,2.0) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=dirt Tex=Dustroad.tga Max=128 Scale=(0.5,0.5,0.5) Growth=(0.8,0.8,0.8) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Reaction=softskid Tex=skidbrown.tga Max=1024 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Materials=grvl
------------------------------------------------
Here is the original gtrp (Gravel Trap)
// Gravel Trap
[FEEDBACK]
Dry=0.75 Wet=0.60 Resistance=50000.00 BumpAmp=0.070 BumpWavelen=3 Legal=false Spring=0.0 Damper=0.0 CollFrict=2 Sparks=0 Sink=0.02 Sound=gravel
Reaction=dust Tex=Dustroad.tga Max=128 Scale=(1.0,1.0,1.0) Growth=(4.0,3.0,2.0) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Reaction=softskid Tex=skidbrown.tga Max=1024 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Materials=gtrp
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What concerns how a gravel or sand trap works is Resistance, BumpAmp, BumpWavelen, CollFrict and Sink.
If you want more resistance, increase the Resistance value (how hard it is to push through the gravel/sand)
If you want big bumps, increase the BumpAmp value (bump amplitude is the height of the bumps) Decrease for smaller bumps.
If you want longer bumps, increase the BumpWavelen value (the length of the bumps) Decrease for shorter bumps.
If you want it to slow you down faster, increase CollFrict (collide friction, I'm not 100% sure what this does but going by it's name, I think it is how fast it slows you when you are moving)
If you want it deeper, increase the Sink value (how deep the car sinks into the gravel).