Pea gravel on top of hardpan?! I wouldn't remove it because it sounds like you may have to depend on it for providing drainage - in fact, you may want to add more gravel to so that you have 1 1/2 to 2" depth to insure good drainage. Build your lasagna on top, and give it an edging so that you will have extra depth. In fact, I'd give it two edgings - one that rises 6" to 8" above ground level, and another snugged horizontally on the ground for your mower wheels so that you won't add trimming to your chore list. 40 feet is quite a length so start at one end, and expect to do only one or two 3' squares at a time. You will be amazed at the sheer volume of material required to build a proper lasagna -- followed by amazement at the comparatively thin layer of rich soil it becomes after a while.

This is assuming your 3x40 bed is meant for annuals and/or perennials. If you have hopes of miniature or dwarf fruit trees, then yes, you'd better remove the gravel and then pick-axe at least 2 feet down to provide room for the roots to spread. OTOH, miniature trees do reasonably well in humongous pots and very well in raised beds that provide at least 12" inches soil below the root ball.

Meldy,
Thanks for the wise words....! I think the gravel was merely an attempt by the previous owners to hide the plain hard dirt/rock mix that is un-diggable. The gravel is red, in an attempt to match the redwood-stain on the fence.

Last year I broke out my Mantis tiller in an attempt to dig a vegetable bed. What a pain! The tines are indeed unbreakable, however, they did bend and the motor got tourqued out of position...! So I built a couple boxes and had 2 "square foot gardens." These went okay until the neighbor's cats rendered it unfit for human consumption.

I'm planing on laying some type of fencing over the lasaga garden, and looking into the motion-activated sprinkler...

Very astute about the fruit trees... I'm planning for an Improved Meyers and Eureka Lemon, and possibly a Kaffir Lime.... These are going into large pots, only because my wife may landscape in the next year or two.... I'm hoping to impress her with the lasana gardening...

Sorry for the ramble....! Have a great day...!

Beeks

If the gravel is directly on top of the hardpan, don't bother removing it. It won't do anything, good or bad, if left in place, not even improve drainage.

Beyond that, removing the gravel would help only if there was suitable plant-growing soil between the gravel and the hardpan.