I don’t know if Donald Trump can build a 1,900-mile wall along the Mexico border. But I do know that I can build a 110-foot retaining wall around my cottage up in northeast Pennsylvania. And I think — emphasize think — it will still be standing next spring after it endures the stress test of what could be another brutal winter.

Trump says a wall will be easy for him, since he’s a builder and that’s what he does. Maybe so. But, based on what I have learned about wall building, I wish Trump a lot of luck with his project (if he is ever in position to move ahead with it).

I didn’t plan to be a wall builder. The old cottage was set a few feet off the ground on nine cinder block piers. It was surrounded on two sides with a wooden deck, that functioned as both an outdoor sitting area and a convenient step-down to the natural terrain of the lot. After the old cottage was torn down (and hauled away in a dump truck) the new cottage was built on a concrete slab. Lo and behold, it ended up at almost exactly the elevation of the old one. From the floor level of the cottage the ground sloped away fairly quickly.

I decided to build a retaining wall about 12 feet away from the cottage, fill in the space between the wall and the cottage with some dirt, and have a relatively level terrace surrounding the cottage. To retain that dirt, I would need a retaining wall. No big deal, I figured, since at its highest point the wall would only be about 28 inches high, at many other points it would be just a little over a foot.

On the advice of a neighbor (who grew up in nearby Susquehanna, Pennsylvania) I consulted a few experts about how best to build such a wall. One thing Trump will discover: All sorts of people will have different opinions on how best to build the wall.

In my case everyone agreed it would be best to build it “dry” — without mortar holding the individual stones in place. Some, though, were sure that the force of the winter frost is such that the wall would require a four-foot deep poured concrete foundation (just like the cottage itself has around the perimeter of that slab). Others said no: Digging down to the original terrain would be enough, with a four-inch gravel base into which the first course of stones could be set. Instead of fighting nature, this approach works with it — let everything go up and down with the frost at once.

You can guess which approach I chose (and did I mention that the four-foot foundation would have cost thousands of dollars?).

But even the four-inch bed of gravel requires a little more material than you first guess — another lesson that Trump no doubt already knows but might be worth keeping in mind. In the case of my wall, I figured it would be a small job. The front of the cottage is barely 30 feet and I planned to wrap the wall around one side about 20 feet and only about five feet on the other. Total wall length: 55 feet. But I needed to figure in the 12 feet the front wall was placed beyond the cottage. The wall’s length increased by 24. And then I designed one funky angle, and in three places I had to curve the wall around some existing trees. Total length: 110 feet.

At that length the requirement of gravel alone (4 inches deep, 18 inches wide, and 110 feet long) came to about three tons. I ordered five tons, used up most of it, and then added another five tons when I realized I wanted to fill the cavity between the wall and the dirt with more gravel.

Everyone agreed that the gravel was the right material for the job. But the choice of material to sit on top of the gravel was subject of debate. I wanted to use natural fieldstone that matched the stone used in the external chimney on the side of the house. Others argued that the only material that would hold together year in and year out would be a system like Versa-lok, a manufactured stone that comes with a hole-to-slot pinning system that enables blocks to be dry stacked without the need for a frost footing. Sounds great, I admitted, and noted that you see it used at shopping malls and corporate centers. Not exactly right for a cottage at the lake. (And not cheap — I saw some website chats that priced the material at between $25 and $50 per square foot — my approximately 220 square feet of retaining wall could cost up to $10,000 or so.

All my challenges of course pale in comparison to a wall the entire length of the U.S.-Mexico border. There has already been an effort to build a barrier along parts of the border, and the experts already have argued about the best approach.

Only about 600 miles of border now have any kind of fence. Most of what now exists is known as “landing mat” fence, described by Popular Mechanics magazine in 2010 as “the oldest border fence still in use. Construction is corrugated steel, usually about 10 feet high” and the name comes from their former use as “portable touchdown pads for helicopters operating in Vietnam.”

The more desirable fence is double-layer fencing, but it is now in place along only 36 miles of border.

Trump promises not a fence but a wall, maybe 50 feet tall — so high it’s tough to climb up and not so easy to get down either. He mentioned that pre-cast concrete slabs could be used. Sure enough, some experts have disputed that, noting that the rugged terrain would pose expensive challenges for creating the proper footings.

Here’s my chance to offer some advice, based on my first hand knowledge. Consider using Versa-lok, since it wouldn’t require a deep footing and its manufacturer claims the interlocking system will support walls up to 50 feet or high. It will be pricey — based on that low-end $25 a square foot it might run nearly $7 million a mile.

That’s about what the Department of Homeland Security has estimated is the cost for the wire fencing currently being used. But the DHS estimate includes design, construction, construction oversight, real estate acquisition, environment planning, compliance and mitigation and contract support.”

My estimate is just for the cost of the Versa-lok. Maybe the Donald — he’s a builder, you know — can come up with an ingenious way to do it less expensively. Maybe he can save on labor. Maybe he knows a source of cheap labor, located not far from where he would build this great wall.

The next Republican presidential debate is Wednesday, September 16. Can’t wait.

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