What Is Under Your Yard in Northwest Arkansas, and Why Does It Change the Build?

Because everything you build outside sits on it, and the ground here is not the same from one town to the next. A patio, a retaining wall, a pergola footing and a pool shell all transfer their weight and their water into whatever is underneath. If that material moves, holds water or hides a void, the finish on top does not matter.

Most hardscape advice online is written for a generic yard with generic soil. Northwest Arkansas is not generic. Two towns twenty minutes apart can be two different engineering problems, and the same construction detail that works perfectly in Rogers can move in Fayetteville. Knowing which ground you are on is the first thing we establish on a site visit, before anyone talks about materials.

The Springfield Plateau is the limestone upland that covers Benton County and the northernmost part of Washington County. Its surface rock is the Mississippian Boone Formation, limestone interbedded with chert, the hard flint-like rock you find in every flower bed north of Fayetteville.

The Arkansas Geological Survey describes the Boone as probably the most widespread and karst susceptible formation in the Ozark Plateaus, with abundant sinkholes, pits, springs and caves. Karst simply means rock that dissolves, so water has been cutting channels and voids through it for a very long time. Where the limestone has dissolved away, what is left is red clay containing chert fragments. The Survey notes that sinkholes typically hold exactly that.

Towns on the plateau include Bentonville, Rogers, Springdale, Bella Vista, Centerton, Cave Springs, Lowell, Pea Ridge, Gravette, Highfill, Siloam Springs and Tontitown. Tontitown is in Washington County but still on the plateau, which surprises people. Springdale straddles the county line.

The Boston Mountains start around Fayetteville and run south, and they are built of sandstone and shale rather than limestone. The shale that matters most is the Fayetteville shale, named for the place it outcrops.

University of Arkansas geology professor Walter Manger summed up the difference in an interview with the Northwest Arkansas Business Journal: "Limestone is good. Shale is bad." His explanation was that "shale expands and contracts, caused from the clay's weathering." Wet it and it swells. Dry it and it shrinks. Repeat through an Arkansas year of wet springs and dry Augusts and anything sitting on it moves.

Towns on this ground include Fayetteville, Farmington, Prairie Grove and Elkins. Hills are the other feature, and on shale a slope adds downhill creep to the swelling and shrinking.

The table below is the general picture, and it is a starting point rather than a survey of your lot. Local conditions vary, fill from earlier construction is common in newer subdivisions, and the boundary between the two grounds is not a neat line.

It makes excavation the line most likely to be wrong on a quote. Rock that turns up a foot down when the plan assumed three changes the method, not just the hours. A mini excavator that sails through clay stops at chert, and breaking or working around rock is a different job with different equipment.

It also makes the dig uneven. One end of a patio can be solid rock and the other end a pocket of soft red clay that used to be limestone. Build a uniform base over a non-uniform subgrade without dealing with the soft pocket and the patio settles over that one spot. The fix is to find the soft areas during excavation, dig them out and replace them with compacted aggregate.

Then there is the water. In karst, water finds a route underground, and it does not consult your site plan. Springs appear after heavy rain, and runoff that disappears into the ground on one lot reappears somewhere else. Drainage on the plateau has to be designed around where the water actually goes, which a February site visit with the leaves off shows far better than a July one.

The good news is that sound limestone is excellent bearing. Where a footing lands on solid rock, it is not going anywhere. That is exactly what Manger meant.

It moves, so the job is keeping water away from it. Shale that is kept at a steady moisture content behaves. Shale that is soaked by a downspout every spring and baked every August swells and shrinks, and a slab or paver base laid directly on it goes along for the ride.

In practice that means excavating deeper than the minimum, putting a properly compacted aggregate base between the surface and the shale, sloping every surface away from the house, and taking roof water somewhere it cannot soak back under what you just built. ICPI guidance for paver bases notes that greater base thickness is often used over expansive soils. On shale that is not an upgrade, it is the job.

Footings for covered structures and pergolas need the same thinking. Taking them down to stable material and keeping the ground around them drained is what stops a post from moving independently of the roof it carries.

Manger's advice for anyone buying on a Fayetteville hillside is worth repeating exactly: "If the trees are straight up and down, there's no problem. But if the trees are tilting, you've got a problem." Tilting trees on a slope point to ground that is creeping downhill. Take it seriously before designing a retaining wall into it.

Each element meets the ground differently, so the ground changes each of them in a different way.

• Retaining walls. On the plateau, rock can make the footing trench and the geogrid layout harder to dig. On shale, water behind the wall is the enemy twice over, because saturated shale both weighs more and swells. Drainage aggregate and a drain line that daylights are not optional on either
• Pools. On the plateau, rock is the headline: the shell has to go somewhere, and what comes out of the hole decides a large part of the programme. On shale, what matters is the deck and the ground around it staying still as the seasons change
• Footings. Sound rock is ideal bearing. Soft clay pockets and shale need the footing designed for them rather than a standard hole
• Patios. Plateau patios fail over soft pockets and hidden water. Shale patios fail from subgrade movement. Different cause, same cure: a deeper, better-drained base

It moves the parts of a quote that nobody sees, which is why an honest quote on this ground has to be written after a site visit rather than before one.

On the plateau, the uncertain line is excavation. Nobody can see through the lawn, and chert at eight inches versus clay to three feet is a different day of work. A sensible quote says what depth has been allowed for, what happens if rock turns up sooner, and how that is priced. A quote that is silent on it has not removed the risk, it has handed it to you.

On the shale, the uncertain lines are base depth and drainage. Doing them properly costs more than doing them to a national minimum, and it is the difference between a patio that stays flat and one that rocks. If two Fayetteville quotes differ a lot, start by comparing what each one puts under the surface.

Weather affects both grounds in the same way. Saturated clay and wet shale cannot be compacted properly, so a wet spring delays more digging here than a cold winter does. Dry cold is good working weather. The National Weather Service records for Fayetteville put the average first hard freeze around the start of November and the last around the start of April, and a lot of good ground work gets done in the dry spells between them.

None of this is a reason to be nervous about building. It is a reason to buy from someone who has dug in both grounds and will tell you, before the machine arrives, what they expect to find and what they will do if they are wrong.

Start with where you are, then look, then dig carefully. Your town tells you which ground you are probably on. The lot itself tells you more: exposed chert and rock in the beds, a spring or wet spot that never dries, a depression that holds water, or trees leaning on a slope.

Before anyone digs, Arkansas 811 has to be called. It is free, and utilities need two full business days to mark their lines. On a site visit we look at grade, water, access and whatever the ground is showing at the surface, and on anything large the first excavation tells the rest of the story.

No sand is found naturally in Northwest Arkansas, either. It is brought in by rail, which is one more reason the base materials here are specified and bought rather than dug from the next field.

If you want someone who builds to the ground rather than to a standard detail, call (479) 379-2226. Projects start around seven and a half thousand dollars, and financing is available through Acorn Finance.