Retaining Wall Shear Key Design Step by Step - 10 ft CMU Wall with Driveway Surcharge (TMS 402-22)

Intro

0:00Retaining wall design. A ten-foot concrete block wall under a driveway, with a stepped stem and a shear key, designed step by step in StructSuite.

The project: a 10 ft wall under a driveway

0:09Here is the project. A new driveway is cut along a slope beside a house, and a ten-foot block wall holds it up. The driveway runs along the top of the wall, so it adds a surcharge. Below, the yard will be landscaped, and planting beds may be dug a foot deep, so that foot of soil in front cannot be trusted for passive resistance. The wall is backfilled with imported clean gravel, drained, over native sand. There is no soils report, so we use the IBC's presumptive values.

0:35The site is in Seismic Design Category C.

What you will get

0:42Here is what you will get by the end of this video. The earth and surcharge pressure, the passive pressure on the shear key, and the bearing pressure. And the reinforcement detail of a stepped block stem. Now let's build it, input by input.

Design basis: sliding & stepped stems

0:57The codes: the IBC 2024 and ASCE 7-22 for the loads and stability, TMS 402-22 for the masonry stem, and ACI 318-25 for the footing and the key. A tall wall is where sliding becomes hard: friction and passive soil in front must hold it, with a safety factor of one and a half. And a stem that steps down must be checked twice, at the base and at the step, including where its bars may stop.

Step 1 — Geometry & stepped CMU stem

1:27Step one, the geometry, per foot of wall. The stem is ten feet tall, and it retains ten feet: the eight-foot grade change, plus two feet of soil over the toe. A concrete block stem, solid grouted, designed to TMS 402-22. At the base, where the moment is largest, twelve-inch block. The moment drops fast up the wall, so the stem can step down. The hint explains the rule: a stem stepped once is checked at the base, and again at the step. Trial one steps at four feet, to eight-inch block above.

1:55An eighteen-inch footing, a two-and-a-half-foot toe, and a five-foot heel: a footing seven and a half feet wide, a bit over half the total height. Two feet of soil over the toe, a level backfill, and no shear key, for now. The cross-section shows the step on the toe face; the earth face stays flat.

Step 2 — Soil & disregarded passive

2:15Step two, the soil. Behind a tall wall it pays to import a clean, free-draining backfill. This one is a well-graded gravel, GW, so IBC Table 1610.1 gives thirty psf per foot, active. One hundred twenty pounds per cubic foot, drained. The native soil under the footing is sand, so Table 1806.2 class four: two thousand psf of bearing, one hundred fifty psf per foot of passive, and a friction coefficient of zero point two five.

2:43The yard in front of the wall will be landscaped, and planting beds can be dug a foot deep. So the top foot of soil over the toe should not be counted for passive resistance. Enter one foot here. Now the app asks how to read that. The book icon explains: if the soil may be removed, it is excavated, and the passive pressure restarts from zero at the lower surface. If it stays, but loose, it is left in place: its own slice gives nothing, but its weight still presses on the soil below.

3:10Excavated is always the lower number. Planting beds can be dug out, so we choose excavated.

Step 3 — Driveway surcharge

3:18Step three. The driveway at the top of the wall puts a uniform surcharge on the backfill. Tick the box. For this light residential driveway, we use one hundred pounds per square foot. Is it variable, or permanent? Cars come and go, so it is variable. That matters: its push on the wall always counts, but its weight on the heel is never counted as holding the wall down. A permanent surcharge, like a slab, would count both ways.

3:41The site is in Seismic Design Category C, so no seismic earth pressure. The other videos in this series show a wall where it is required.

Step 4 — Masonry, footing & bars

3:50Step four. The masonry: f prime m of two thousand psi, grade sixty bars, one-hundred-twenty-five-pound block, fully grouted, and the bars at the earth face. Number six bars are larger than number five, so TMS asks for two inches of masonry cover. Seismic Design Category C. The book icon quotes TMS Chapter 7: in category C, a wall spanning vertically needs at least one number four bar every one hundred twenty inches. Step five checks it.

4:17The concrete footing: three thousand psi, grade sixty, one hundred fifty pounds per cubic foot, three inches of bottom cover and two at the top.

4:30Stem bars: number six at sixteen inches, every second cell. Above the step, the app asks which bars continue. Every second one, so number six at thirty-two inches; the others stop at the step, and Step five checks where they may stop.

4:46In the footing: number six at twelve on top of the heel, ending straight. Number five at nine along the bottom of the toe, enough for the minimum steel of an eighteen-inch footing, with a ninety-degree hook at the toe end. And number four at twelve along the wall.

Step 5 — Trial 1 fails: sliding & bar cutoff

5:16Step five. Trial one fails twice. Sliding, at one point two five. And the upper stem, above the step. Open the sliding row. Passive acts on the soil and the footing in front: two feet plus one and a half, less the foot we disregarded, is two and a half feet. That gives only zero point four six nine kips per foot. With friction, the resistance is two point eight four.

5:40The soil pushes with one point nine eight four. The driveway adds zero point two eight seven. Two point eight four over two point two seven one is one point two five. Now the upper stem. The stresses at the step are fine: twenty-one thousand psi in the steel, against thirty-two thousand. The problem is the bars that stop. TMS section 6.1.10.1.3 says a bar must run past the point where it is no longer needed. The continuing bars can carry the wall alone only from three point two four feet up, plus nine and a quarter inches:

6:11four point zero one feet. Our step is at four. So raise the step, or carry more bars through.

Passive: excavated vs left in place

6:17Would the other passive reading save it? Switch to left in place. Now passive is zero point eight four four, instead of zero point four six nine. But sliding only reaches one point four two. Still not enough, and our planting beds really can be dug out. So back to excavated.

Revision: shear key & higher step

6:48The real fix is a shear key. In Step one, tick shear key: twelve inches wide, eighteen inches deep. The key pushes into the soil below the footing, so the passive block gets deeper. The offset places it. Zero lines the key's face up with the stem's front face, right under the stem. Positive moves it toward the heel, negative toward the toe. And raise the step to six feet: nine courses of block, well above the four point zero one we need.

Step 5 — Final design passes

7:13Run the checks. Everything passes. Overturning, five point two four. Sliding, one point six one. Bearing, one thousand three hundred twenty-six psf, against two thousand. The base of the stem works at zero point seven nine, the upper stem at zero point two one, and the key easily carries its passive push. Open sliding. With the key, the passive block is four feet deep, and passive grows to one point two kips per foot. One point six one.

7:40The bars stopping at the step now run far enough. And the seismic minimum for category C: at least one number four bar at not more than one hundred twenty inches. Number six at sixteen is far more.

Step 6 — Pressure diagrams & rebar detail

7:57Step six draws the wall. The earth pressure and the driveway surcharge on the back, the passive pressure in front, down the face of the key, and the bearing pressure under the footing. Pick any load case on the left, or switch to resultants and lever arms, to see each force and where it acts.

8:20And the reinforcement detail, to scale: the twelve-inch block stepping to eight, the stem bars and dowels, the heel and toe mats with the toe hook, and the shear key. Close the step. All six summaries are blue, and the wall prints as a complete calculation package.

Recap

8:39To recap. A ten-foot block wall under a driveway. Trial one failed sliding, and its bars stopped too low at the step. The other passive reading was not enough. A shear key under the stem, and the step raised to six feet, fixed both. Design your own wall, free, at StructSuite dot com.