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A CHB Fence Is Not a Retaining Wall: The Numbers

By 6 min readUpdated

It happens on many subdivisions and provincial lots. The lot is lower than the road, so it is filled up with soil. The boundary fence of hollow blocks (CHB) is already there, so it is made to hold the fill. After a few years the fence leans, cracks open at the base, and after a heavy rain part of it falls into the neighbour's yard.

"It has stood for years" is the usual answer. Here is why it stood, and why it may not keep standing.

A fence and a retaining wall do different jobs

A fence carries its own weight and a little wind. A retaining wall carries the sideways push of the soil behind it, every day, forever. That push grows with the square of the height: soil twice as high pushes about four times as hard, and the overturning effect grows even faster.

For a level, drained backfill, the push is estimated with the Rankine active pressure coefficient:

Ka = (1 − sin φ) / (1 + sin φ)

For a typical compacted soil with φ = 32°, Ka ≈ 0.31. With soil at 18 kN/m³, the pressure increases by about 5.5 kPa for every metre of depth. Add a small yard surcharge (people, a parked motorcycle, stored materials) and it grows further.

Now add water. If the backfill gets saturated and there is nowhere for the water to go, the water alone pushes with about 9.8 kPa per metre of depth, on top of the soil. That is why so many of these walls fail right after a typhoon.

The test case

I ran two walls through our RC Retaining Wall Designer (NSCP 2015) for the same site:

  • 2.0 m of soil retained, level backfill, 10 kPa yard surcharge
  • Soil 18 kN/m³, φ = 32°, allowable bearing 150 kPa
  • Seismic Zone 4 (kh = 0.20), f'c = 28 MPa, drained backfill

Case A: the fence-type wall

Stem 150 mm thick, 2.0 m tall, on a small wall footing 0.60 m wide × 0.25 m (toe 0.15 m, heel 0.30 m), 10 mm bars @ 400 mm.

I modelled it as reinforced concrete, so this is the best case. A real CHB wall is weaker still: hollow blocks have almost no tensile strength, and the cells are often not fully grouted.

Case A: a fence-type wall holding 2.0 m of soil (section, to scale) Section drawn to scale of a 150 mm thick, 2.0 m tall stem on a 0.60 m wide by 0.25 m thick wall footing (toe 0.15 m, heel 0.30 m) with 10 mm bars at 400 mm. The level backfill carries a 10 kPa yard surcharge. The soil push on the back of the stem grows by about 5.5 kPa per metre of depth (Ka about 0.31, phi 32 degrees, 18 kN per cubic metre) to about 11 kPa at 2.0 m, plus about 3.1 kPa from the surcharge. An arrow shows the wall tipping about its toe. Overturning factor of safety 0.41 against 1.5 needed: FAIL. 10 kPa yard surcharge 2.0 m 0.25 m wall tips over about the toe toe stem 150 mm 10 mm bars @ 400 mm toe 0.15 heel 0.30 0.60 m Case A: fence-type wall Surcharge: Ka × 10 kPa ≈ 3.1 kPa at every depth Soil: ≈ 5.5 kPa more for every metre of depth Ka ≈ 0.31 · φ = 32° · 18 kN/m³ OVERTURNING FS = 0.41 needed 1.5 FAIL ≈ 11 kPa at 2.0 m depth Modelled as reinforced concrete: the best case. A hollow-block wall is weaker still.
Figure 1. Case A drawn to scale: the soil push, about 11 kPa at 2.0 m depth plus about 3.1 kPa from the yard surcharge, tips the wall about its toe, with a factor of safety of 0.41 against the 1.5 needed.
Check Needed Case A Result
Overturning, factor of safety1.50.41FAIL
Seismic overturning1.5 (sheet minimum)0.25FAIL
Seismic sliding1.50.31FAIL
Bending at the base of the stemMu ≤ φMn31.5 vs 6.9 kN·m/mFAIL, 4.6 times over

Overall: REVISE WALL.

Case A in the RC Retaining Wall Designer, NSCP 2015 edition: the top of the sheet's Design Check Summary, where the overturning, seismic and stem bending rows quoted above are marked FAIL.
Figure 2. Case A in the RC Retaining Wall Designer, NSCP 2015 edition: the top of the sheet's Design Check Summary, where the overturning, seismic and stem bending rows quoted above are marked FAIL.

A factor of safety of 0.41 means the soil's overturning moment is more than twice what the wall's weight can resist, on paper. So why do some of these walls stand for years? Usually because the real soil is stiffer or partly cohesive, the fill was never fully saturated, and the CHB fence is tied into columns and neighbouring walls. Those are luck, not design. One wet season can take them away.

Case B: a designed cantilever retaining wall

Stem 250 mm RC, base 2.85 m wide (toe 0.60 m, heel 2.00 m, 0.40 m thick), a shear key 0.30 × 0.70 m under the base, 12 mm bars @ 150 mm in the stem, heel and toe.

Case B: a designed cantilever retaining wall (section, to scale) Section drawn to scale of a cantilever retaining wall: 250 mm reinforced concrete stem, 2.0 m tall, on a base 2.85 m wide (toe 0.60 m, heel 2.00 m) and 0.40 m thick, with a 0.30 by 0.70 m shear key under the base and 12 mm bars at 150 mm in the stem, heel and toe. The block of soil standing on the heel, 2.0 m by 2.0 m at 18 kN per cubic metre, is shaded: about 72 kN per metre of wall holding the wall down. Result: overturning factor of safety 8.55 against 1.5 needed, WALL ADEQUATE. For comparison the case A fence-type wall is drawn beside it at the same scale, with its 0.30 m heel. 10 kPa yard surcharge Soil on the heel 2.0 m × 2.0 m × 18 kN/m³ ≈ 72 kN per metre of wall holds the wall down 250 mm 2.0 m 0.40 m 12 mm bars @ 150 mm in stem, heel, toe shear key 0.30 × 0.70 m toe 0.60 heel 2.00 2.85 m Case B: designed cantilever retaining wall PASS · WALL ADEQUATE overturning FS 8.55, needed 1.5 Case A, same scale heel 0.30
Figure 3. Case B drawn to scale, with case A beside it at the same scale: the 2.0 m heel carries about 72 kN of soil per metre of wall, which holds the wall down, and the shear key under the base helps against sliding.
Check Needed Case B Result
Overturning1.58.55PASS
Sliding1.52.81PASS
Seismic overturning1.5 (sheet minimum)3.79PASS
Seismic sliding1.51.52PASS (just)
Bearing pressure≤ 150 kPa52 kPaPASS
Stem bendingMu ≤ φMn32.5 vs 53.4 kN·m/m (0.61)PASS

Overall: WALL ADEQUATE.

Bending at the base of the stem: demand Mu against capacity phi Mn Bar chart in kN m per metre of wall. Case A, the fence-type wall: Mu 31.5 against phi Mn 6.9, FAIL, 4.6 times over. Case B, the designed wall: Mu 32.5 against phi Mn 53.4, ratio 0.61, PASS. Bending at the base of the stem Mu = moment from the soil · φMn = what the stem can carry · kN·m per metre of wall 0 10 20 30 40 50 60 kN·m/m Case A fence-type wall FAIL · 4.6 times over Mu 31.5 φMn 6.9 Case B designed wall PASS · Mu/φMn = 0.61 Mu 32.5 φMn 53.4 Mu: demand φMn: capacity
Figure 4. Bending at the base of the stem: in case A the soil moment is 4.6 times what the stem can carry, while in case B it uses 0.61 of the capacity.

The lesson: the heel does the work

Look at what changed. The stem is only 100 mm thicker. The big difference is the 2.0 m heel under the backfill.

The soil sitting on the heel is heavy: 2.0 m × 2.0 m × 18 kN/m³ is about 72 kN for every metre of wall. That weight holds the wall down and back. The soil is used against itself. A thick wall with no heel is far weaker than a thinner wall with a long heel.

The shear key under the base digs into the ground and helps against sliding, which is what governed under the earthquake load (1.52 against 1.5).

Drainage is part of the structure

Case B assumed a drained backfill. That assumption must be built, not just written:

  • Weep holes (for example 75 mm PVC at about 1.5 m spacing each way) near the bottom of the stem
  • A gravel drain layer behind the wall with filter fabric so the soil does not clog it
  • A sloped surface or canal at the top so rain runs away instead of soaking in

Without drainage, the water pressure alone can be larger than the soil pressure the wall was designed for.

Drainage behind a retaining wall, and why it matters Schematic section, not to scale, of a cantilever retaining wall with weep holes near the bottom of the stem (for example 75 mm PVC at about 1.5 m spacing each way), a gravel drain layer behind the stem with filter fabric on the soil side, and a sloped surface with a canal at the top so rain runs away. Beside it, the push on the back of the wall over 2.0 m of depth: drained soil about 5.5 kPa per metre, about 11 kPa at 2.0 m; water alone about 9.8 kPa per metre, about 19.6 kPa at 2.0 m. Drainage behind the wall schematic, not to scale Canal and sloped surface at the top: rain runs away Weep holes near the bottom, e.g. 75 mm PVC at about 1.5 m spacing each way Gravel drain layer Filter fabric keeps the soil out of the gravel Push on the back of the wall over 2.0 m of depth (values at 2.0 m) back of the wall 0 2.0 m ≈ 11 kPa ≈ 19.6 kPa Soil, drained: ≈ 5.5 kPa per m Water alone: ≈ 9.8 kPa per m
Figure 5. Drainage behind the wall, schematic and not to scale. Water alone pushes about 9.8 kPa per metre of depth, more than the drained soil at about 5.5 kPa per metre.

What to do if you already have a fence holding fill

  1. Do not add more fill behind it, and keep heavy loads (vehicles, stockpiles) away from the top.
  2. Open weep holes at the bottom if there are none.
  3. Watch for leaning, horizontal cracks near the base, and soil coming through joints. These are warnings.
  4. Have a civil or structural engineer check it, and plan for a proper retaining wall, especially above about 1 m of retained soil.

General information only. A retaining wall that holds back a neighbour's property or a road should always be designed and signed by a licensed engineer.