Ecology Block Wall: Are the gaps between blocks sufficient to relieve hydrostatic pressure? 2

[jocose]

I have a situation with an existing ecology block wall. The blocks are 2ft x 2ft x 6ft. The math checks out for holding up the slope provided there is no hydrostatic pressure ( Used Rankine Formula ). However, worse case scenario, the wall fails if water builds up the full height of the wall.

I'm wondering if this is a legitimate concern given that every 6 feet there is a break 1/4 - 1/2 gap between the blocks that water can seep through. There is no drain rock behind the wall, just silty/sandy soil.

 

[oldestguy]

This post is another asking a question but not giving full info, expecting a usable reply. Topography, up hill slope, ground water table, rainfall history, age of wall, height of wall, slope of wall, consequences of failure, seepage noted, size gradation of soil, etc. Without any of these, I'd say "probably fine, depending".

 

[jocose]

Apologies for the lack of details.

Blocks: 2ft x 2ft x 6ft 3,600 lbs /block

The soil is silty sand. The angle of repose is 34 degree. The slope above the wall is 30 degrees. Slope rises to a height of 8 feet above the top of the wall.

Wall uses one below grade block. Has two above grade. Total height is 6 feet including buried block, 4 feet above grade.

Blocks are installed correctly, keyed together, and staggered like bricks, and tilted back slightly into the hillside.

Wall is 50 feet long.

The region is in a rain shadow so surface water is of little concern. However, man-made sources of water are a concern —- water utilities and a drain system hooked to a residence basement 14 feet directly behind the center of the wall could potentially inject massive amounts of water into the soil if they drained unchecked for a prolonged period of time.

The wall is relatively new (2 years old). To date, no water seepage has been noted. The soil is about 10% clay, 20% silt, and 70% sand.

A major concern is a pipe break in neighbors basement or water main leak that might build up a large volume of water quickly.

If the wall were to fail it would destroy a car garage that has yet to be constructed on the lot below.


Again blocks were installed directly in contact with the silty sandy soil. No drain rock was used.

 

[oldestguy]

Without trying a stability analysis and assuming normal conditions, I'd not be concerned about the wall. The possibility of a deluge however, might be evaluated on the basis of how long the worst conditions are present. Again, I'd guess that these occurrences will not involve serious backfill saturation, so don't worry. This is not a high wall by any stretch and the sheer bulk has a lot going for it. If it were higher, I'd tie in some geogrid to provide resistance to failure.

 

[oldestguy]

OG again. At 30% P-200 sieve material, this is rather poor as to permeability, not letting water pass readily. Chances are the slope will shed most what appears on it.

 

[cvg]

"30% P-200" material may not hold much water, but if dry and desiccated, may develop cracks. these cracks could transmit large amounts of water which would not all shed off during rainfall. Just because a region is in a rain shadow, doesn't mean it never rains.

 

[jocose]

The soil hasn't ever exhibited cracking, even in the driest months. It is also covered in grass. This is a good point though, thanks cvg.

Oldestguy's point about the soil not being saturated is a very good one. The soil is never saturated. We have one of the lowest 100-year storm rainfall events in the country. Topping out at on 1" in one hour, and 1.5" in 3 hours.

The entire property is also up on a hill elevated well above any groundwater.

The primary concern here is a deluge event from either a utility line or the adjacent properties drain system.

However, this goes back to my original question about the ecology blocks and the gaps between them. My assumption is that these block will not let enough water through to sufficiently relieve hydrostatic pressure build up behind the wall. Is this correct? I ask because I know rock retaining walls with larger gaps typically rely on those gaps to let water pass through I just wanted to make sure this effect wasn't applicable to block walls such as this.

 

[oldestguy]

jocose: You have it right about saturation raising the pressure on the wall. Take the water at 62.5 and the submerged active (or at rest) making the lateral pressure at total equivalent fluid density of maybe 90 or 100 #/cf. That may be three times what is there now and to allow for that slope above these numbers get increased maybe 10 to 20 percent at the most. However, those open joints will affect things, maybe quite well. And, this gravity wall is darn secure now with a good safety factor, so it starts off with a lot going for it if saturation occurs. I'd not worry about it down the road. Oh, where does the term ecology blocks come from. These likely are concrete emptied out of ready-mix trucks as they return to the plant. Heck one could be making roads or sidewalks out of it, or in my day, just more paved land next to the shop out in a swamp.

 

[cvg]

mafia blocks, jimmy hoffa is probably in one
I live in the desert. our total, annual rainfall is about 8 inches. they build dams out of silty sand here and transverse cracking of silty sand soils due to dessication is a big problem, so are collapsible soils. If there is drainage channel taking the water away from the slope, somewhere above the wall, then I would be satisfied. otherwise, is the runoff flowing down over the blocks and through the cracks in the blocks? Of so, than you will build up some hydrostatic pressure.