Minimum Backfill Width for Basements

[abarker]

Is anyone aware of recommendations for a minimum backfill width for basements to allow a) proper drainage and b) to allow the use of the lateral earth pressure values of the backfill instead of native soil? I've not to date commented about this in my reports, but a client showed me a report which had recommendations for the backfill to extend beyond a 60 degree plane (as measured from the horizontal) extending outward from the base of the basement. I understand the thought process behind this, but I have not seen this in a report in our area before, and I have not been able to find a cross reference for this recommendation. With this in mind, I'm wondering what the "standard engineering practice" is in other areas of the US concerning this topic (I'm concerned that I'm maintaining my "standard of care&quot.

 

[kmerl]

I have never heard of any sort of recommendations of this type, however the values you talk of are determined by the qualities of the native soils adn backfill materials.

Thickness of materials for determination of proper drainage depends on the expected base wall water load. It is simialr to sizing pipes for stormwater flow. The capacity of a drainage system needs to be sized for the amount of project water intake at the basement soil contact area. Parameters would include the groundwater elevation, depth of basement, groundwater flow rate, and sources of surface water infiltration to the system. There are general guildlines to follow, but I usally refer to manufactures suggestions for that type generic general specifications.

It looks like to me that the report writer specified particular backfill material that mobilized the earth pressure wedge along the 60 degree inclination. I base that number strictly on the soil qualities, i.e. internal strength parameters (cohesion and friction angle). Theorteically only the soil wedge against the basement wall acts upon the wall, therefore if you can replace the entire soil wedge with manufactured materials you could control the loading against the wall, by conrtoling the inter strength parameters. I seems like a relatively high friction angle (phi), i.e. manufactured materials.

I have experenced this when designers want to minimize retaining wall mass ans steel thereby reducing overall project costs. Depending on your situation it many or may not reduce the overall cost of construction.

My take on this....

I think in order to maintain or "standard of care" 1) for basement wall drainage if there is encountered water just refer to manufactures suggestions, unless there is governing circumstances that make the system very important to move a certain amount of water 2) as far as the soil filling wedge I'm not sure that is an issue unless you attempting to control pressures against your basement walls. I have never attempted to remark to this extent due to the extra cost needed to backfill with manufactured materials as compared to natural.

I would be interested in more detail about the report you reference above.

Keithe J. Merl

 

[Focht3]

This is a brief post - short of time right now. Let me know if this needs to be fleshed out.

I have used a similar recommendation in the past - two reasons:[ol][li]You need to be sure the wall drain will function as intended, so thin drains are a no-no in some circumstances.[li]If you provide a reduced equivalent fluid weight for the fill in comparison to the native soil, you need to be sure that the lower pressure will likely occur.[/ol]Hope this helps -



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[abarker]

kmerl and Focht3,
Thanks for the comments. You comments are in line with my thoughts (which is reassuring!). kmerl, I come from clay/silt country so we always recommend replacing the native soils with free draining sand soils to reduce the pressure on the wall. Manufactured material is sometimes used in our area, but the report did not specifically recommend it as structural fill or backfill. The concern with the noted project was that it extended 3 stories below grade, so backfilling to outside the zone of influence would have meant about a 20' wide backfill zone! Not too practical in my book. My preference would have been to give lateral pressure values closer to that of the native soil.

 

[Focht3]

...so backfilling to outside the zone of influence would have meant about a 20' wide backfill zone! Not too practical in my book.

Hmmm,

It's really a question of economy given the local practice, the chosen method of construction (open cut vs soldier pile & lagging), site constraints (can you store the excavated spoil on site?), cost of local material (is granular fill cheap and close by?), etc. I usually give both numbers - and still hear complaints from the structural engineer, architect and/or construction manager about how "impractical" one - or the other - recommendation is. I have learned to interpret "impractical" as "unfamiliar." And on a number of those jobs, the winning bid was based in part on my "impractical" recommendation...



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[abarker]

Point well taken. I guess I grow tired of seeing "cookie cutter" reports from my colleagues where the word "impractical" could be replaced with the word "obsurd" because the writer has not thoroughly thought through the recommendations being made.

 

[cdh61]

abaker,

The thickness of the back fill material will depend on the soil type you are in, and I agree with Focht3 that a thin drain is a no-no. Some standard engineering practices in Canada and US will be similar, and a minimum drain width of 12 inches should be considerd, but will depend on soil type and seepage conditions.

The use of fill to replace native soil to reduce lateral earth pressure would normally be given for walls with little support. How deep is this basement that a change is soil type is required to reduce lateral pressures, and what is the diffenance in lateral pressure between the fill and native soil?

Regards

 

[Focht3]

Ah! If it's a cookie cutter report...well, that's an entirely different story. They're the bane of our profession.

When I first began practicing geotechnical engineering in Houston some 22+ years ago (am I really that old?), one of the established "bottom feeder" firms didn't even bother to re-type their reports. They'd take the last one, make a photocopy, "white out" the design bearing capacity, client name, project number, date, etc., type in the new information (which looked bad since it was typed onto the "white out&quot, reproduce it, sign and seal it - and send it to the client along with the bill. Needless to say, the reports were absolutely awful (in more ways than one.) But they were dirt cheap.

In this age of word processors and laser printers, the end product from the "bottom feeders" looks a hell of a lot better, but it still has that old, familiar stench...



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[kmerl]

I have read too many obvious "cookie cutters" from our local compost spreaders. Another problem with some firms in my area is that they own there own drilling equipment, which translates to increased cost mostly due to the need to keep drillers working, rather thought into the problems and solutions at hand.

A three story deep structure in clays does require more consideration to lateral pressures. In which case the bare minimum is not the way to go, however I agree that the twenty feet of materials is rather excessive.

As always a joy.


Keithe J. Merl

 

[abarker]

cdh61 - I'd typically recommend an at rest equivalent fluid pressure of between 80 and 90 psf for the typical native clay soil in our area, for our recommended sand fill I'd typically recommend between 45 and 55 psf. As you can see, that's a fairly major difference for the wall design. Because of this, most commercial/industrial projects in our area replace the native soil with granular backfill. The problem with the report I read is they did not give an option for what to do if it is not possible to achieve the noted setback (because of site constraints).

By the way, the lowest level of the structure was going to be about 40' below finished grade, would be within about 15' of an existing roadway in a portion of the project, and has upper lean clay and silt soils overlying deeper sand. Because of the site constraints, they were also looking at excavation bracing so they did not want the excavation wider than necessary. My preferred option would have been to give them a value somewhere between the sand and clay values for the upper clayey/silty zones.

 

[BigH]

I'll try to jump in here - lots of good points made. Some comments are applicable to open cut scenario; some for shored cut. Need to look at which is used, and if the later, is the shoring (say soldier pile and lagging) left in place?
The most critical point, in my view, is to ensure no hydrostatic pressure against your wall after backfilling. You definitely need a drain designed to ensure this - say 10-20 crushed stone against the wall with suitable filter between it and the native soil. I'd probably use minimum 600mm for the vertical drainage blanket - extra for filter zones. Conversely, there is a geotextile mat - almost like a wick drain that you could paste against the wall and it will drain all the water you want (sorry, forgot the name of it - my personal references aren't with me).
With respect to the earth pressure - if it is a basement wall, we usually assumed a ko condition (say ko=0.5) in our geotechnical reports - seems like many are wanting to give active coefficients of earth pressure. Basement in your post is three levels and the pressures are likely to be sufficiently restrained - by each level's floor slab to minimize soil movement - hence the movement required to reach ka value. In clayey soils, you do have the tension crack phenomenon - reduces the positive acting pressue (ensure no water in the tension crack).
One other thought - if you make a deep trench and fill with sand up against your likely very stiff clayey soil (else you'd be in wide open cut or supported excavation), depending on the exact nature of the clay - could your trench not be acting like in a silo effect??? or a cushion to absorb the movement of the clayey wall? - think of a wall between rock and wall or between an older retwall (or left in place excavation support wall) and the new one.
I suppose this is why we just went with ko for its ease in application - and does it mean that much extra steel in the walls, etc.???

 

[Focht3]

[blue]BigH[/blue]:

There may be a silo effect for some period of time; but it won't last for the 50 to 100 year life of the structure. I would expect the sand to "stiffen" over time - resulting in a fairly rigid transfer of stress from the clay to the wall. And the silo effect won't happen if the sands are compacted with even moderate effort.

If the sand mass isn't drained, water can collect in the sand, wetting the clay soils adjacent to the sand backfill. The wetted clay could swell and cause lots of problems...



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[abarker]

BigH,
Interesting thought about ka possibly being suitable because of the levels of restraint, I had not thought about that one. I've recommended the composite vertical drainage mat in reports, but I've never had anyone bite on that alternative (us Midwesterners are sometimes way too conservative to try anything new - hence, Focht3's comment about "unfamiliar"!).

Focht3 - I also thought about the "sand cushion" idea and was wondering if the act of "stiffening" the sand overtime would move the clay pressure from the at-rest state to the active state? In other words, is it more likely that the clay relaxes and pushes on the sand (compressing it horizontally), or that vertical water flow through the sand compresses it vertically? If it's the former, then I would think using the active clay pressure could be appropriate.

I agree with everyone that drainage is the biggest concern for this type of project. By the way, I also ran across many conversation threads in my internet search concerning which pressure coefficient to use for basement wall design. It varied from many that, like BigH mentioned, used the active pressure to a few that even recommended using the passive pressure (because it's conservative!). I hope with these threads, we give the appropriate impression that recommendations and designs should start with the "at-rest" pressure, unless serious thought can justify otherwise (at least in my humble opinion).

Thanks for everyone's thoughts!

 

[Focht3]

This is a good thread!

I, too, feel that the "at rest" condition should be the starting point for most basement walls. Values should go down - or up - depending on the construction techniques, sequence of the work, soil types, backfill materials, weather effects, groundwater, ...



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[cdh61]

abaker,

The depth of the basement and the potential of high laterial pressures does mean you will need a wider drain. Without completing an assessment, BigH comments about 600 mm sounds more in keeping, but is there significant moisture (seepage) from the silt? The drain material will have to be course to ensure no hydrostatic pressure developes in the drain, and the hydraulic conductivity difference between clean gravel (10-20 mm dia) and lean clay is well within the order of magnitude of 1000. This should be sufficent to prevent any build-up of porewater pressure in the drain.

As for the value of ko=0.5, this is also typical of where I start and the value used. For this application, most likely your clayey soils will compress the sand, given it is not practial to excavate and the drain width. On projects like this, I also stat it is extreamly important to ensure the drain is free draining, and the fill adjacent the wall be moderately compacted, hard compaction also developes stress on the basement wall.

As for the swelling, this should not be a major problem unless the lean clay is on the dry side (less than the PL)and water ponds in the drain.

regards

 

[BigH]

Note the 600mm I mentioned "as a start" was for drain material of 10-20 clear size - this should be sufficiently permeable to prevent buildup of hydrostatic pressure.

 

[kmerl]

I think cdh61 brought up a good point, how is the system to be drained. You may be successful in directing the water and contorolling hydrostatic pressure, but it needs to be reomved form the system.

Is there a gravity system to remove the water from the system, or a mechanical (pump) that requires electric. At the depths you are talking I wonder if gravity is not possible. This may become a factor if you need to worry about storm events filling up you drain, thereby introducing the hydrostatic loads against the basement. Also by placing a drain directly against the wall you could increase the propensity for storm water runoff to flow into your drain from roof runoff etc, along the basement wall.

I would think that to install the drain it would be neccessary to ensure you are not increaseing the water inflitrated at the drain into the subsurface. This can be accomplished (in my opinion) by a three foot clay cap, pavement, or first floor roof over hangs.


Keithe J. Merl

 

[abarker]

To all - water table is in the lower sand soils, silts and clays are above the water table. Lower sand is also pretty clean (less than 5% P200) so build-up in upper layers is not too likely. However, keeping the water table depressed in the sand layer does require free draining material (and no failure of the drainage system!). Our client was looking into gravity vs. sump pump, the site is situated on a ridge so the storm sewers are relatively deep in the area. My recommendation in these situations is typically gravity drainage is best (although in a bigger city, backup of the storm sewer system is also a concern), but if you put in sump pumps you should include an emergency backup generator to make sure the system continues to work during a power outage. I also typically recommend the clay cap (usually I say 2', but 3' would be fine also) in unpaved areas and tieing roof drains into the storm sewer system.

Finally, I also am not too concerned with clay swelling in this particular instance because the clay is pretty "lean" and it has reasonable in-situ moistures.

 

[Focht3]

[blue]abarker[/blue] -
I think you have a good handle on things -

I'm disinclined to use a true active pressure on a below grade (basement) wall for any reason. Again, too much can happen in the 50 to 100 year life of the structure. The risk is too great for the limited reward. I would consider using a reduced wall pressure when I have two basement walls close to one another, with the gap filled with a sand or gravel backfill. That truly presents a viable "silo" effect that could be incorporated into the design. But I wouldn't do this for clays.



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