DRAFT – Comments Welcome
I just can’t help but talk about retaining walls in this course. A retaining wall retains (keeps out) something, typically backfill soil, earth, etc., but we could have a retaining wall retain other things (a flood flow, chemicals, whatever). In the context of this course we will be dealing with walls that retain soil, gravel, etc. backfill. The earth being retained by the wall could be sloped, or we could have a `surcharge’ load of, say, a logging truck parked up behind the wall. In this lesson we will talk about the kinds of walls, and issues involved, but not really do any `designs’. Maybe later.
Actually, we have already talked about retaining walls when we talked about basement retaining walls (here). And, in some sense, probably just about all perimeter foundation walls are retaining walls, in that they probably hold out at least a little bit of soil. But there are other retaining walls that I want to talk about; in fact I need to talk about, since I talked with the other Profs and they said they don’t cover what I’m going to talk about. And since you are the `Master Builder’ – you need to know. There are basement retaining walls, and there are the retaining walls that are outside the structure. The ones outside the structure might adjoin the structure, or might not. I see a lot of retaining walls since where I live and work a lot of the land is not flat, so to build on it we need to incorporate retaining walls, … for the structures, for the landscaping, and perhaps to get a driveway to the structure and landscaping.
2. Basement Retaining Walls
Basement retaining walls, recall, fall into several types.
- One-way vertical slab supported laterally by basement floor at bottom and a (Main) floor on top.
- Two-way vertical slab incorporating support at the bottom, support on the sides by way of `returns’ and/or buttresses, with support not necessarily provided at the top.
- (Rare for basements …) Cantilever walls made externally stable by big footings and internally stable with lots of rebar, that also do not require support at the top.
And being basement walls they have superstructure on top of them. And are constructed of reinforced concrete.
3. Outside Walls
The two main `players’ of what I call `outside walls’ are:
- reinforced concrete (generally cantilever or gravity walls, but sometimes buttressed 2-way walls) and
- segmented block (masonry) walls.
The cantilever retaining wall will not be covered here. Suffice to say they generally have big footings and lots of reinforcement. A ten foot tall wall, for example, will typically have a six foot wide footing. Good detailing of these walls is important as `mistakes are visible’ (in your face as you look at the wall). Control joints will be necessary at intervals approximating the wall height, and for really long walls expansion/contraction joints may be necessary.
These walls must be `engineered’ and they should be constructed by contractors who `know what they are doing’.
Made of reinforced concrete, which is `rigid’, these walls must be founded below frost depth.
Furthermore, they must be designed to resist `At-Rest’ pressures. (We will talk more about that later.)
Segmented block walls are becoming more and more popular for `outside’ retaining wall use. They are basically built stacking blocks. Stability of short block walls is provided by the weight of the blocks themselves; taller block walls use soil reinforcing in the backfill.
4. Stability Issues
For any retaining wall we need to look at stability issues. I break them down into three categories with subcategories.
- External Stability
- Internal Stability
- Global Stability
When we look at external stability we are making sure that, as a whole, the wall does not …
- tip over
- fail the bearing soil underneath (typically under the toe)
Short walls that successfully utilize the weight of the wall itself to not tip over are called `gravity’ walls. Taller walls must use some of the retained soil to stabilize them with regard to tipping. For a cantilever a bit footing (heel) is required to `grab’ enough soil above it to counterbalance the tipping action of the soil pushing on the soil side of the wall.
For block walls, it is kind of the same. Short block walls utilize their own weight to keep the from tipping. Taller walls are connected to reinforced backfill with `geogrid’ or some other soil reinforcement; the whole reinforced soil mass provides the stability.
Sliding is resisted by some or all of the following means:
- friction between wall and base material,
- the support provided by a (huge) slab at the toe. This slab must be huge enough so that it doesn’t slide, and it must be butted directly up against the wall. A big reinforced concrete slab driveway might be able to serve in this regard,
- a `key’ into the founding soil that would provide passive earth pressure resistance,
- embedment of the `toe’ of the wall into solid soil that may provide passive resistance.
We have already talked about the basement slab providing this resistance to sliding in the context of basement walls.
Use of a key is cumbersome, but sometimes needed. A key is only applicable to a reinforced concrete wall (not a segmented block wall).
FAILURE OF BEARING SOIL AT TOE
Probably the biggest `player’ in design parameters for a wall is the allowable bearing soil pressure at the toe. In terms of `number crunching’ the design process will proportion the details (dimensions) of the wall so that the applied pressure at the toe doesn’t exceed the allowable pressure (as the wall wants to tip). In terms of real life walls, if the condition of the bearing material is poor under the toe – the soil will fail, and the wall will tip. THIS IS NOT GOOD! How do you fix a tipped over wall? You don’t. You replace it. Bad, costly, embarrassing. My colleague and I have seen walls as short as one foot tall fail (tip) because of poor bearing soil. So, make sure you deal with the bearing pressure under the toe in your calcs, and make sure on site that you have good, quality bearing material there.
We need to make sure the wall itself is stable, internally. So, we check:
- Reinforcement development, connectivity, etc.
Global Stability (Sloped Terrain)
in addition to making sure that the wall is externally and internally stable, we also need to make sure that the whole site is stable. In other words, we don’t want the whole hillside to give way and end up down at the neighbors. Here are two rules of thumb for global stability:
- generally no combination of walls, terraces, slopes, flat spots, etc. can be overall steeper than the natural terrain in the overall surroundings, and
- if there is any question, hire a Geotechnical Engineer to deal with global (and slope) stability issues.
5. Segmented Block Walls
Segmented block retaining walls (SBRWs) are cool. They are built block by block and thus can be built essentially by hand. They can be really long. They can be really high where reinforced. Structurally they are `flexible’. This means that we don’t need to worry about them cracking. Also, the wall `gives’ … can settle a bit. Whereas reinforced concrete walls must always be founded below frost depth (even outside walls), the SBRWs need not be. Their flexible nature can generally accommodate some frost heave action.
Types of Segmented Block walls are:
- Reinforced Backfill
The gravity walls are made stable externally by their own weight. Generally they are internally stable through block-to-block friction. The main issue with gravity walls is the bearing material under the toe, as discussed earlier. Ridiculously short walls can tip over if founded on poor soil. Tall walls can stand `forever’ on good soil.
Reinforced backfill walls are walls where the blocks are connected to geogrid or other soil reinforcement. Externally the weight of blocks and reinforced backfill keep it from tipping or sliding. Again, toe bearing condition is critical. Internally the geogrid must be strong enough to tie the blocks to the soil, and must be at sufficient intervals to keep blocks from sliding or building out the face of the wall. Like rebar, the soil reinforcement bond with the soil must be `developed’. This is accomplished by compacting the soil around the reinforcement and making sure the reinforcement has proper (additional) embedment. Connection of the reinforcement to the blocks is accomplished by pins, friction, or other means.
Blocks are generally cast concrete. They range in size from a cubic foot or less to … well, giant blocks that need to be lifted into place with a crane. Reinforcement is generally a geosynthetic material or in some applications welded wire fabric.
Rigid reinforced concrete walls generally become un-economical above 10 or 15 ft. SBRWs can be built very tall. I would like to say in excess of 50 to 75 feet, but there are probably much taller ones already in place.
An advantage design-wise of SBRWs is that their design does not require `At-Rest’ pressure calcs. Since the wall is flexible, from a soils standpoint, we are allowed to use `Active’ pressures, which are lower. As such, we might be able to get a gravity SBRW just a bit higher without reinforcement, or, if reinforced, the lengths and backfill might be less.
6. Soil Pressure
There are three types of soil pressure we deal with in retaining wall design. (These are all lateral pressures exerted sideways on the face of the wall, kind of like a fluid pressure.)
The At-Rest pressure is the pressure the soil is assumed to exert essentially horizontally on the wall where the wall and soil do not move. At Rest pressures are greater than Active pressures. For example, the International Building Code prescribes an At-Rest pressure of 100 psf/ft for silt-clay soils (IBC Table 1610.1). This means that for every foot we go down the pressure increases 100 psf. So, ten feet down the lateral soil pressure on the wall is 10 x 100 = 1000 psf. Huge! That will mean big wall features and lots of reinforcement. On a project of significance you should have the Geotechnical Engineer provide a design At-Rest pressure. And it might end up being less than the presumably conservative `prescriptive’ value in the Code.
Active Pressure is the pressure that the soil exerts on the wall as it is allowed to `move a little’. Active pressures are less. For silt-clay soils IBC 1610.1 prescribes 60 psf/ft. A lot less. But we can only use this pressure on walls that also are allowed to `move a little’ … flexible walls … SBRWs.
Passive Pressures are developed where, alternately, something pushes on the soil. If a toe of footing key is deeply embedded in undisturbed or properly compacted soil, then the resistance it provides to being pushed can be used in the design of the wall. Even though passive pressures are large, it takes pretty deep embedment to start taking advantage of them. For example, the passive pressure for silt-clay soil may be as high as 300 psf/ft, but often the first foot of embedment isn’t even counted, and further `feet down’ means more and more excavation and/or compaction.
The one thing I need to emphasize with the reinforced SBRW is that the reinforced backfill must be inspected as to proper compaction to develop the bond with the reinforcement. Inspection = $.
Here are some issues that I need to cover, so I’ll do it here.
- Engineering … generally the building codes require that walls taller than 4 ft be `engineered’. Or, if the wall retains a slope, or significant surcharge, engineering may be required for even shorter heights. AND NOTE: wall height is from bottom of wall to top of wall, regardless of embedment (and not `net height’).
- Water … water kills retaining walls. Make sure the wall is well-drained. SBRW manufacturers will boast that their walls drain automatically. Fine if they do. I generally also detail an actual drain just like it is a basement wall.
- Slope … it should be obvious, but if the wall retains a sloped hillside, the pressures will be greater and the design will need to be more robust.
- Batter … batter is better. Batter is the slope of the face of the wall backward toward the backfill. Batter makes the wall more stable. And batter makes the wall LOOK more stable, especially if it begins to tip a bit. (Recall that we design the SBRW so that it moves a bit; as such, batter is super important.)
- Gravity versus reinforced … for modest block sizes (say, that can be placed by hand … 100 lb each, range) gravity walls can be built to heights of 4 or 5 ft. With bigger blocks (dropped in by crane), even higher. Maybe way higher. Reinforced walls seem nearly unlimited in height.
A lot may be at stake with designing a retaining wall, so either follow the prescriptive provisions of the Code, for example, with a one-way reinforced concrete basement retaining wall, or hire an engineer. For SBRWs … hire an engineer. But on short walls where engineering is not required, you may want to design one yourself. Or, for taller walls, you may want to do a preliminary design yourself. The manufacturers of the blocks for these walls so want you to buy their blocks that they will often provide you with design software. Now you know what it’s all about … now you know the issues, have been exposed to the terminology, and so on. So give it a try. The providers of the software will generally not provide the engineering (stamp). You will need to provide (or hire) your own.
Design of Basement Retaining Walls, Jeff Filler, Associated Content
International Building Code, International Code Council, 4051 West Flossmoor Road, Country Club Hills, IL 60478.