There is a river at about 5 m from the base of hill. What special care must be given during construction of retaining wall so that construction of small units of house (about 500 sq. ft.) could be safely made on the top of that hill? Soil type is stiff clay. During monsoon river reach the base of retaining wall.
Since safety is an apparent concern, review of situation by a geologist with soils and hydrology background should help identify if there are added issues to consider such as unstable geology, soils or river erosion or stability to consider. Activities and conditions in the watershed could also have some bearing to consider. It is not appropriate to prescribe methods when life and property may be involved without a full understanding of the circumstances, although I am sure it happens all too often. I have to commend you for looking into this first, weigh options and evaluate before building.
This may add information on retaining wall design and applications.
A gravity wall will eventually suffer from erosional undercutting of its foundation, especially during the monsoon period. That will cause the wall to either slide downslope or overturn. A wall consisting of cast in place RC piles or a sheet pile wall with anchored head may be considered as long as good quality geotechnical data to at least 5 meters below the expected pile tip are obtained. Slope stability analysis should also be performed since stiff clay is not much of a guarantee for strength and durability through time. Cheap solutions frequently lead to failure.
Geocell walls are the best solution for the conditions you have mentioned.
Dear Madhavi, geocells will not resist the erosional force of a river flow that reaches maximum level at the base of the under consideration slope (and the wall in that sense). Stiff clay also, once exposed to the dry - wet conditions of the area, will produce desiccation cracks which develop with depth year in - year out, accelerating thus the loss of strength of the slope. So, any type of slope retaining structure should be safe from undercutting of the toe. The other solution is to abandon this development scheme as not safe, if the cost of securing that slope is considered too big.
First, I agree with William Hansen that nothing any of us say here should do anything but guide you to the appropriate geotechinical professionals that can address this specific site in detail.
Second, you were completely correct to recognize that gravity and cantilever retaining walls are two of your options. In the case of retaining walls, the cantilever is, in fact, a vertical element that resists horizontal forces by transferring them to the ground as vertical forces. The terminology is well established in geotechnical literature and practice.
Third, as has been introduced above, those two general retaining wall types are far from your only options. There are numerous additional options, although one might be able to categorized many of them as gravity or cantilever in a broad sense
Without knowing more about the geometry of both the slope and the channel, anything we say here is conjecture and speculative. To give further insight we need that kind of detail and also information about the range of river stage and velocities that are experienced. Photos would go a long way to informing out conjecture. Another question that comes to mind is whether you are actually looking to provide structural support for the slope, or are you more looking to provide erosion protection for the river bank so as to halt channel migration?
In the absence of these details, I can add that the foundation of whatever retaining structure might be built (as was pointed out the appropriate choice might be no wall or houses at all) will need to be embedded below the level that any channel scour or bank erosion is possible. To do otherwise would likely guarantee that your structure will be undermined and eventually fail. How deep that depth is will influence the choice of the type of retaining wall structure that best suites the situation. Depending on the river in question, that depth might be 1m or 20m.
My parting comment is that in my part of the world, 10m gravity retaining walls that support ground that will be bearing a building of some sort have essentially ceased to be used. Gravity walls tend to be used nearly exclusively in landscaping settings where a failure will not directly effect any building. Having said that, there are still plenty of old gravity retaining walls that are performing fine. It is just that there are now better engineering solutions for most situations that will be supporting a building.
Please be sure to bring in a qualified, local geotechnical professional to deal with the situation you have described. They will be able to suggest the safest and most economical solution for your site.
If there are no geometrical restrictions, a gravity wall constructed using reinforced earth with gabions could be a good solution. The gabions of the foundation of the wall should be extended in front of the wall in order to avoid any erosion by the river. An example is given in the attached photo. For the 10m high hill the RC pile wall will require, probably, anchoring/tiebacks (hence monitoring and maintenance) and will definitely be more expensive than the gabions' wall. In any case, gabions should be used in front of any wall to avoid erosion.
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