SLOPES AND RETAINING WALLS.

Pictures....grrrhhh!, wish the copy and paste was possible....

PS/the only reason we're talking about this is because landslides can cause loss of life and damage to property of the human.
And when they go off, they are like huge unstoppable monsters that dont give precise warning signs of their arrival.
However, when you see animals and birds darting off days earlier, the warning signs are good enough.

Bringing you up to speed!
First, see how devastating a land slide can be.
http://sorisomail.com/email/42722/ja-viram-desmoronar-uma-montanha.html

Sometimes it can be so obvious, Kampala city surburb, Uganda (the author calling it 'catastrophe in waiting')
http://www.monitor.co.ug/News/National/-/688334/896256/-/view/printVersion/-/xfo1yv/-/index.html

Also this one.
land slide manner

And this one below actually caused serious loss of precious life in Bududa, Uganda. Bududa(March 2010)
http://www.nation.co.ke/News/africa/-/1066/871740/-/122ljwsz/-/index.html
Residents of Bududa look at what remained after the landslide that left 80 dead and 100 missing.

Now you've got the picture of what we are talking about.

The simplified concept of a land slide
A land slide is when the soil along and near the slope fails to sustain its own weight and the weight of objects(trees, vegetation, houses, vehicles) above it.
These weights bring about stresses in the soil. The concept is simple, when these stresses exceed the soil's cohesive ability then the soils break up along made up slope lines causing a land slide. The soil's cohesive ability is in simple terms its strength or its stability.
These slope lines are called slip planes.
In such a case, the slope is said to be unstable....an unstable slope.
Do not ask the engineer to precisely predict slip planes, soils can have many inherent little fracture planes.
The most unpredictable engineering materials is soil.


clay landslide morphology land slide effigy




Categorising slopes
We do have built-slopes and natural slopes.
Built slopes are normally found around homes, business premises and factories.
The figure below is a styled up built-slope good for a home/business area.

Built-slopes are what we commonly we refer to as retaining walls.

Natural slopes are those around villages, country homes and roads, schools factories but also on hills or mountainous areas.

Image credit: http://farm1.static.flickr.com/3/3295992_af658c0623_o.jpg http://www.leo.lehigh.edu/projects/seismic/pictures/6.jpg land slide mannerism


land slide form


Moving around Kampala-Uganda, the potential for land slides is just way to much. And the most areas I have seen are in residential zones for those putting up homes on hills. The situation is almost unlike other cities in other countries. Kampala is hilly throughout, that's one of its unique features. Some of us have on various occasions been asked to propose solutions for these steep slopes and the methods we've proposed have been called expensive as they get to a tune of about 15% of the construction of a home. In money terms, that's about 10m Ushs(5,000$) for ordinary homes(three bedroomed). And this value can go up to over 100m Ushs (50,000$) for bigger homes and organisations.

Likewise unprotected deep excavations for storied buildings within Kampala are still going on despite the recent soil collapses killing people. And the Ugandan engineering community (UIPE) has already talked about it but it has not sunk in well into the mainstream life.
This is where you, the information distributors, come in. And the way forward is simple. Big-styled buildings needing deep excavations will require that the contractors present construction methods to the consultant and then to the council including details designs/drawings of how to protect/brace these steep slopes arising from deep excavations...but that's for another day. Key issue is, this is a man-made slope that can easily fail causing life-loss to construction workers. And by the way, similarly around the world...however, the HSE(health, safety, environment) institutions in some countries, say, scotland, have some strength...case in point, a contractor last month was persecuted for failing to support a 1.8m excavation within the given deadline, click the link...link 1, link 2

piles

Cause of unstability in existing or made slopes
Three main causes that have to be curbed are
1. The human
2. Water,
3 Earthquakes/tremors
The biggest cause of instability for existing slopes is water. All water has to be directed.
Water has a tripple effect on soils.
1. It reduces the soil's cohesive strength
2. It adds its own weight to the soil.
3. It adds pore-water pressure within the soil.
Earthquakes enhance slip planes or create new ones and likewise reduce soil stability through direct tug&pull.
Looking from the engineering viewpoint, the human is also a menace when it makes deep excavations and many judgmental errors. The human has to be curbed through awareness because unlike the other two, it has a mind and can WILL...but if stubborn, then the law is needed when the problem is community-affecting or letting it be a victim of its own fate if it(this human) ridicules all your efforts to help it.

Most of land slides occur during unending rainy seasons and strongly increased by tremors and built-up environments, such as paved yards, roads and most especially roof tops. A strong down pour is not as harmful but those unending soft-pouring rains are.
Whenever it rains, the cohesive ability in soils is reduced. The more it rains the more the reduction in cohesion.
This is because water infiltrates within the soils through the soil pores making it waterlogged.
In otherwords, the more waterlogged the soils, the more its weakness in strength.
If your house is situated near a slope, never give chance for the slope to ever become waterlogged by choice.
This water has to be drained out as fast as possible.
That's why most people put up hidden drains or weep holes(see figure below) along built slopes or retaining walls (see figure below).
http://www.burkesbackyard.com.au/img/archive/1814/retainingwall_22.gif
Built slopes without weep-holes WILL fail.
When you move around, you will see constructors including weep holes on retaining structures.
Slopes rarely fail on their own weight, and most especially for soils in Uganda.

Are people aware?
I honestly do not know if you look at the picture below.

PHOTO BY ISMAIL KEZAALA. from www.monitor.co.ug.


Taming Land slides
So this is where the engineer's assessment, design and mode of construction comes in.
The engineer does not work alone...for community projects and beyond, a full complete team is needed. Briefly outlined below
1. Site Investigation(investors, construction consultants, economists, politicians, enviromentalists, geologists,... list goes on as needed)
Construction consultants here include architects, engineers, hydrologist and surveyors
2. Benefit (investors, economists, politicians, enviromentalists,.. list goes on as needed)
3. Planners(consultants...)
4. Soil properties (geologist)
5. Models & Simulations(consultants)
6. Hazard rating and mapping (consultants, council)
7. Risk and cost analysis (consultants, economist)
8. Finalised engineering design&details.(consultants)
9. Construction.(consultants, builder)

When the soils are highly cohesive, you do little on steep slopes, less expensive.
When the soils are less cohesive, you're advised to make the slopes less steep or built-up properly.

Key factors.
1. Stay away from slopes.
2. But if you cant contain (1), then stay at a distance from the slope equal to its height.
3. Do not reside at the foot of a slope, be above it and at a distance away from the slope equivalent to its depth.
4. If you cant contain (3) & (1), and are that upclose to the slope, build an engineered retaining wall and for God's sake provide drainage options for stormwater esp. that off your roof.
You can turn a nuisance steep slope into an architecturally looking beautiful retaining walls around your homes.
All you have to do is ask the architect for beauty and the engineer for structural feasibility and other stakeholders for related feasibility(s).


That second pic is a honey combed wall. Common if you walk around.

Designs exist, freely given if needed.

5. Also do not forget that for big slopes, needed are anchor rods or dowels (figure below) + chainlink(or strong wire mesh or geosynthetics, see figure below) along the slope + lime-stabilised soils + and proper drainage of stormwater channels.
rock slope stabilization Vegetated steep slope application photo
http://www.terrafixgeo.com/uploads/60_SierraScape_7M.jpg

6. With inclusion of some concepts in item 5, you can beautify your retaining wall with a serious stone facading.
Retaining walls and vegetated steep slopes - system drawings

Retaining wall application photo

6. Soil stabilisation(see below) also somehow tames land slides as seen in the deep hills of Kigezi, Uganda.


okay.

Nathaniel. Ddumba
MSc. Str.Eng, BSc.Civ.Eng.
M'IABSE, M'UIPE, M'SEAINT.