If Your's is a Sloppy Site, You Need to Know this Before you Build - Kenneth Oigo

Flash floods are with us again and, even though they present a big headache to urban dwellers, the greatest danger lies in rural, on-slope settlements.

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However, builders can avoid the risk of collapsing structures by building retaining walls, which are designed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil.

Retaining walls are built in order to hold back ground, which would otherwise move downwards.

Their purpose is to stabilise slopes and provide useful areas at different elevations, for example terraces for agriculture, buildings, roads and railways, dams, flower boxes, and basement walls.

Due to the constant lateral pressure that the retained soil applies on the wall, coupled with the ever-dynamic surcharge and water pressures, retaining walls are always under assault by these forces and, if not properly constructed or designed, they may fail, putting the lives of anyone in the vicinity at risk.

Basement walls may collapse, burying people alive, or dams may fail, sweeping entire populations under a massive wave of sudden floods in river towns downstream.

The main types of retaining walls are:

Gravity walls: These are the most basic retaining walls available in the design range. They depend on the weight of their mass to resist pressure from behind and often have the slight setback of improving stability by leaning back into the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units. Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing.

Cantilevered walls: Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry, often in the shape of an inverted. These walls cantilever loads like a beam to a large, structural footing, converting horizontal pressure from behind the wall to vertical pressure on the ground below. Sometimes cantilevered walls are buttressed on the front, or include a counter fort on the back, to improve their strength, resisting high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.

Sheet piling: Sheet pile retaining walls are usually used in soft soils and tight spaces. They are made out of steel, vinyl, or wood planks which are driven into the ground. Tall sheet pile walls will need a tie-back anchor, placed in the soil a distance behind the face of the wall that is tied to it, usually by a cable or a rod. Anchors are placed behind the potential failure plane in the soil.

Anchored walls: An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it. Anchors, usually driven into the material with boring, are then expanded at the end of the cable, either by mechanical means or often by injecting pressurised concrete, which expands to form a bulb in the soil. Technically complex, this method is useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.

There are basically two major ways through which retaining walls fail. These are:

Slip cycle failure: This is when the entire soil mass behind and under the structure becomes saturated with water, making the wall to slide in a circular manner as it gives way to the push of the lateral pressures behind it.

Failure by overturning: The wall is forced to overturn when the lateral pressure is more than the counteracting forces. This mainly happens when surcharge forces are added. This may affect basement walls that are subjected to access surcharge forces from vehicles parked too close to the wall, hence exerting too much pressure.

For maximum safety, the following points must be considered when installing a retaining wall:

When choosing materials, select the type that is best suited for the desired result. Many long-lasting materials are available in the market.

The type of wall you choose should be determined by need. Decide if you need a mass concrete wall or a less expensive pre-cast wall.

Most retaining walls fail because of pressure against the wall caused by water or soil-moisture build up behind the wall. All walls should provide for the back-of-wall water to freely drain down and away from the wall.

This is accomplished with gravel backfill or manufactured drainage blankets and drain pipes. Structural walls require “weep” holes to allow water to drain from behind the wall.

A wall is only as good as its foundation and all retaining walls should be built on structurally sound, compacted foundation sub-base material.

Levelled and compacted earth or gravel fills are acceptable. The foundation material should extend at least one foot beyond the front and back of the base width of the wall.

When building dry-laid stone walls, place the largest, most stable stones at the bottom of the wall and be aware that the base width may need to be as wide as the wall is high.

Walls are more stable and structurally secure if they slope back into the retained slope.

Timber walls and other walls of solid horizontal materials usually have “T” anchors extending back into the slope of undisturbed earth.

This helps walls to resist pressures that force them forward or cause them to pivot on footing material.

A good rule of thumb is to provide at least one anchor per 16 square feet of exposed wall face.

Source: Daily Nation

A Great Architectural Marvel in Nyeri's Wilderness - Treetops

After covering the War Memorial church located in Nyeri, the Architecture Kenya team must have thought that was the last they would cover of the town located at the central highlands of Kenya. But that was not to be, another marvel came about, though located 17 kilometres from the town, it can still be said to be in Nyeri.



As the name suggests, Treetops Hotel was literally built into the tops of the trees of Aberdare National Park. The idea was to offer guests a very close view of local wildlife in complete safety. This concept was borrowed from an Indian tradition called Shikar, which was a form of hunting in colonial India, where hunting was done on a platform mounted on a tree in complete safety and comfort.

The hotel, initially run by Eric Walker, was operational in 1932 starting with a modest two roomed tree house to the fifty rooms it has today. The original structure was burned down by African guerrillas during the 1954 Mau Mau uprising, but the hotel was later rebuilt near the same waterhole where it was initially.



It rises straight out of the ground on stilts and has four decks and a roof top viewing platform. It has a rustic external feel, with matching interior decor. Large timber framed windows dot the internal spaces and a rooftop terrace provides a perfect location for viewing the wildlife below.

English Point Marina Revisited

When hotelier Alnoor Kanji acquired a four-acre plot along Tudor creek near the Mombasa Showgrounds in 2007, he thought to himself: Why not put up some apartments and sell them off for a quick buck?

But that was before he took his time off one evening and sat there, studying the environment. “At the time we finalised the sale process, I hadn’t seriously thought of the type of investment we were going to put up.

“But after sitting there for a whole three hours, enjoying the luxury of the view, the tranquillity and the ambience, I knew I was sitting on not only a water-front property, but also a monument with huge potential,” he says.

“I went home, called my brother in London and told him we needed to change our game plan. My thinking was that, if we put up apartments and sold them, it would be a disservice to our tourist city.

“We had to come up with a facility that had a public component and boosted our tourism industry” says Kanji, who owns Pinewood Village Beach Resort in South coast.

Having lived in Vancouver, Canada — a town known for its water-front life — for over 10 years in the late 1970s and early 1980s, Kanji was convinced that the best land use they would make of the property was a marina.

From that point on, they started the hunt for the best architects to draw up plans for an international five-star marina complex.

Because of the complex nature of the project, the duo opted for the services of Broadway Malyan, an architectural firm based in London.

And so in two years’ time, the English Point Marina — for that is what the two brothers decided to name their project — will change the sea-front near Fort Jesus, Mombasa.

Estimated to cost Sh4 billion, the extraordinary development will move Mombasa higher up on the world map, and transform the tourist town into a luxury boats facility and water sports destination.

The complex’s four four-storey blocks consisting of 96 three-bedroom apartments and eight five-bedroom penthouses is designed in such a manner that all rooms have a sea view.

The apartment blocks will also be built in an elevated position on the creek overlooking Mombasa Old Town and the 16th-Century Fort Jesus, two important tourist attractions in the region.

The complex will also house a 26-room hotel, a spa, a gym, a five-star international marina, a water sports centre, restaurants and an underground parking lot for about 200 vehicles.

Kanji says the developer will manage the marina, while investors will provide accommodation by renting out their apartments.

The project design indicates that the marina will also be connected to Mombasa Island to ease traffic congestion at the Nyali Bridge, and will also be accessible to the public.

This convenient link with Mombasa Island will enable people to shuttle between Nyali and the town within a very short period of time by just taking a boat from the site Island, where a jetty has existed for centuries.

Keen investors are already lining up to own a section of the marina. Just two months after the contractor broke the ground to lay the foundation for the four apartment blocks, over 20 investors have committed themselves to acquire some of the 96 lifestyle apartments.

With each apartment going for over Sh50 million and the penthouse in the upwards of Sh150 million, Kanji figured that it would be difficult to convince somebody to cough all that money, so he invested in a model apartment adjacent to the property.

The marina will be the first of its kind along the east coast of Africa, and will join two others along the entire eastern coastline, from Cape to Cairo — one in Egypt and the other in South Africa.

It will have a capacity to accommodate 88 boats, with all facilities to cater for the boats that will include service and an oil jetty which, according to tourism industry marketers, will position Kenya, known for its safaris and pristine beaches, as one of the most favourable destinations for high-end tourists.

Source: Daily Nation