8/2013 – No space for a slope?
Developers and Architects are often faced with the challenge of restricted or even no space at steep or small building sites. This is when retaining walls with the smallest possible footprint on plan have to be designed to make it all possible.
Various design and construction methods exist to achieve this objective:
A. Conventional reinforced concrete or hollow blocks filled with reinforced concrete, on a reinforced concrete foundation, large enough to cantilever and resist the bending moment exerted by the retained soil. Interlocking concrete retaining blocks (CRB’s) are often used in these applications, provided they lend themselves to accommodate reinforcing steel extending from the foundation.
Vertical Terraforce wall at a residence replacing a collapsed brick wall
The client loved the charcoal colour and the rockface finish
B. Single or multiple layers of CRB’s in combination with cement stabilised backfill, are occasionally specified, at times in a mixed design of method A. and B., to resist lateral earth pressure.
The blocks are secured to stand without soil at the back with concrete reinforcing
The wall was raised above the normal ground level to give the client more privacy
C. The third option is the most popular method in developed countries. It is generally referred to as the Mechanically Stabilised Earth (MSE) or Composite Earth Retaining method. This method utilises reinforcing sheets of geo-synthetics, also known as geo-grids (woven or extruded) incorporated in layers into an engineered body of backfill and clamped into the fascia of CRB’s. The integrated nature of the fascia and the abutting body of reinforced soil acts as a monolithic unit and thereby supports the applied earth forces.
Another vertical wall at a residential home
Vertical wall next to a parking area
By virtue of their inherent flexibility, such retaining walls can accommodate moderate movement and settlement that is vital to mobilise the necessary resisting forces. This characteristic explains why it is advisable to build MSE walls with a small inclination, say 87 degrees instead of 90 degrees. A small setback of 10 mm per row of blocks will suffice. It also explains why this method is extremely popular in earthquake prone countries or where unstable ground conditions occur.
Whatever method is used for any particular vertical retaining wall, there are some other elements that have to be considered. Apart from the professional input from the design side, (geotechnical report and structural design) it is vitally important to have an installer on site with the necessary experience to carry out the job.
A vertical wall that acts as a noise barrier
Vertical wall acting as bridge abutment
It is also very important that the system to be specified has been evaluated and tested to meet all these conditions. Block crushing strengths, inter-block shear resistance and geo-grid connection testing are some of the tests required. The examples used here all use the Terraforce Retaining System, that has been evaluated and tested extensively, locally and abroad (ICBO Evaluation Report ER-5448 according to design methodology in accordance with Terraforce and NCMA design manuals, ASTM C 90-99 and section 1802 of the International Building Code (2000).