Put simply, a retaining wall is designed to withstand, or retain, the pressure from materials; (e.g. soil, rocks, etc.) This can be either when digging vertically down for a basement or substructure, or when digging into a slope or cliff face to support an angled structure. Retaining walls come in a variety of different styles and levels of complexity. The main four are Sheet Piles; Diaphragm Walls; King Post Walls; Secant Piles.
Sheet piles are long sections of interlocking vertical sheet material; usually made from steel, reinforced concrete or timber. These piles are then anchored by steel rebar or beams, (En.wikipedia.org, 2017), to help them to not fall under the weight and pressure of the soil.
Diaphragm walls are made from reinforced concrete and contain waterproofing inner walls. The diaphragm wall also starts by building a guide wall. A bentonite mixture is then poured in during excavation to help reinforcement, (Piling Contractors, 2014). Steel or reinforced concrete walls are inserted, which may contain a rubber water bar to help with waterproofing. A prefabricated reinforced cage is inserted to help reinforce the concrete which is then poured in.
King post walls very simply designed. Steel columns are equally spaced, usually 1-3m apart (Aarsleff, 2017), along the path of the retaining wall. The gaps in between the columns are then filled with concrete. As I said, very simple.
Finally, we have the secant piles which are shown in the brief as figure 1. Secant piles are created by intersecting primary and secondary reinforced concrete piles, (Deepexcavation.com, 2004). A shallow guide wall is initially built to estimate the location of each pile. This wall is shaped to include alcoves on either side for the piles to be closely compacted into the wall. The primary (also known as female) piles are put in first. This can either be via augering or by driven methods. After sufficient curing time, the secondary (or male) piles are cut into them.
I would use secant piles for the Design and build firm from the brief. The advantages of using secant over the other forms of retaining walls are that they are stronger than other types of piles, (Deepexcavation.com, 2004). As all the male and female reinforced concrete piles overlap they have a greater combined strength and then, therefore, can be used to withstand a greater weight. They also have a quieter installation compared to other piles meaning they can be used in residential areas in cities, which from figure 1 I would estimate our site is located.