| RETAINING WALLS (Contractor) | | | | concrete, timber, steel, stone, brick, |
| | | | high-strength polymers, UV-resistant vinyl |
| By staff of ACS Distance Education | | | | and other new materials. Latest trends in |
| | | | retaining walls construction has developed in |
| A retaining wall is any structure that is put | | | | favour of segmental walls instead of |
| into place to retain soil moving down a | | | | poured-over concrete, as they are quicker and |
| slope. As the soil tends to move with | | | | more economical to put in place than the |
| gravity, the retaining wall will have to | | | | latter. A mixture of classical materials and |
| resist the pressure created by the mass of | | | | segmented construction is the modular |
| soil and structural water against its upward | | | | retaining wall technology of stone-filled |
| side. The pressure is smaller at the top of | | | | gabions. Other newly developed systems |
| the wall, and it increases as we approach the | | | | consist of timber crib walls and engineered |
| wall base. Retaining walls can be quite | | | | reinforced soil systems. |
| small, as those used for landscaping small | | | | |
| areas, or be civil engineering works, as | | | | MASONRY AND ROCK WALLS The two main methods |
| those used in road building. A special case | | | | of building masonry or stone walls are: |
| of retaining wall are dams, where they have | | | | |
| to retain large masses of water, and port, | | | | 1. Dry Walls Dry walls have been built |
| seaside, canals and river civil works, used | | | | since the Stone Age in many cultures, and |
| to protect land areas from flooding and wave | | | | there are remarkable works done around the |
| action. In the latter case they are referred | | | | world that have lasted centuries. They are |
| as bulkheads. In all cases, retaining walls | | | | built by stacking rocks or blocks one on top |
| have to be able to endure the pressure of the | | | | of another without using concrete or any |
| soil or water behind it and the water at the | | | | other `joining' material to stick them |
| other side, in ports and dams, for long | | | | together. The individual units need to be |
| periods of time. In the case of soil | | | | stacked in a way that they interlock as much |
| retaining walls, they have to be built | | | | as possible and with a proper batter, a good |
| considering the additional factor of the | | | | stability is achieved. The base of this type |
| varying degrees of soil water content during | | | | of wall should be twice as wide as the main |
| seasonal rainy period or stormy weather. | | | | section of the wall, and when building up |
| | | | soil should be propped against the stone for |
| There are several uses for retaining walls: | | | | better stability. This spreads the weight and |
| Small and medium size: • As a garden | | | | helps prevent the wall sinking. They should |
| design feature, to define garden beds, stairs | | | | have also drainage. Common materials |
| and multileveled spaces • As a | | | | available are granite, limestone, marble, |
| physical barrier when required as a | | | | sandstone, slate and quartzite. For |
| alternative to hedges • To retain soil | | | | maintenance and repair of historical dry |
| and make a surface level • To maintain | | | | walls it is advisable to contact a |
| privacy from lower levelled observing points | | | | specialist. Large dry walls can be used as |
| in relation to the garden, as from streets or | | | | retaining walls in slopes when large stone |
| public spaces below hill houses. | | | | boulders are used. Work is done by |
| | | | specialists companies, and locally quarried |
| Large works: • Road construction | | | | stone is used as transport costs of such |
| • Ports and sea flooding prone areas, | | | | heavy and large material don't allow for |
| seaside roads and urban spaces. • | | | | great distances from the quarry. Heavy |
| Canals and rivers • Dams | | | | machinery is used in handling the boulders, |
| | | | as they may weight between hundred kilos and |
| Here we are going to describe mainly soil | | | | several tonnes. |
| retaining walls, but we would like to mention | | | | |
| one very important point in water retaining | | | | 2. Wet Walls The stones or blocks in this |
| walls, especially salty or sea water. When | | | | wall are concreted together. A strip |
| building for special conditions as those, | | | | foundation should be laid first, with steel |
| special concrete mixes should be used that: | | | | reinforcing set in concrete. The stones |
| • Dry faster • Resist salt water | | | | blocks are then laid on top. To further |
| chemical and physical erosion. Assistance | | | | strengthen the wall, sections of wall are run |
| from specialists in the area is essential to | | | | back into the embankment at occasional |
| ensure the quality of the end results. | | | | intervals. These walls can be effectively |
| Any retaining wall design should consider the | | | | cemented with a mortar mix of 3 parts fine |
| following: | | | | sand to 1 part cement. |
| | | | |
| 1. Shape and Substance of the wall. The wall | | | | Wet walls can be built with different |
| must follow the shape of the embankment it is | | | | materials: stone, stone or masonry blocks and |
| retaining, as close as possible, to avoid | | | | bricks. When using stone, there are three |
| carting large quantities of soil either in or | | | | main types depending on the type of stone |
| out of the work site. The substance which the | | | | used: • Rubble is when uncut stone is |
| wall is built from is largely determined by | | | | used, • Ashlar uses cut stone, and |
| what materials are available. Dry walls | | | | • Veneer consists on a layer of stones |
| (without cementing) need a solid base | | | | that are cemented to the face of a previously |
| foundation. A mortared wall needs a concrete | | | | constructed wall. |
| strip foundation and weep holes for drainage. | | | | |
| The design should also consider that with | | | | CONCRETE Concrete blocks are quite a |
| different seasonal temperatures, there will | | | | versatile material for building walls with as |
| be lateral expansion of the wall; therefore | | | | they can come in many colours and different |
| expansion joints should be incorporated. If | | | | sizes. Some blocks should be built on a |
| the wall is longer than 10 m then it may be | | | | concrete foundation but needs no buttressing. |
| necessary to provide expansion joints as it | | | | If extra strength is desired it can be |
| may be liable to subsidence and contraction | | | | reinforced with rods running from the |
| or expansion and crack. A gap of 1/2 inch, in | | | | foundation up through the hollow concrete |
| a staggered fashion, should be sufficient to | | | | blocks and concreted in place. Capping is |
| allow movement and not spoil the appearance | | | | best done with a pre cast slab. There are in |
| of the wall. | | | | the market nowadays solid concrete blocks |
| | | | than have interlocking lips which provide |
| 2. Positioning of the wall to minimise soil | | | | strong bonds between blocks without the use |
| movement. All walls should slope back into | | | | of mortar or cement foundations, as a type of |
| the embankment. This slope is called "the | | | | modern dry wall. They are useful for |
| batter". A minimum batter should be approx. | | | | retaining walls below 100 cm. With these |
| 1 cm for every 6 cm in height. Ideally, the | | | | materials building retaining walls is |
| ground at both the top and bottom of a wall | | | | simpler, quicker and costs are lower. They |
| should be fairly flat, to minimise erosion. | | | | come in many colours and in smooth or rough |
| The batter is essential for wall stability in | | | | quarried-stone looks, and the shapes that can |
| all retaining walls above 1 m height. | | | | be created range from straight classical |
| | | | lines to gentle innovative curves. |
| 3. Drainage both above and below the wall. | | | | Nonetheless, care should be taken preparing |
| This factor is obviously more critical in | | | | the surface where the wall will be installed, |
| clay soils. A spoon drain may be built at | | | | to avoid future problems with ground level |
| both the top and bottom of the wall. | | | | movements, and with drainage. Similar |
| Subsurface drains and rubble back filling to | | | | blocks in bigger sizes, more depth and weight |
| facilitate drainage has also to be used in | | | | (up to 55 Kgs each) can be used, in |
| these positions. If surface drainage is | | | | conjunction with geosynthetic reinforcement, |
| allowed to run over the top of the wall, it | | | | to build walls of any height and small |
| can cause bad erosion behind and at the base | | | | batters. |
| of the wall, very quickly. | | | | |
| | | | OTHER MATERIALS |
| There are different types of retaining walls. | | | | |
| Gravity walls rely on the weight and size of | | | | Innovative products for retaining walls have |
| the materials to retain the soil, and they | | | | appeared recently in the market that can |
| may have batters to increase stability. These | | | | offer economic solutions when large retaining |
| are the most common ones nowadays in | | | | walls are needed, and they are offered in a |
| landscaping works. They are built with | | | | range of appearances, from soil like looks to |
| 'classic' materials like stone or masonry, or | | | | veneer type facing. Facings can be done with |
| with new materials such as high strength | | | | shotcrete, pressure treated wood, or sheet |
| polymers. Gravity walls must be at least 50 | | | | metal panels. One of the options is with |
| to 60 % as thick as the walls' height, or | | | | material made of recycled car tyres. This |
| thicker if there are surcharges on the wall. | | | | option is good for large areas as well as for |
| Cantilever walls rely on the weight of the | | | | smaller ones, although they need to be |
| soil on top of a large wall footage to | | | | installed by professional companies. Walls |
| compensate for the soil pressure against the | | | | can be built with many shapes and wall |
| vertical wall. They may have additionally | | | | heights, allowing for design flexibility. If |
| buttresses spaced to reinforce the wall. This | | | | you are also keen to favour environmental |
| technique is not as used now as it was | | | | solutions, this can be one of them. The |
| before. Sheet pilling is a technique offered | | | | product is relatively new, but it has been |
| to build waterfront retaining walls with | | | | used already in a variety of projects, such |
| materials like vinyl, polymer coated timber | | | | as ponds and animals enclosure, soil |
| and steel fasteners. Materials used for | | | | retaining walls for canals and road drainage. |
| building retaining walls are masonry, | | | | |