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As a Construction Management Consultant of I-Consult Ltd appointed by I- Build Sdn Bhd, I am required to propose different types of retaining wall system available. My proposal focuses on the site investigation, importance of retaining wall, construction methods, design concepts and preventive measures to mitigate functional failures. I am required to propose an appropriate selection of the most suitable retaining wall system used to the Board of Directors, in which the project is located at hilly area which consists of 20 units of luxury bungalows.
In this proposal, I am going to discuss:
the importance of site investigation,
the importance of retaining wall,
the concept of retaining wall,
types of retaining wall
the most suitable retaining wall for this hilly project.
The Importance of Site investigation
Every man-made civil engineering structure is found on, in or with ground. The structure we put on the ground is man-made. We can control to design every item that goes into our structure, such as concrete, reinforcement, bricks and even the last wall plug. However, the ground below on which this structure will stand is not man-made. We usually have less knowledge about it and we undoubtedly cannot design or control its behavior. The ground below will determine the economies of how tall, how heavy, how safe the final structure is going to be and not the other way round. In short, the safety and economics of every civil engineering structure is influenced by the properties of the ground on which this structure will stand. These properties of the ground below or sub-soil properties are obtained via Site Investigation.
In any site investigation work, the questions which should be resolved in determining the investigation program are:
What type of investigation is needed,
Why they are needed,
Where the actual field works should be performed,
How the work is to be done.
Whether the investigation is sufficient or too much.
Site investigation is normally crucial and carried out prior to the commencement of design of a construction project. Site investigation has been defined as investigation of the physical characteristics of the site and includes documentary studies, site surveys and ground investigation. It is also refers to the actual surface or subsurface investigation, including on site and laboratory tests. In broad sense, study of the site history and environment, interpretation and analyses of all available data, and making recommendations on the favorable/unfavorable locations, economic and safe design, and prediction of potential risks should also be included in site investigations.
First and foremost, a desk study to reveal data which may already exist concerning the site, its geology and history, together with a site reconnaissance, is invaluable and can be done before purchase at minimal cost. This may be followed by trial pitting, probing or boring as appropriate to the conditions revealed by preceding studies and in the light of the development proposals. It is also a precaution to minimize damage that could be caused by these soils could also be communicated at this stage. At least home dwellers would be alerted to potential problems (and the associated costs) from the outset, thus enabling them to make informed decisions regarding the most appropriate foundation system for their homes.
Site investigation is designed to identify the characteristics of soils or fill materials which lie beneath the site, the groundwater conditions and the existence and extent of other physical features or contaminants which may be present. This information influences the selection and design of an appropriate structural form for the proposed building. It is a process which should continue on an iterative basis throughout the design and construction phases. Post-construction monitoring can also be of importance in many instances as part of a validation process.
The combined building loads indicated in figure 1 must be safely supported by the subsoil and also ensure that unreasonable movements of the building do not occur. If the supporting soil is sufficient resistant and its characteristics under load are likely to remain satisfactory, the problems of support and movement will be easily resolved. However, few soils other than rock can resist these concentrated loads and it is usually necessary to collect the resolved loads at their lowest point and transfer them to adequate bearing soil known to be available on a particular site (figure 2).
Figure 1: Combined building loads
Figure 2: Method of transferring combined building loads to supporting soil.
The general distribution of soil types in the United Kingdom is indicated in figure 3; the soils include peat, clay, slit, sand and gravel. Corresponding safe bearing pressures are also given.
Figure 3: simplified distribution of various types of supporting soils
Lastly, site investigation should be undertaken by professional specialists, such as surveyors, geotechnical engineer and ground investigation contractor, and in a phased manner. The ground investigation contractor is responsible for providing reliable factual data. The geotechnical consultant should responsible for the planning & execution of the investigation program, interpretation and analyses of results, and making appropriate design recommendations to avoid over design as well as unsafe design.
Failure of Site Investigation
Due to lack of or inadequacy of guide/code requirement regarding the extent as well as quality of site investigation work, geotechnical failures often occurred. These failures sometime led to catastrophic disaster and imposed serious threat to public safety.
For the Highland Incident in Kuala Lumpur in 1993, the Architect appointed qualified civil engineer to be the consulting engineer for Highland Towers. Initially, civil engineer's scope of works was restricted to the structural aspect of the three blocks. But subsequently, the civil engineer was engaged by the developer to submit proposals over the drainage of the area. His drainage plan was approved. He was also retained by the developer to design and supervise the construction of two retaining walls on the Highland Towers site. The Plaintiffs claimed that the civil engineer was negligent for the following reasons:
(i) Designing unsuitable foundations;
(ii) Lack of care and concern of the hill and slope;
(iii) Issuing a notice to the authorities confirming the drainage works was completed when only a fraction of it was done.
By the above acts of preparing, designing and supervising the construction of Highland Towers and the drainage system of the Highland Towers site, he was negligent and had caused nuisance to them. The civil engineer had used rail piles welded together as foundation to support the three apartment blocks. This type of piles, which was considered inferior to concrete piles, was accepted in the engineering and building industry to support high-rise buildings at the material time. Thus, no fault can be attributed to the civil engineer in using the rail piles as he was only adhering to the accepted professional practice at that time. However, there was lack of consideration by the civil engineer to the hill and the slope directly behind the three blocks. The court ruled that the civil engineer should have reasonably foreseen the danger of a landslide producing a lateral load against the foundation of the building. For this, he should have exercised care to either design or construct a foundation to accommodate the lateral load or ensure that the slope was reasonably stable. Failure to do so is a breach of his duty of care he owes to the Plaintiffs since his duty was to ensure the safety of the buildings he designed and built. The civil engineer's attempt to deny liability on the ground that he relied on the developer to ensure that other retaining walls were constructed properly was unsuccessful. The judge found that it was incumbent upon the civil engineer to enquire and ascertain whether the work was that of a qualified professional and what its impact might be on the safety of his own building.
Figure 4: Highland Incident in Kuala Lumpur in 1993
What is Retaining Wall?
A retaining wall is a stabilizing structure designed and constructed to retain soil at a slope that is greater than it would naturally assume, usually at a vertical or near-vertical position. Besides, the retaining wall used to prevent the erosion and the movement of soil. The function of retaining wall is to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the natural slope taken by the soil which is called the angle of repose of the soil. The retaining wall is the wedge of soil resting on this upper plane of the angle of repose that a retaining wall has to support.
It is also designed with weep holes which allow collected water to escape. This releases the additional pressure created by a accumulated water and helps in stabilizing the retaining wall.
The walls are designed to offer the necessary resistance by using their own mass to resist the thrust or relying upon the principles of leverage. The terminology used in retaining wall construction is shown on figure 5:
Figure 5: Terminology of Retaining Wall
Constructing a retaining wall, several types of materials can be used. Stone and concrete are the most common materials used in constructing a retaining wall. Besides, there are also special retaining wall blocks crafted from aggregate materials and light concrete which are designed for this purpose. Because each block fits securely with the next, some styles interconnect, making building simpler, less costly and time required. These blocks do not require the addition of mortar due to the fit of these blocks is secure.
A retaining wall can be tiers or a series of "steps", which allow more efficient erosion control as well as a more sophisticated design. Different types of plantings, flowers or materials in each tier can be included in the design to bring more color, texture and interest to the area. By breaking down the amount of soil and pressure, a tiered design also give a better erosion control held by each division of the retaining wall instead of adding the aesthetic value.
Today, there are several styles and types of retaining wall blocks, and most people choose preformed blocks. Unlike today, large stones and railroad ties were often used to construct a tiered retaining wall in the past. Performed blocks are affordable and user- friendly, making other methods out of date at all, except the look of rough stone or wood is preferred.
The Concept of Retaining Wall
The design of any retaining wall is basically concerned with the lateral pressures of the retained soil and any subsoil water. The purposes to construct a retaining wall are shown as below:
It is difficult to precisely identify the properties of any soil because they are inconsistent materials. The calculation of pressure exerted at any point on the wall is a task for the expert, who must take into account the following factors:
Nature and type of soil;
Height of water table;
Subsoil water movements;
Type of wall;
Materials used in the construction of the wall.
Design calculations related to the resultant thrust of retained material behind a 1m length of wall. There are two well-established methods can be used to determine the resultant thrust:
Coulomb's graphical representation or wedge theory.
The designer is mainly concerned with the effect of two forms of earth pressure:
ACTIVE EARTH PRESSURE are those that tend to move or overturn the retaining wall, and are composed of the earth wedge being retained together with any hydrostatic pressure caused by the presence of groundwater. The latter can be reduced by the use of subsoil drainage behind the wall, or by inserting drainage openings called weep holes through the thickness of the stem, enabling the water to drain away.
PASSIVE EARTH RESISTANCES are reactionary pressures that will react in the form of a resistance to movement of the wall. If the wall tends to move forward, the earth in front of the toe to counteract the forward movement. This pressure can be increased by enlarging the depth of the toe or by forming a rib on the underside of the base.
Active earth pressures must equal to passive earth resistances in order to avoid overturning and circular slip. Typical examples of these pressures are shown in figure 6 and figure 7.
Figure 6: Active and Passive Earth Pressures act on Mass Retaining Wall
Figure 7: Active and Passive Earth Pressures act on Cantilever Retaining Wall
The overall stability of a retaining wall is governed by the result of the action and reaction of a number of loads:
EFFECT OF WATER
The design and stability can be affected by ground water behind a retaining wall whether static or percolating through subsoil. The pressure on the back of the wall will be increased. By reducing the soil shear strength, the bearing capacity of the soil can be reduced; it can reduce the frictional resistance between the base and the soil and reduce possible passive pressure in front of the wall. As a result, the issue of drainage of the water behind the retaining wall is the utmost important in the design.
SLIP CIRCLE FAILURE
Slip circle failure (shown in figure 8) is sometimes encountered with retaining wall in clay soils, particularly where there is a heavy surcharge of retained material. It takes the form of a rotational movement of the soil and wall along a circular arc. The arc commences behind the wall and passes under the base, resulting in a tilting and forward movement of the wall. Further movement can be prevented by driving sheet piles into ground in front of the toe, to a depth that will cut the slip circles arc.
Moment due to weight of retained earth and wall above slip circle arc about O is greater than restoring moment RM.
RM=permissible shear stress x length of arc ABC x arc radius OC
Result: mass above ABC rotates about O. Wall tilts forward and earth heaves in front.
Figure 8: Retaining wall failure due to rotational movement.
The Importance of a Retaining Wall
A retaining wall might be built on property for various reasons. Some level of dirt and soil will be held away from home, garden, pool or play area by building a retaining wall.
The sheer aesthetics might also be added on it. A more usable land will be provided if there have a lot of rolling and slopping yard.
The importance of retaining wall is stated as below:
Retaining Walls Hold Back the Earth
Buildings might be built on a hill or in a valley between several hills. Retaining wall will hold back the earth once the hillside has been dug out to construct buildings. Dirt does back in from the sides of the wall with each shovel full. The more you shovel, the dirt begins to fall back in. the sides are loose and weak, so they will crumble at the slightest provocation.
The dirt which remains has to discharge when the side of a hill is dug out. If left alone, it will eventually comes tumbling down. The dirt will be held back and the safety will be ensured.
Retaining Walls Provide More Usable Land
A walk-out basement, patio, playground, garden, tennis court or swimming pool might be installed into proposed development. If the land is rolling or hilly, work cannot be begun until the ground is leveled off.
A sort of cliff, where a part of the proposed land which is not dug out is higher than the lower flat land, will then be left out. Adding a retaining wall will have more usable land as well as will add structure and beauty to your new area. Retaining wall can be used as steps into the pool where pool equipments to be held.
Retaining Walls Avoid the Ground from Washing Away
If the area gets a lot of rainfall or near water such as lake, a retaining wall will keep the water from eroding the soil of your landscaping and around your foundation. This could be a safety measure to prevent landslides and river of dirt from floating towards the buildings.
Types of Wall
Mass retaining walls
Mass retaining walls also known as gravity walls. They rely upon their own mass together with the friction on the underside of the base to overcome the tendency to slide or overturn. They are generally economic only up to a height of 1.800m. Mass walls can be constructed of semi-engineering quality bricks bedded in a 1:3 cement mortar or of mass concrete. Mass concrete could have some light fabric reinforcement to control surface cracking. Natural stone is suitable for small walls up to 1.000m high, but generally it is used as a facing material for walls up to 1.000m high, but generally it is used as a facing material for wall over 1.000m.
Typical examples of mass retaining walls are shown in figure 9 and 10:
Figure 9: Brick Mass Retaining Walls
Figure 10: Mass Concrete Retaining Wall with Stone Facings
Cantilever walls are usually of reinforced concrete, and work on the principles of leverage. Two basic forms can be considered: a base with a large heel so that the mass of the wall with a large toe (figure 10). The figure shows typical sections and patterns of reinforcement encountered with these basic forms of cantilever retaining wall. The main steel occurs on the tension face of the wall, and nominal steel (0.15% of the cross-sectional area of the wall) is very often included in the opposite face to control the shrinkage cracking that occurs in in-situ concrete work. Reinforcement requirements, bending, fabricating and placing are dealt with in detail in the section on the reinforced concrete.
Reinforced cantilever walls have an economic height range of 1.200-6.000m. Walls in excess of this height have been economically constructed using prestressing techniques. Any durable facing material may be applied to the surface to improve the appearance of the wall, but it must be remembered that such finishes are decorative and add nothing to the structural strength of the wall.
Figure 10: Reinforced Concrete Cantilever Retaining Walls
Counterfort Retaining Walls
These walls can be constructed of reinforced or prestressed concrete, and are considered suitable if the height is over 4.500m. The counterforts are triangular beams placed at suitable centres behind the stem and above the base to enable the stem and base to act as slabs spanning horizontally over or under the counterforts. Figure 11 and figure 12 show a typical section and pattern of reinforcement for a counterofrt retaining wall.
If the counterforts are placed on the face of the stem they are termed buttresses, and the whole arrangement is called a buttress-retaining wall. The design and construction principles are similar in the two formats.
Figure 11: Reinforced Concrete Counterfort Retaining Wall
Figure 12: Reinforced Concrete Counterfort Retaining Wall
Reinforced masonry retaining walls
Steel reinforcement may be used in brick retaining walls to resist tensile forces and to prevent the effects of shear. A brick bonding arrangement known as Quetta bond is used to create a uniform distribution of vertical voids. Vertical steel reinforcement is tied to the foundation reinforcement and spaced to coincide with purpose-made voids. The voids are filled with concrete to produce a series of reinforced concrete mini-columns within the wall when the brickwork is completed.
Where appearance is not important, or the wall is to receive a surface treatment, reinforcement and in-situ concrete within hollow concrete block work provide for economical and functional construction. Figure 13 shows the application of standard-profile, hollow, dense concrete blocks lay in stretcher bond as permanent formwork to continuous vertical columns.
Figure 13: Reinforced Concrete Block Retaining Wall
The height potential and slenderness ratio (effective height to width) for reinforced masonry walls can be enhanced by post-tensioning the structure. For purposes of brick walls there are a number of construction options, including:
Quetta bond with steel bars concrete in the voids;
Stretcher-bonded wide cavity with reinforced steel bars coated for corrosion protection;
Solid wall of perforated bricks with continuous voids containing grouted steel reinforcement bars.
Some examples are shown in figure 14:
Figure 14: Post-tensioned Brick Retaining Walls
Based on discussion shown on above, I would like to select Cantilever retaining wall as the most suitable retaining wall for the project where is located at hilly area which consists of 20 units of luxury bungalows.
As shown as above, a cantilever retaining wall is a form of masonry installation that holds a large amount of earth in place. The cantilever design is just one of several variations on a retaining wall design, using various principles to manage earth loads. Cantilever retaining wall has an economic height range of 1.200-6.000m. This type of retaining wall is suitable for bungalows because the height is sufficient to support the bungalows.
A concrete cantilever retaining wall uses a relatively thin stem of steel-reinforced, cast-in-place, concrete or mortared masonry. In a cantilever retaining wall design, an earth pressure vector acts horizontally against the side of the wall. The bottom part of the wall presents a gravity vector downward. That gravity vector produces an opposite force upward. The resulting vector counters the earth pressure vector, and pushes back against the earth load. This type of retaining wall is more stable compared to the other three types which stated on above in order to build 20 units of luxury bungalows because the loads are distributed equally.
Retaining wall design is evaluated to moderate the effects of a landslide. Retaining walls can be helpful in protecting soil against the kinds of movement associated with these natural disasters. Looking at landslide risk and the risk of liquefaction, the water saturation of earth, is part of assessing how a retaining wall works. Cantilever retaining wall is one that consists of a uniform thickness wall which is tied to a footing. It located at the basement of bungalows. Thus, cantilever retaining wall will hold back the earth. Soil erosion, landslides and environment disasters are less likely to be occurred. In other words, cracking and collapse of 20 units of luxury bungalows are avoided.
Weep holes are designed in the cantilever retaining wall to release the additional pressure created by a accumulated water and helps in stabilizing the retaining wall. This ensures the building will not collapse due to the fast flow of water on the hill.
In conclusion, a suitable retaining wall is able to stabilize the soil and avoid overturning and sliding of building. Beside, a suitable retaining wall can also help in saving the construction cost. Therefore, cantilever retaining wall is the most suitable retaining wall for 20 units of luxury bungalows which are located on the hilly area.