Theory of structures

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Information about Theory of structures

Published on March 6, 2014

Author: hamzaaaaaah



RETAINING WALL    Retaining walls are used to retain earth or other materials which have the tendency to slide and repose at a particular inclination. They provide lateral support to the earthfill, embankments or other materials in order to hold them in a vertical position. Retaining walls also have application in buildings and bridges such as basement, foundation wall, bridge abutment etc.

The primary geotechnical concern in design and installation of retaining walls is that the retained material is attempting to move forward and down slope due to gravity. This creates soil pressure behind the wall, which can be analysed based on the angle of internal friction (φ) and the cohesive strength (c) of the material and the amount of allowable movement of the wall. This pressure is smallest at the top and increases toward the bottom in a manner similar to hydraulic pressure, and tends to push the wall forward and overturn it.  Groundwater behind the wall that is not dissipated by a drainage system causes an additional horizontal hydraulic pressure on the wall.

STRUCTURE OF RETAINING WALLS (1) (2) (3) (4) (5) Base Heel and Toe Stem Batter: the outer face of the wall which is built inward to prevent the wall tipping over. Backfill: The material placed behind a retaining wall.

COMMON MATERIALS USED FOR RETAINING WALLS :        Wood sheets Steel and plastic interlocking sheets Reinforced concrete sheets Precast concrete elements (crib walls and block walls) Closely spaced in-situ soil-cement piles Wire-mesh boxes (gabions) Anchors into the soil or rock mass (soil nailing)

EARTH PRESSURE ON RETAINING WALL The effect of 2 forms of earth pressure need to be considered during the process of designing the retaining wall that is:  Active Earth Pressure “ It is the pressure that at all times are tending to move or overturn the retaining wall”  Passive Earth Pressure “It is reactionary pressures that will react in the form of a resistance to movement of the wall.

ACTIVE EARTH PRESSURE The active state occurs when a soil mass is allowed to relax or move outward to the point of reaching the limiting strength of the soil; that is, the soil is at the failure condition in extension.  Thus it is the minimum lateral soil pressure that may be exerted.  It is composed of the earth wedge being retained together with any hydrostatic pressure caused by the presence of groundwater.  This pressure can be reduced by:  The use of subsoil drainage behind the wall  Inserting drainage openings called weep holes through the thickness of the stem to enable the water to drain away. 

PASSIVE EARTH PRESSURE     The passive state occurs when a soil mass is externally forced to the limiting strength (that is, failure) of the soil in compression. It is the maximum lateral soil pressure that may be exerted. This pressure build up in front of the toe to resist the movement of the wall if it tries to move forward. This pressure can be increased by enlarging the depth of the toe or by forming a rib on the underside of the base.


SHEET PILE WALLS    Used to build continuous walls for waterfront structures and for temporary construction wall heights > 6 m if used with anchors. Can be made of steel, plastics, wood, pre-cast concrete. ADVANTAGES 1. Provides higher resistance to driving stresses; 2. Is of an overall lighter weight; 3. Can be reused on several projects; 4. Provides a long service life above or below the water table; 5. Easy to adapt the pile length by either welding or bolting; 6. Their joints are less apt to deform during driving.

Soldier Piles and Lagging Walls This type of wall was known to Roman military engineers, and was used for deep excavations.  It is a relatively inexpensive system, easy and fast to construct.  It is primarily limited to temporary construction, and cannot be used in high water table conditions without extensive dewatering and expense. 

Slurry Walls         A slurry wall refers to the method of construction. Specifically, the digging of a deep trench with a special bucket and crane. As the trench becomes deeper, the soil is prevented from collapsing into the trench by keeping the hole filled with a “slurry”. This slurry is a mixture of water with bentonite The bentonite makes the slurry thick, but liquid. This keeps the soil lateral walls from collapsing into the excavation. When the excavation reaches the intended depth, the slurry filled excavation is reinforced with steel and carefully filled with concrete. These walls have been built to 100 foot depths and range from 2 feet to 4 feet in thickness. The panels are typically 15 feet to 25 feet long, and are linked with one another through tongue and groove type seals (to prevent the intrusion of groundwater into the future underground site. Slurry walls have the advantage of being stiffer than sheet pile walls, and hold back the soil better than soldier piles, lagging and steel sheeting.

Secant Pile Walls        These walls are formed by the intersection of individual reinforced concrete piles. These piles are built by using drilling mud (bentonite) and augering. The secant piles overlap by about 3 inches. An alternative are the tangent pile walls, where the piles do not have any overlap. These piles are constructed flush with each other. The important advantage of secant and tangent walls is the increased alignment flexibility. The walls also may have increased stiffness, and the construction process is less noisy. The disadvantage is that waterproofing is difficult to obtain at the joints, their higher cost, and that vertical tolerances are hard to achieve for the deeper piles.


GRAVITY RETAINING WALL       A gravity wall is made of plain concrete or brick masonry. The stability of the wall is maintained by its weight. It is generally made up to a height of 3 m of wall. Gravity walls are the earliest known retaining structures. They are built from solid concrete or rock rubble mortared together. The lateral forces from backfill is resisted by the weight of wall itself, and due to their massive nature, they develop little or no tension. Therefore, they are usually not reinforced with steel.


CANTILEVER RETAINING WALL    It consists of a vertical wall, heal slab and a toe slab which act as cantilever beams. Its stability is maintained by the weight of the retaining wall and the weight of the earth on the base of the retaining wall. It is generally made when the height of the earth on the base is in the range of 3 m to 8 m.

COUNTERFORT RETAINING WALL     When the height of retaining wall is more than 6-8m, it is economical to tie the vertical wall with the heel slab by counter forts at some spacing. The counterforts act as tension members to support the vertical wall and reduces bending moment in it. It also provides support to heel slab. Generally, counterforts are spaced at approximately one-third of the height of wall.


BUTTRESS RETAINING WALL   It is similar to counter fort wall except that the vertical wall is tied with the toe of the retaining wall at some spacing. It acts as a compression member to support the vertical wall and reduces bending moment in it.

CRIB WALLS Crib walls are made up of interlocking individual boxes made from timber or pre-cast concrete.  The boxes are then filled with crushed stone or other coarse granular materials to create a free draining structure.  There are two basic types of crib wall: - Timber, and - Reinforced pre-cast concrete. 

GABBION WALLS   Gabbions are multi-celled, welded wire or rectangular wire mesh boxes, which are then rockfilled, and used for construction of erosion control structures and to stabilize steep slopes. Their applications include, - Retaining walls, - Bridge abutments, -Wing walls, - Culvert headwalls, - Outlet aprons, - Shore and beach protection walls, and - Temporary check dams.

MECHANICALLY STABILISED EARTH     Mechanically stabilized earth or MSE is soil constructed with artificial reinforcing. It can be used for retaining walls, bridge abutments, dams, seawalls, and dikes. Although the basic principles of MSE has been used throughout history, MSE was developed in its current form in the 1960s. The reinforcing elements used can vary but include steel and geosynthetics.

MSE Mechanically stabilized earth (MSE) retaining walls combine geosynthetic-reinforced earth layers (such as geogrids) and a fascia of geocell, wire mesh or concrete blocks to create high-strength, versatile earth retention systems.  The geogrid reinforces the soil while the fascia protects the wall face.  The versatility of MSE walls makes them suitable for a variety of applications, from road structures to golf courses. 

 Mechanically Stabilized Earth Walls Facing There are several facing systems used with mechanically stabilized earth systems. Some of the common facing options are:  Segmental precast concrete panels- Precast panels in the form of square, rectangle, diamond or hexagon, connected with shear pins.  Modular block wall – Solid or with cores filled with aggregates during their installation. Usually name Keystone, Versa-Lok, etc.  Metallic Facings – lighter facing finished, made of galvanized steel split into half cylinders.  Gabion Facing- can be connected to the wall systems.  Geosynthetic Facing- Geotextile reinforcement, forming the face of the retaining wall. They can be exposed to vandalism, weather inclemency and other types of surfacing problems.  Post construction Facing – Shot Crete, cast-in place, spray on, plastering or other techniques can also be used as a compliment facing for the mechanically stabilized earth wall facing. It adds some cost to the construction but will provide an aesthetic finish surface for the MSE walls.

REINFORMENT IN MSE     The reinforcement materials of MSE can vary. Originally, long steel strips 50 to 120 mm (2 to 5 in) wide were used as reinforcement. These strips are sometimes ribbed, although not always, to provide added friction. Steel grids or meshes are also used as reinforcement. Several types of geosynthetics can be used including geogrids and geotextiles. The reinforcing geosynthetics can be made of high density polyethylene, polyester, and polypropylene.

ADVANTAGES OF MECHANICALLY STABILISED RETAINING WALLS             Simple construction. The use of heavy equipment is reduced. Faster construction than traditional concrete walls. Do not require specialized labor. Reduces the need for wall finishing. Less site prep is needed. Can be built in confined areas or areas where a concrete wall is almost impossible to be constructed. Mechanically Stabilized Earth walls are susceptible to elastic deformation. High seismic load resistance. MSE walls can be used as tall structures, exceeding more than 60 feet of wall height. Can be combined with other products. Various shapes and forms can be made.

DISADVANTAGES OF MSE      Require a relatively large space behind the wall or outward face to obtain enough wall width for internal and external stability. MSEW require select granular fill. (At sites where there is a lack of granular soils, the cost of importing suitable fill material may render the system uneconomical). Requirements for RSS are typically less restrictive. Suitable design criteria are required to address corrosion of steel reinforcing elements, deterioration of certain types of exposed facing elements such as geosynthetics by ultra violet rays, and potential degradation of polymer reinforcement in the ground. Since design and construction practice of all reinforced systems are still evolving, specifications and contracting practices have not been fully standardized.

WHAT IS GEOSYNTHETIC…? Geosynthetics is the term used to describe a range of generally polymeric products used to solve civil engineering problems.  The term is generally regarded to encompass eight main product categories:  geotextiles, geogrids, geonets, geomembranes, geosynthetic clay liners, geofoam, geocells (cellular confinement) and geocomposites.  The polymeric nature of the products makes them suitable for use in the ground where high levels of durability are required.  These products have a wide range of applications and are currently used in many civil, geotechnical, transportation, geoenvironmental,hydraulic, and private development applications. 

TYPES OF GEOSYNTHETICS USED FOR SOIL REINFORCEMENT    Geotextiles (particularly woven geotextiles), geogrids and geocells. Geotextiles are continuous sheets of woven, nonwoven, knitted or stitch-bonded fibres or yarns. The sheets are flexible and permeable and generally have the appearance of a fabric. Geogrids have a uniformly distributed array of apertures between their longitudinal and transverse elements. These apertures allow direct contact between soil particles on either side of the sheet.

 Geocells are relatively thick, three-dimensional networks constructed from strips of polymeric sheet. The strips are joined together to form interconnected cells that are in filled with soil and sometimes concrete. In some cases 0.5 m to 1 m wide strips of polyolefin geogrids have been linked together with vertical polymeric rods used to form deep geocells layers called geomattresses.

In-Situ Walls     These walls do not rely on their mass to retain the soil. They rely on their flexural strengths to retain the soil. They are supported by penetration into the soil or by anchoring systems. For example, soil nailing.

Soil nailing    Soil nailing is a technique whereby in-situ soil is reinforced by the insertion of steel rods, 20–30 mm in diameter, However, because of the possible corrosion of steel bars, reinforcement coatings with high resistance to corrosion are being developed and fibre-glass nails have been used recently . The technique is employed to enhance the performance of granular soils and stiff clays.

BASEMENT / FOUNDATION WALL    Retaining wall restrained at the bottom by the basement floor slab and at the top by the first floor slab is known as basement wall. It is subjected to lateral earth pressure exerted by earth fill and the vertical load from the super structure. The lateral support to the basement wall is provided by the basement floor and first floor slabs.

BRIDGE RETAINING WALL   Its behaviour is similar to that of the basement or foundation wall. The bridge superstructure induces horizontal as well as vertical loads that alter the normal cantilever behaviour.


MSE PRECAST PANEL RETAINING WALL Reinforced Earth retaining walls are coherent gravity structures consisting of alternating layers of granular backfill and high-strength, inextensible discrete steel strip soil reinforcement with a modular precast concrete facing.  They are used extensively in highway projects for retaining walls and bridge abutments, in seawalls, dams, bulk storage facilities, and in supporting various types of railway transit. 

Retained Earth – MSE Precast Panel Retaining Wall Retained Earth retaining walls are coherent gravity structures consisting of alternating layers of granular backfill and high-strength, inextensible, high-adherence welded wire bar mat soil reinforcement with a modular precast concrete facing.  They are used extensively in highway projects for retaining walls and bridge abutments, in seawalls, dams, bulk storage facilities, and in supporting various types of railway transit. 

Terratrel– MSE Wire Faced Retaining Wall    Terratrel is a wire-faced MSE wall system that combines the technology of Reinforced Earth and Retained Earth with the economy of wire facing. The system provides a cost-effective alternative for wall applications where aesthetics are not critical, a temporary retaining structure is required or differential and overall settlement exceeds the limits for a one-stage precast panel MSE wall.  The Terratrel system can be designed to utilize discrete steel strip, welded wire bar mat or high-tenacity polyester based geostrap soil reinforcements.

TechSpan – Precast Arch    TechSpan is a state-of-the-art three-hinged precast arch system which can be used in conjunction with The Reinforced Earth MSE products.  It used for the construction of bridges, culverts, rail and roadway tunnels, mining and industrial tunnels and undercrossings.  Advanced design procedures and unique fabrication capabilities provide the most efficient use of concrete and steel.  

GeoMega– MSE Precast Panel Retaining Wall  GeoMega retaining walls are coherent gravity structures consisting of alternating layers of granular backfill and a high-tenacity polyester based geostrap soil reinforcement with a modular precast concrete facing.  The Omega strip consists of high tenacity polyester fibers encased in a polyethylene sheath which makes it ideal for use in the construction of walls in where sea water or other chemically aggressive environments preclude the use of galvanized steel soil reinforcements.

Piano Wall – MSE Precast Retaining Wall with Traffic Barriera MSE wall system which combines the  Piano Wall is technology of a Reinforced Earth retaining wall topped with an integral traffic barrier in a single engineered unit.  This unique solution was developed for projects that require a Jersey-type traffic barrier atop a five- to tenfoot high Reinforced Earth retaining wall.   It is ideal for projects requiring low-height grade separations adjacent to bi-level roadways.

TechWall Precast Counter fort Retaining Wal l TechWall is a full height panel which combines a precast counter fort and wall facing into one unit.  When the traditional design advantages and benefits of a typical counter fort retaining wall are recommended due to items such as a severe right-ofway restriction, large utility conflict, construction in cut conditions, and roadway widening TechWall is an effective solution which encompasses the quality and efficiency of precast concrete.

Fan wall  - Noise wall / Protective Barrier Fan wall is a precast modular permanent barrier wall system which can be a freestanding trapezoidal structure or where right-of-way is minimal, a post and panel alternative is available.    A unique, rotatable and interlocking connection system and limitless architectural finishes makes Fanwall extremely versatile.   This engineered barrier wall system is available for nearly every noise or security problem found in the transportation, utility, industrial, and commercial markets. 

RE/Tension - Precast Counter fort Retaining Wall RE/Tension consists of integral precast box attached to the back of a standard MSE wall facing panel, the box serves as a form for a cast-in-place concrete counterfort.   This system is ideally suited for retaining walls with right-of-way constraints, large utility conflicts and full or partial rock cut situations.   Since the facing is comprised of standard MSE wall panels the RE/Tension system can be easily combined with a Reinforced Earth retaining wall within the same structure, while maintaining an identical aesthetic appearance.      

Advantages of retaining walls  A retaining wall provides advantages such as preventing damage by soil falling or sliding away from a foundation. This can preserve ground area on the property or even add to it.

   DISADVANTAGES OF RETAINING WALLS Vertical Cracks  Some retaining walls begin developing vertical cracks, such as in poured concrete. This is often due to excessive pressure or wide changes in temperature. In order to prevent these vertical cracks, provide plenty of good drainage, which can often be expensive to accomplish. Failing Foundation  Brick retaining walls often fail under pressure.  Retaining walls need extensive drainage resources to prevent moisture, but they also require a strong foundation. Some retaining walls will not be able to retain high amounts of pressure from soil, such as bricks walls. This often results in crumbling bricks or a failing foundation. Termites  Retaining walls also can attract termites to the property, as is often the case when timber is used as a retaining wall. Termites can nest inside rotting timber and near molded areas. Correcting this problem can be expensive, if you have to use termite monitoring and treatment devices.

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