Temporary Earth Retaining System

(Copyrighted Content by Aung Myo Myint – Not for Re-use)

Deep Excavations are increasing day by day as less space over the surface of the earth is available for construction and as a result we are forced to go underground for more space and better utilization of the space. Deep excavations need temporary earth retaining structures which ensures that the movements in the ground should not cause damage to the neighboring structures. Temporary Earth Retaining Structures enables to cut sides of the excavation to vertical or near vertical keeping the surroundings stable and unable the sidings to collapse or damage.  An excavation can be done vertically or horizontally. When doing horizontally the earth retaining system has to support the top layer and then only the bottom can be excavated and when going vertically we need to provide strength to the sidings by various earth retaining systems. The temporary earth retaining system is basically built to enable the surrounding ground or soil to hold.

There are various types of Temporary Earth Retaining System. While choosing an Earth Retaining System a number of conditions should be taken into consideration

ü  Safety: Human & surrounding should be safe and secure and no harm should be caused due to excavation. Safety should be taken against failure, breakdown, vibration, change in soil composition, undercutting & premature removal of retaining structures.

ü  Economics: Cost effectiveness is a very crucial aspect as there is a range of methods that can be used so one must choose more efficient and cost effective method.

ü  Soil Conditions: Soil conditions like hard rock are very stable until disturbed. Soft Soil is an unstable soil which should be given rigid support for the excavation and so and so forth.

ü  Ground Water: Ground Water can cause instability in an excavation as the force generated by water is immense and water levels in all the areas are very different.

ü  Underground Obstructions: Underground obstructions are generally boulders, trees, logs etc. Which can cause problems in excavation.

ü  Overhead Obstructions: Overhead obstructions like bridges, power lines, buildings etc. Can cause difficulties in excavation.

ü  Environmental: In Urban and Suburban areas sound, noise, vibrations are constraints to excavation and temporary earth retaining system.

Excavation Bracing are majorly used in a narrow excavation in which firstly Waler a horizontal member is placed against the sole support and after Waler, Struts are placed between the Waler as vertical bracings. The whalers are fixed on to the laggings which are fixed on the soldier beams which are I shaped and directly fixed to the soil surface. This kind of structure needs a proper study of the type of soil, seismic forces and other integral factors.

In Temporary Earth Retaining Systems there is no method which is fit for all because the method to be used depends on a lot of factors such as geology, cost, technology available etc. Slope failure only occurs when the forces driving exists the resisting forces. So a study of the driving forces is very necessary to define how much resisting force is required.

Cheap and cost effective ways of preventing soil erosion are spray on products and plastic coverings which enables the soil to maintain its integrity and does not let the moisture or driving forces to act on against the plastic or spray on product.

While excavating and using Temporary Earth Retaining Systems de-watering is a very crucial aspect to be considered as lowering groundwater in the vicinity helps in maintaining the effective stress of the soil hence settlement of the ground.

Raker Bracing is a system in which a deep cut is made through the center and a foundation slab is placed. Then rakers or inclined struts are used to be braced from the foundation slab to the side walls which are covered in satin and the whalers are put to fix the inclined struts or rakers.

Tieback Systems are a very general system in which a combination of Slurry trench method and anchors are used. The slurry trench method is used to build a wall of concrete after in which post-tensioned tieback is inserted in the soil at an angle of 10 or 20 degrees. Usually the tieback or soil nails are cut only after the permanent structure can bear the load.

Soil Nailing is a system in which array of soil nails is used to create a grid which function to create a mass of soil which is stable. The mass of the soil created by the nails retains the less stable material behind it. Soil nail is an economical and fast way of rationing system. Soil nailing requires proper craftsmanship, hand work and Geotechnical knowledge to construct.

While excavating and building temporary earth retaining systems there are various Do’s and Don’ts which are

Do’s:

• Having proper knowledge of the soil.
• Ensuring correct grade of bolts for all connections.
• Ensuring the struts is correctly and adequately located and supported on the wailers.
• Proper Inspection.
• In case of Nailing then ensuring right size and strength of nails are used.

Don’ts:

• Not following adequate safety measures.
• Not keeping a track of the weather conditions.
• Using sub graded materials.
• Improper removal of temporary earth retaining systems.
• Inadequate knowledge of Geotechnical subject.

A lot of professionals devote their lives to improve the Temporary Earth Retaining Systems and are working towards developing new systems which are more cost effective, fast and more safe. As there have been numerous incidents around the world causing harm to human life because of improper Temporary Earth retaining Systems. To conclude it will be worthwhile mentioning that these systems are very important for the growth of theinfrastructural and building needs we are moving towards as day by day we require more and more buildings to accommodate our homes, offices, bridges, roads etc. So we need these systems to be in the continuous development phase.