Retaining systems are required in excavations where the site does not allow for sides to be safely sloped back. Typically this applies to excavation greater than 8 feet or deeper in soil or unstable rock.
There are several systems typically used depending on the conditions. In soil, common systems include H-Beam and wood lagging, and soil nailing. In rock, protection can be as simple as chain link fence to contain loose rocks or the more positive system or friction rock stabilizers (spit sets) and shot Crete face.
Top down construction of walls is the process of completing the retaining or exterior wall from the top down. This process is completed in conjunction with soil nailing or rock nailing. The soil nails serve the function of tying back the excavation and supporting the vertical loads of the wall. The advantages include significant cost and time savings, the flexibility to accommodate design modifications during construction, and a “one-wall” system that supports construction and provides a permanent wall.
Soil nailing is a procedure to reinforce and strengthen ground adjacent to an excavation by installing closely spaced steel bars, called nails as construction proceeds from the top down. The process is effective in cohesive soil, broken rock, shale or kurst. This technique permits felxibiity to meet almost any site-specific condition to conform a variety of geometric shapes to meet specific site needs, spacing, placement angles, and nail lengths.
Both in-situ and precast piling systems can be used to provide fast and effective foundation solutions.
In-situ techniques available include:
• Bored piling with permanent or temporary casings
• Continuous Flight Augured Piles (CFA)
• Continuous Helical Displacement Piling (CHD)
These piling techniques are regarded as non-displacement meaning there is less risk of ground heave. The length of in-situ piles can be readily varied to suit ground conditions and they can be installed with very large diameters or long lengths. An additional advantage is the low level of noise and vibration produced during piling.
Precast concrete piles are used in the construction of foundations for a wide range of different structures in the civil engineering and building sectors. As precast piles are suitable for all applications and ground conditions, they provide a very cost-effective piling solution. They are quick to install without producing spoil or arising material in the process, providing a further saving on waste disposal costs.
Factory production techniques, using high performance precast concrete and rigid reinforcement cages, means that precast concrete piles can tolerate high loadings. Generally, design safety factors for piling are taken to be loadings in the range of 2.0 to 3.0. It is normal to use safety factors at the lower end of the range for precast concrete piles because of the:
• Enhanced quality control procedures operable in the factory manufacturing environment.
• Availability of a full product inspection to ensure that no defects are present prior to installation of the pile.
Precast piles are generally top driven into the ground using hydraulic drop hammers. Sound levels from modern drop hammers are comparable to other piling systems such as continuous flight auger (CFA). Noise during piling can be further reduced by shrouding the drop hammer. The variety of segment lengths available, along with specialist piling equipment, makes precast piles particularly suited to restricted access and low headroom sites.
Precast mini-piles and precast concrete beams can be used for underpinning existing wall foundations to repair and improve the stability of housing and other structures. Specialized installation techniques and equipment have been developed to enable underpinning to take place in areas of difficult and restricted access with minimal disruption to the building occupants, and to any neighboring building.
Hydro Tek Bags
Bags filled with concrete is one of the simplest forms of earth retention. Jensen’s has experience filling and placing these bags on site.
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