Soil retention systems revision notes


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Soil retention systems revision notes

  1. 1. SOIL RETENTION SYSTEMS Deep excavations for basements and cut and cover structures require Secure Earth Groundwater Retention Technique used will depend on method of substructure construction 4 categories Open excavation with face of excavation unsupported Open excavation with face of excavation supported Bottom up excavation Top down excavation Open Excavation - Used where sufficient space on site and possible to put safe slope on soil. Retention method is to put embankment at gradient of 45◦ - Soil must then be assessed - If space restricted, support may need to be considered Soil Nailing Gabion Walls Toe Walls Bottom Up Excavation - where excavation is temporarily support laterally as excavation proceeds. Support is not part of the final structure. Methods Permanent or temp retaining walls Steel sheet piling with temp propping Retaining walls using ground anchors Flying shore props and frame installed once excavation is complete Top Down Excavation – uses permanent walls and floors progressively to maintain retention of soil and groundwater Advantages - Reduces temp works - Allows simultaneous substructure and superstructure construction - Also has better control of lateral movement and settlement - Perimeter foundation walls are constructed using the concrete diaphragm or secant wall methods. Load-bearing elements or drilled shafts are installed and the building columns brought up to grade level. Ground level and first basement slabs are poured, with access holes left to allow
  2. 2. excavation beneath. As each subsequent subgrade level is completed, the floors act as lateral bracing for the perimeter wall system. Above grade construction can proceed while the subgrade work is ongoing since the building’s structural support is already in place. Types of retaining walls depends on Soil type Required depth Whether temp or permanent Examples of Retaining Systems: 1. Sheet Piles 2. King Post Walls 3. Contiguous Bored Pile Walls 4. Secant Pile Walls 5. Diaphragm Walls Sheet Piles • Installed easily using Percussion Hammer or Hydraulic Drivers • Can be used as both temporary closure for excavation works or permanent structure as retaining wall • Suitable for all soil types except boulder beds • Larseen, Frodingham and straight web piles have interlocking joints to form a water seal Erection: o Frame should be constructed to be used as a guide for positioning o Sheet pile lifted using eyes on top of pile o Once in position piles are driven in pairs using percussion hammer or hydraulic drivers o When extet of wall has been driven into position, excavation can be carried out o If temp piles, should be well greased to allow for easy extraction Disadvantages: o Vibration & Noise due to driving process may be unacceptable o Costs may be high if not reusable King Post Walls o Walls for temp soil support during construction using: 1. Soldiers 2. King posts of steel section with horizontal timber spanning between them 3. Reinforced concrete walls spanning between posts
  3. 3. o King posts tied into soil using ground anchors o Used in €60m development in Tallaght Contiguous Bored Pile Walls o Closely spaced insitu concrete piles o Installed using an auger or CFA (Continuous Flight Auger) o Where ground water is likely to seep through gaps, may be necessary to plug them with concrete or jet grouting behind the piles o Contiguous bored piles must be lined with a reinforced concrete wall if there is a risk of water ingress or loss of loose soil between piles. If so – use secant piles Secant Pile Walls o Installing piles on a hit & miss basis at pile centres slightly less than pile diameter o Initial piles (female piles) have weaker concrete mix allowing male piles to cut the area of teh female pile cross section with less effort o Male & Female piles sit side by side with male piles supporting female piles and female piles preventing water seepage. Male piles have reinforcing steel and stronger mix o Preferable over contiguous piles in granular water bearing soil o Very popular in basement construction for soil retention Diaphragm Walls o A wide cavity wall in which two block leaves are tied together by 100mm block diaphragm or ribs of 1 – 1.25m along the wall o The two leaves act as flanges and the ribs as the web of a stiff box section o Diaphragms are preferably bonded to the leaves but can be tied either using stainless steel ties o Wall formed on normal strip foundation (175-250mm thick) o Bases of cavity filled with weak concrete to act as an anchor at the base of teh wall o Overall thickness of wall (300-700mm) depending on pressure o Under heavy pressure may need to fill wall with concrete o Used in Dublin Port Tunnel