DT117-4

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. 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. 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