What should be the factors that need to be considered while selecting a type of retaining wall? What data are required for design of a retaining structure? Please give an insight on design methods and factors to be taken in to consideration while designing.
Selection of an earth retaining structure should be carried out taking the following factors in to consideration.
1. Ground conditions.
2. Height of the retaining wall.
3. Ground water / tidal condition
4. Location of the wall and the overall space available.
5. External live loading
6. Life and maintenance
7. Materials available
8. Acceptable ground movement and its effects.
An economic comparison should be carried out in case more than one is suitable for a location. Carbon footprint may be calculated if the client requires a carbon minimizing design.
Design of a retaining structure shall require a load of information on site characteristics, soil properties and load conditions. It is essentially required to conduct a site investigation to gather information on site topography, ground water conditions, and physical conditions in the vicinity. Boreholes should be made to ascertain the soil condition and variability along the length of the wall. Safe depth for borehole would be three times the height of wall. In-situ test such as Standard Penetration Test or Stiffness test should be carried out to gather information on soil properties. Details of foundations of adjacent structures should also be collected. Existence of geological faults, joints and tendency of site to creep or settle should be established.
Design will require details of ground water conditions including seepage pressures and existence of any hydrostatic uplift pressures. Install piezometers where necessary to gather ground water conditions. Possibility of flooding should also be ascertained. Presence of corrosive chemicals in soil or ground water should be examined, so that corrosion prevention measures can be taken. In case of waterfront structure, maximum tidal range, possible surge waves and flooding conditions should be established.
Climatic variation such as temperature changes and rainfall variations along with its effect on earth pressures should be examined. During site investigation presence of trees or shrubs in the vicinity or at site should be noted. This may need to be removed if at close proximity. Soil samples for lab tests (Shear Box test, Tri-axial test etc) should be collected as per the requirements. See the relevant IS codes for details.
All necessary details regarding externally applied loads whether static or dynamic should also be collected.
Design should take in to consideration the different design situations. To list some of them
1. Worst case scenario of loads and its combinations
2. Geometry of structure required in worst condition
3. Material characteristics
4. Unplanned excavation or surcharge
5. Water pressure regime
7. Chemical corrosion
11. Tolerance to deformation
12. Change in ground water levels and seepage
13. Water pressure in tension cracks
14. Effect of time on strength
15. Long term effect of fault, cavities and joints
16. Effect of new structure or service of existing structure in future.
Provide expansion and contraction joints 10mm to 20mm thick at joints and suitable locations.
Hydrostatic uplift should be taken in to account at horizontal joints.
Where bearing quality is poor and soil likely to be saturated Gabions can be used as retaining structure. In such cases the density of fill material can be safely taken as 60% of the fill material.