What Uplift pressure means?
An uplift pressure is any pressure exerted beneath a structure (e.g. A retaining wall) that has the potential to raise the structure higher relative to its surroundings.
Most common uplift pressures come from water pressures present around the structure. Permitting flow through a permeable stratum will reduce the hydrostatic pressure in the water due to energy losses.
However, some of this water will remain under any impermeable structure and produce forces vertically to the structure (see picture below).
The excess pore water pressure remaining will produce an uplift beneath the structure and an uplift of the whole structure can occur.
However, if the structure has sufficient deadweight or appropriate anchorage system ( Eurocode 7 check required) the uplift pressures will be balance and no failure will occur.
How to calculate Uplift pressures.
Calculating Uplift pressures is much easier than you thought
Pore water pressure "u" is :
u = γwH
γw = Unit weight of water = 10 kN/m3
H = Height = in metres (m)
Since pore water pressure acts equally in all directions ( hydrostatic pressure) uplift pressure equals the pore water pressure but at the underside of an impermeable structure.
Uplift Water Pressure = Pore Water Pressure (kN/m2)
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What does Eurocode 7 say about Uplift Pressure Failure Check?
Example of a design for a Retaining Wall
According to Eurocode 7 - BS EN 1997-1 (Part 1: General rules),
at Section 6.5.2.1 Bearing Resistance:
Where:
Vd: Shall include the weight of the foundation, the weight of any backfill material and all earth pressures, either favourable or unfavourable.
Rd: Soil Bearing Capacity calculated from the Geotechnical Investigation Report.
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