Failure by uplift (UPL)

10.2.1. General

Clause 10.2(1)P Failure by uplift is checked by comparing the sum, G_{dst, d}and Q_{dst, d}, of the design values of the destabilizing permanent and variable vertical actions, i.e. the sum of the water pressures under the structure (permanent and variable parts) and any other upwards forces, with the sum, G_{stb, d}and R_d, of the design values of the stabilizing permanent vertical actions and the design value of any additional resistance to uplift provided, for example, by tension piles, ground anchorages or friction forces on the side of a buried structure:

Clause 2.4.7.4(1)P G_{dst, d}+ Q_{dst, d}£ Gstb, d+ R_d (2.8)

Clause 2.4.7.4(3)P

This general inequality is applied when checking the stability of submerged structures against uplift failure and the stability against uplift failure of impermeable layers in excavations. The design procedure for a structure where tension piles are required to provide additional stabilizing resistance is presented in a worked example in this guide (see Example 7.5). For persistent and transient situations, the values of the partial factors on the destabilizing and stabilizing actions should be selected from Table A.15, and the values of the partial factors on the ground strength parameters or the resistance of geotechnical elements should be selected from Table A.16.

Clause 2.4.7.4(2) According to clause 2.4.7.4(2), the additional resistance to uplift due to tension piles, ground anchors or friction forces may be treated as a stabilizing permanent vertical action

Varved clay

Sand

(a) (b)

Fig. 10.1. Examples of hydraulic failure mechanisms due to different soil conditions: (a) failure by uplift and (b) failure by heave

rather than as a resistance, and hence the design value will be obtained by applying the partial factor on permanent favourable actions to it (of which the recommended value is 0.9).

If this procedure is adopted and the partial factor values on actions recommended in Table A.15 are applied, the resulting UPL design will be less conservative than if the partial soil parameter values in Table A.16 are applied to the additional resistance. This is because applying the partial factor in Table A.16 to the additional tensile pile resistance is equivalent to multiplying the resistance by 0.71 (or, in the case of friction forces, to multiplying the ground strength parameters by 0.8). It is clear from this that, if clause 2.4.7.4(2) is applied in the case of the resistance from tension piles, ground anchors or friction, then the GEO ultimate limit state should also be checked.

10.2.2. Submerged structures

As an example of a design against failure by uplift, it may be useful to consider a tunnel completely below the groundwater level, as illustrated in Fig. 10.2. With the design values of the destabilizing uplift action U_dof the water and the design values of the stabilizing actions of the weight G_{str, d}of the structure, the weight G_{soil, d}of the ground on top of the structure and the side friction force T_don the vertical walls, inequality (2.8) becomes

U_d£ G_{str, d}+ G_{soil, d}+ T_d (D10.1)

Clause 2.4.2(9)P If a structure is completely below the groundwater level, the water pressure acting on the

top of the structure could be regarded as a stabilizing action and the water pressure acting on the bottom as a destabilizing action. As the stabilizing and destabilizing actions are multiplied by different partial factor values, the safety against uplift would then depend on the water depth above the structure. Therefore, the principle described in the note to clause 2.4.2(9)P may be applied in this situation: that is, a single partial factor may be applied to the difference between these actions, i.e. to the difference between the characteristic permanent actions of the water pressure acting on the top and the bottom of the structure. The difference between the characteristic destabilizing actions due to the water pressures is

U_K=γw(H₂– H₁)A =γwHA (D10.2)

where A is the base area of the tunnel and the other symbols are as illustrated in Fig. 10.2.

The design value of the destabilizing actions is then

G_{soil, d}

Gstr, d

T _d T _d

H 1

H ₂

U_d

H

Fig. 10.2. Tunnel below the groundwater table

U_d=γG, dstγwHA (D10.3) It should be noted that, when calculating the stabilizing action, the weight of the soil below the groundwater level should be calculated using the effective weight densityγ¢ of the soil.

As indicated in inequality (D10.1), friction forces, e.g. on the walls of submerged structures, may be taken into account. They can be derived from

T_d/A = Kσ¢vtanδd (D10.4)

The wall friction angleδdshould be determined by dividing tanδkbyγϕfrom Table A.16 to obtain a conservative low friction force. If K, the coefficient of earth pressure, is calculated from the angle of shearing resistanceϕ¢, an appropriately cautious value should be selected forϕ¢.

10.2.3. Design against uplift of an impermeable layer

If friction forces are neglected, the design against uplift of an impermeable layer where there is no seepage through the layer, e.g. at the bottom of, or below, an excavated building pit (Fig. 10.3), can use stresses instead of forces. In this case, the design value of the destabilizing total water pressure u_dacting at the interface between the two layers must be less than or equal to the stabilizing total vertical stressσstb, ddue to the total weight of soil above the interface

u_{dst, d}£ σstb, d (D10.5)

Using the partial factors specified in Table A.15 and the symbols of Fig. 10.3, inequality (2.8) for design failure by uplift becomes

γG, dstγwH_k£ γG, stbγd (D10.6)

Using the values for the partial factors given in Annex A.4, the method of EN 1997-1 is equivalent to an overall factor of safety (OFS) against uplift given by

OFS =γG, dst/γG, stb= 1.00/0.90 = 1.11 (D10.7)

10.2.4. Worked example of a design against uplift

An example of a design against uplift of a structure with tension piles is presented in Example 7.5 of this guide.

H_k

Water

Impermeable layer

d

Permeable layer

Fig. 10.3. Uplift of an impermeable layer