M

Ed,stb

Design is unacceptable if degree of utilization is > 100%

© It is assumed that the uplift on the base of the wall can be represented by a simple triangular distribution, reducing from a maximum pressure at the heel to zero at the toe.

© For sliding resistance, vertical actions on the wall heel are favourable and so variable actions are ignored. However, the uplift due to the water pressure is unfavourable and results in a lower effective vertical force for DA1-1 compared with DA1-2. This results in a lower design resistance for DA1-1 compared with DA1-2, even though the design shear strength is lower for DA1-2.

© The introduction of ground water in the back fill results in a wider wall (B = 4.3m) being required to provide adequate sliding and bearing resistance compared with Examples 11.1 and 11.2 (for which B = 3.0m). For this example, sliding governs the design according to Combination 1. To improve the design account could be taken of passive resistance in front of the wall and/or a shear key provided.

© The results for Design Approaches 2 and 3 are presented in summary only. The full calculations are available from the book's website at www.decodingeurocode7.com.

Design Approach 2 applies factors greater than 1.0 to actions and resistance. Sliding is critical in DA2 and the degree of utilization (109%) is more than Eurocode 7 allows.

Design Approach 3 applies factors greater than 1.0 to structural actions (i.e. the self-weight of the concrete) and material properties. Sliding is again critical with a degree of utilization (83%), but is less onerous than for Design Approach 1.

11.11.4 Mass concrete wall retaining dry fill

Example 11.4 considers the design of a mass concrete wall retaining dry ground, as shown in Figure 11.12.

The wall, which has sloping front and back faces, sits on a strong rock and retains granular backfill. The ground surface behind the wall slopes upwards and carries a surcharge loading.

Figure 11.12. Mass concrete wall retaining dry ground

Notes on Example 11.4

O Similar comments are applicable to aspects of the calculation to those given for the undrained example, the reader is referred to this example for additional guidance.

© The selection of an appropriate angle of interface friction between the wall and the backfill is a matter of engineering judgement. The constant volume angle of friction, 9cv, is the minimum likely value for a soil and thus it is considered that it would be unreasonable to adopt a design value of 9 lower than 9cv.

© The selection of an appropriate angle of interface friction between the wall base and the underlying ground is also a matter of engineering judgement. For a rock/concrete interface this may be taken as the design angle of friction for the rock.

© The values of earth pressure coefficients have been calculated using the equations given in Annex C of EN 1997-1 and discussed in Chapter 12.

Example 11.4 Mass concrete wall retaining granular fill Verification of strength (limit state GEO)

Design situation

Consider a mass concrete gravity wall, B = 2.0m wide, which retains H = 4.0m of granular fill and sits upon a strong rock (so bearing failure is not a design issue). The top of the wall (which is symmetrical) is b = 1.0m kN

wide.The weight density of unreinforced concrete is Yck = 24-(as per m

EN 1991-1-1 Table A.1). The backfill has characteristic drained strength kN

parameters ^ = 36°, c'k = 0kPa, and weight density Yk = 19-. The fill's m constant volume angle of shearing resistance is pcv k = 30°. The characteristic angle of shearing resistance of the rock beneath the wall base is Pk fdn = 40°. The ground behind the wall slopes upwards at a slope of 1m vertically to h = 4m horizontally, i.e. at an angle p = tan i ( !m 1

variable surcharge qQk = 10kPa acts on this ground surface during persistent and transient situations.®

Design Approach 1

Geometrical parameters

There is no need to consider an unplanned excavation

Inclination of wall surface (virtual plane) 0 = ^ = 7.2° B - b

Actions

Characteristic self-weight of wall Wgk = Yck

Characteristic moment about toe (stabilizing)

Material properties Partial factors from Sets

VM2 y are y

Design shearing resistance of backfill 9d = tan tan

0 0

Post a comment