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Eurocode 7 design procedures for braced excavations

In 1975, the Commission of the European Community decided on an action program in the field of construction that aimed to eliminate technical obstacles to trade and harmonize technical specifications across member states. Within this action program, a set of harmonized technical rules for the design of construction works was established which from 2010 serve as the basis of design in each Member State. Design of braced excavations is mainly governed by the geotechnical standard that is Eurocode 7 (EN 1997 herein EC7). However, a designer has to also consider applications of Eurocode 2 for concrete members, Eurocode 3 for steel structures, and Eurocode 8 for earthquake resistant design.

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EC7 specifies that, when applicable, the following Ultimate Limit States should not be exceeded:

  • EQU:   Loss of equilibrium of the structure or the ground, considered as a rigid body, in which the strengths of structural materials and the ground are insignificant in providing resistance.
  • STR:    Internal failure or excessive deformation of the structure or structural elements, including footings, piles or basement walls for instance, in which the strength of structural materials is significant in providing resistance.
  • GEO:   Failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance.
  • UPL:   Loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions.
  • HYD:  Hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients.

In the design of braced excavations, typically the STR, GEO, and HYD checks are of importance. Under all limit states, the designer should verify that:

Ed Rd

Ed =    Design value of the effect of actions (geotechnical, structural, etc)

Rd =    Design value of the resistance to an action.

Partial factors on actions may be applied either to the actions themselves (Frep) or to their effects (E) by applying either one of the following procedures:

Ed = EF Frep; XkM; ad}                                            (EC7 equation 2.6a)

Ed = γE E{Frep; XkM; ad}                                            (EC7 equation 2.6b)

Resistances to actions are determined in a similar manner where partial factors are applied to ground properties (X) or resistances (R) or to both with either one of the following:

Rd =     RF Frep; XkM; ad}                                      (EC7 equation 2.7a)

Rd =     RF Frep; Xk; ad}/γR (EC7 equation 2.7b)

Rd =     RF Frep; XkM; ad}/γR (EC7 equation 2.7c)

Where:

ad =      Design value of geometrical data

γE =      Partial factor for the effect of an action

γF =      Partial factor for an action

γm=      Partial factor for a soil parameter (material property)

γR =     Partial factor for a resistance

Xk =     Characteristic value of a material property (soil friction, effective cohesion, undrained shear strength, etc).

Subsequently, the designer can apply up to three basic design modes that each provide one or more combinations of minimum partial factors that are applied concurrently to actions (A), material properties (M), and resistances (R). Actions on a retaining structure generally originate from external loads and from water or earth pressures. These actions are subsequently categorized as permanent or variable, and as favorable or unfavorable depending on the nature of the load. While EC7 provides suggested values for all the partial factors, each member state is free to adopt other factors. The combination modes are referred to as Design Approaches and are the following (quick designations DA-1/1 etc):

Design Approach 1:    Combination 1:     A1 “+” M1 “+” R1 (DA-1/1)

Combination 2:     A2 “+” M2 “+” R1 (DA-1/2)

Design Approach 2:    Combination:        A1 “+” M1 “+” R2 (DA-2)

Design Approach 3:    Combination:        (A1* or A2†) “+” M2 “+” R3 (DA-3)

*on structural actions, †on geotechnical actions

Where “+” implies: “to be combined with”.

The recommended partial factors by EC7 for actions (A), material properties (M), and resistances (R) are the product of statistical analysis and are summarized in Tables 1 through 3 respectively for each design case. Table 3 also includes recommended partial factors for the resistance of pre-stressed ground anchors since they are often used as external bracing in temporary excavations.

Table 1. Partial factors on actions (γF) or the effects of actions (γE)

Action

Symbol

Set

A1

A2

Permanent

Unfavorable

γG

1.35

1.0

Favorable

1.0

1.0

Variable

Unfavorable

γQ

1.5

1.3

Favorable

0

0

 

Table 2. Partial factors for soil parameters (γM )

Soil parameter

Symbol

Set

M1

M2

Angle of shearing resistance (applied to tan φ)

γf

1.0

1.25

Effective cohesion

γc

1.0

1.25

Undrained shear strength

γcu

1.0

1.4

Unconfined strength

γqu

1.0

1.4

Weight density

γγ

1.0

1.0

 

Table 3. Partial resistance factors for earth resistance, pre-stressed anchors (γR )

Resistance

Symbol

Set

R1

R2

R3

R4

Earth resistance

γR;e

1.0

1.4

1.0

-

Ground anchors (temporary)

γa;t

1.1

1.1

1.0

1.1

Ground anchors (permanent)

γa;p

1.1

1.1

1.0

1.1

An interesting aspect of the above classification is that there is no specific definition for lateral earth actions and for lateral water pressures. However, it is common practice to consider both unfavorable lateral earth and water pressures as permanent loads (Bauduin et. al). Nevertheless, one could conceivably argue that unfavorable water under flow conditions can be considered as a variable load. The following two sections present examples of applying EC7 combinations for limit equilibrium and for nonlinear analysis methods.

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