Eurocode 7 analysis methods

Rana Jdidi
Jan 29, 2024
3 min read

Updated: Feb 15, 2024

Eurocode 7 Analysis Methods

DeepEX implements Eurocode 7 specifications and load combinations for shoring design and analysis. This section examines and presents the Eurocode 7 specifications.

In the US practice excavations are typically designed with a service design approach while a Strength Reduction Approach is used in Europe and in many other parts of the world. Eurocode 7 (strength design, herein EC7) recommends that the designer examines a number of different Design Approaches (DA-1, DA-2, DA-3) so that the most critical condition is determined. In Eurocode 7 soil strengths are readjusted according to the material “M” tables, surcharges and permanent actions are readjusted according to the action “A” tables, and resistances are modified according to the “R” tabulated values. Hence, in a case that may be outlined as “A2” + “M2” + “R2” one would have to apply all the relevant factors to “Actions”, “Materials”, and “Resistances”. A designer still has to perform a service check in addition to all the ultimate design approach cases. Hence, a considerable number of cases will have to be examined unless the most critical condition can be easily established by an experienced engineer. In summary, EC7 provides the following combinations:

Design Approach 1, Combination 1: A1 “+” M1 “+” R1

Design Approach 1, Combination 2: A2 “+” M2 “+” R1

Design Approach 2: A1* “+” M1 “+” R2

Design Approach 3: A1* “+” A2+ “+” M2 “+” R3

A1* = For structural actions or external loads, and A2+= for geotechnical actions

EQK (from EC8): M2 “+” R1

(The Italian code DM08 uses DA1-1, DA1-2, and EQK design approach methods only).

In the old Nonlinear (version 7 and before), the different cases would have been examined in many “Load Histories”. The term “Load History” has been replaced in the new software with the concept of “Design Section”. Each design section can be independent from each other or a Design Section can be linked to a Base Design Section. When a design section is linked, the model and analysis options are directly copied from the Base Design Section with the exception of the Soil Code Options (i.e, Eurocode 7, DM08 etc).

In Eurocode 7, various equilibrium and other type checks are examined:

STR: Structural design/equilibrium checks
GEO: Geotechnical equilibrium checks
HYD: Hydraulic heave cases
UPL: Uplift (on a structure)
EQU: Equilibrium states (applicable to seismic conditions?)

DeepEX handles a number of STR, GEO, and HYD checks while it gives the ability to automatically generate “all” Eurocode 7 cases for a model. Unfortunately, Eurocode 7 as a whole is mostly geared towards traditional limit equilibrium analysis. In more advanced analysis methods (such as in Nonlinear), Eurocode 7 can be handled according to “the letter of the code” only when equal groundwater levels are assumed in both wall sides. However, much doubt exists as to the most appropriate method to be employed when different groundwater levels have to be modeled.

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Safety Parameters for Ultimate Limit State Combinations

Safety Parameters for Ultimate Limit State Combinations

Table 1 lists all safety factors that are used in the new software and also provides the used safety factors according to EC7-2008. The last 4 table columns list the code safety factors for each code case/scenario (i.e. in the first row Case 1 refers to M1, Case 2 refers to M2). Table 2 lists the same factors for the Italian code NTC08, while tables 3, 4, and 5 list the safety factors for the Greek, the French, and the German codes respectively. Last, table 2.6 presents the load combinations employed by AASHTO LRFD 5th edition (2010). AASHTO slightly differs from European standards in that soil strength is not factored.

Table 1: List of safety factors for strength design approach according to Eurocode 7

Table 2: List of safety factors for strength design approach according to Italian NTC 2008 code

Note: F_Wall is not defined in EC7. These parameters can be used in an LRFD approach consistent with USA codes.

Table 3: List of safety factors for strength design approach Greek design code 2007

Note: For slope stability the design approach is equivalent to EQU

Table 4: Partial safety factors for strength design approach with French codes XP240 and XP220

Note: French code standards are particularly important for soil nailing walls.

Table 5: Partial safety factors for strength design approach with German DIN 2005

Note: 1. Case 5, 6, and 7 are only used in slope stability analysis in conjunction with cases 1, 2, and 3 respectively.

2. At-rest earth pressure factor is used in multiplying earth pressures only in limit equilibrium analyses.

Table 6: Partial safety factors for strength design approach with AASHTO LRFD 5th edition 2010

Note: 1. AASHTO recommends that slope stability analysis is performed only with the Service I combination.

2. At-rest and active earth pressure factors are used in multiplying earth pressures only in limit equilibrium analyses when the user has selected a relevant method.

Automatic generation of active and passive lateral earth pressure factors in EC7 type approaches

Automatic generation of active and passive lateral earth pressure factors in EC7 type approaches

The Figure below outlines the calculation logic for determining the active and passive lateral earth pressure coefficients. In conventional analyses, the resistance factor is applied by dividing the resisting lateral earth pressures with a safety factor.

Figure: Calculation logic for determining Ka and Kp and driving and resisting lateral earth pressures.

Water Pressures & Net Water Pressure Actions in DeepEX

Determination of Water Pressures & Net Water Pressure Actions in DeepEX (Conventional Limit Equilibrium Analysis)

The software program offers two possibilities for determining water actions on a wall when EC7 is employed. In the current approach, the actual water pressures or water levels are not modified.

Option 1 (Default): Net water pressure method

In the default option, the program determines the net water pressures on the wall. Subsequently, the net water pressures are multiplied by F_WaterDR and then the net water pressures are applied on the beam action. The net water pressure results are then stored for reference checks. Hence, this method can be outlined with the following equation:

Wnet = (Wdrive-Wresist) x F_WaterDR

Option 2: Water pressures multiplied on driving and resisting sides (This Option is not yet enabled.)

In this option, the program first determines initial net water pressures on the wall. Subsequently, the net water pressures are determined by multiplying the driving water pressures by F_WaterDR and by multiplying the resisting water pressures. The net water pressure results are then stored for reference checks. Hence, this method can be outlined with the following equation:

Wnet= Wdrive x F_WaterDR - Wresist x F_WaterRES

Load behavior and factors when a design approach is used

Load behavior and factors when a design approach is used

When an analysis uses design approach such as EC7, each external load must be categorized as favorable or unfavorable. In the default mode when no load combination is used, the software program automatically categorizes loads as favorable or unfavorable based on their location and direction relative to the wall and the excavation. Hence, loads that push the wall towards the excavation are treated as unfavorable, while loads that push the wall towards the retained soil are treated as favorable. In all design approach methods, favorable variable loads are ignored in the analysis while favorable permanent loads are multiplied by a safety factor equal to 1. Unfavorable loads get typically multiplied with factors ranging from 1 to 1.5 depending on the examined design approach and the load nature (permanent vs. variable).

When a load combination is used, the user has the option to manually select the behavior of each load.