Finnish national design guidelines for ...

Finnish national design guidelines for georeinforcements following Eurocode – “Georeinforced earth structures 2012” K. Koivisto & J. Forsman Ramboll Finland Oy, Espoo, Finland

L. Korkiala-Tanttu Aalto University, Espoo, Finland

P. Salo & T. Perttula Finnish Transport Agency, Helsinki, Finland

ABSTRACT: Due to adopting the new European standard EN 1997-1 in Finland, new national design guidelines for georeinforcements have been created. In the handbook Georeinforced earth structures 2012 by Finnish Transport Agency, general guidelines for design and analysis of synthetic or steel reinforced earth structures are given. The handbook presents design methods for different reinforced structures: retaining wall, steep slope, embankment on soft soil, piled embankment and widening of a road embankment. Design examples following the Eurocode system are also presented in the handbook for the aforementioned structures. This paper presents the most significant changes compared to the old Finnish guidelines concerning georeinforcements. Some design principles concerning widened road embankments and steep slopes are also presented, which are not based on international specifications. 1 INTRODUCTION 1.1 Background of the new guidelines When the new European standard EN 1997-1 (Eurocode 7, Geotechnical Design) was adopted in Finland, it was noted that new national design guidelines for georeinforcements would be required. The partial safety factors and load factors given in Eurocode 7 have not been calibrated for georeinforced structures, and thus it does not cover detailed design of reinforced earth structures. In Finland until now, design has been done according to guidelines given in the 1998 published Synthetic Georeinforcements (Aalto et al. 1998), which was created in Finnish Georeinforcement Research and Development Project (1989-1998). Since Finnish Transport Agency has adopted Eurocode in the design of transport routes, the old guidelines cannot be applied to current or future designing as such. Elsewhere in Europe reinforced earth structures are designed according to national and other standards and guidelines, for example BS 8006 (2010), EBGEO (DGGT 2010), NF P 94-220 (1998) and Nordic Guidelines for Reinforced Soils and Fills (Nordic Geosynthetic Group 2005). However, all of these guidelines use different design approaches from what the Finnish Transport Agency has decided to be used in Finland (see Table 1). This being the case, Finnish Transport Agency commissioned new georeinforcement guidelines Georeinforced

Earth Structures 2012 (GES 2012) in concert with Ramboll Finland and Aalto University. GES 2012 is written from the perspective of roads, but it can also be applied in the design of railways and other infrastructure. 1.2 Reinforcements and structures discussed in the guidelines Both geosynthetics and steel reinforcements are discussed in the new guidelines, as well as gabion structures. Gabion structures have been included due to their absence in all other Finnish design guidelines. Reinforced road pavements or superstructures have not been included in the guidelines. The design methods used for different structures are based on several international or national standards and guidelines as presented in Table 1. The methods and their formulas have been altered to include the requirements (mainly partial factors) in the European standard. GES 2012 contains extensive and specific calculation examples of all the discussed reinforcement structures. 2 DESIGN PRACTICE AND LOADS 2.1 Former design practice Formerly in Finnish design practice, the traffic load in ultimate limit state was assumed to be 10 kPa in

all cases. All design was usually done using overall safety factors, even though it was also possible to design with a method that used partial safety factors. 2.2 Design loads according to Eurocode 2.2.1 External loads on top of structures and embankments The external loads and load combinations needed in design are defined according to Eurocodes SFS-EN 1990, SFS-EN 1991 and SFS-EN 1997-1 and the Finnish national annex’s (Ministry of Transport and Communications) application directives NCCI 1 and NCCI 7 (Finnish Transport Agency 2011a & 2011b). The traffic load used in design is based on load diagram LM1, which is presented in Figure 1. It is composed of uniform load strips and truck loads. (Finnish Transport Agency 2011a) When road foundations are designed, the truck loads in different load strips of load diagram LM1 can be combined with the uniform loads as presented in Figure 2. Thus the uniform load is 84 kN/m2 on the 1st load strip, 56 kN/m2 on the 2nd load strip and 28 kN/m2 on the 3rd load strip. This load combination affects simultaneously on the road’s entire cross section. (Finnish Transport Agency 2011b) Sometimes the load on the top of an embankment has to be modeled as one continuous and uniform load due to limitations of design methods. In those

cases the load distribution presented in Figure 3 can be used, where the driving lanes are burdened with a uniform load of 81 kN/m2 and the 1 m wide edge with a load of 9 kN/m2. Table 1. Standards and guidelines in which the new design methods for different georeinforced structures are based on in Georeinforced Earth Structures 2012 (Finnish Transport Agency). Structure Embankment over soft soil Widened embankment, gentle slope Widened embankment, steep slope Piled or deep column stabilized embankment Retaining wall Steep slope

Standard or guideline of the original design method British Standard 8006-1:2010 Nordic Guidelines for Reinforced Soils and Fills (2005) British Standard 8006-1:2010 Nordic Guidelines for Reinforced Soils and Fills (2005) No standard. Combination case of embankment over soft soil and retaining wall/steep slope British Standard 8006-1:2010 British Standard 8006-1:2010 EBGEO (2010) Synthetic Georeinforcements (1998)

Figure 1. The load diagram LM1 according to the application directive NCCI 1 by Finnish Transport Agency (2011b). In figure “kaista” = load strip.

2.3 Design practice according to Eurocode

Figure 2. Applying the load diagram LM1 as uniformly distributed load in road foundation design according to Finnish Transport Agency (2011b). In figure “kaista” = load strip.

Figure 3. Modeling traffic load as continuous uniform load in Finnish Transport Agency’s projects, if it is not possible to use the load distribution presented in Figure 2. (Finnish Transport Agency 2012a)

When wide scale stability is considered, the characteristic value of the live load is 10 kN/m2. (Finnish Transport Agency 2011b) 2.2.2 Uniform live load inside an embankment structure The uniform live load inside an embankment structure caused by traffic load is determined using the nomogram presented in Figure 4. The nomogram applies only with certain georeinforced structures (not applicable with georeinforced piled embankment) and only when design approach DA2* is used.

In ultimate limit state it must be shown that the design value of load actions is smaller or equal to the design value of strength, Ed Rd. Both internal and external stability of the reinforced structure must be considered. In GES 2012 design is done using Eurocode design approach DA2* in structural design, and design approach DA3 in stability design of earth structures. When the design approach DA2* is used, the impacts are calculated with soil parameters’ characteristic values and the resulting characteristic values of impacts and loads are divided by a partial factor. In the design approach DA3, the partial factors are allocated to live loads and soil parameters in the beginning of calculation. Service limit state is used for example when examining settlements of subsoil and the stresses they produce in the reinforcement. In service limit state the characteristic values are used for both loads and soil properties. 3 GUIDELINES FOR DEFINING THE DESIGN STRENGTH 3.1 Synthetic reinforcements 3.1.1 Design strength The design strength for synthetic reinforcements in service limit state is calculated by reducing the tensile strength (gained from standard tensile test) with material factors, and considering the allowable strains of the reinforcement in question. Compared to the earlier Finnish guidelines, in GES 2012 determination of the design strength was changed by including a new material factor RFW in Equations 1 and 2 that allows for UV exposure of the material at the construction site. )

(1)

(2)

Figure 4. Uniform live load at depth H, caused by traffic load diagram LM1 (Finnish Transport Agency 2011b, extended to embankment heights of 0.5…1 m).

where fd = design strength; Tk (dt) = characteristic tensile strength from long- and short-term tensile tests corresponding the design life; Tchar = characteristic tensile strength from short-term tensile tests; RFCR = material factor of creep (dependent on the polymer type); RFID = material factor of mechanical damage during construction; RFW = material factor of weathering (dependent on UV exposure); RFCH =

material factor of chemical and biological effects; RF = safety factor of the material factors (RFCR, RFID, RFW, RFCH) (depends on the magnitude of economic loss and risk of bodily injury caused by structural damage as well as the design life of the structure) As the traffic loads used by Finnish Transport Agency already contain impulse action, there is no need to separately implement the dynamic effect. 3.1.2 Optimizing the reinforcement design strength in embankments over soft soil and in widened embankments When writing the new guidelines, it was decided that reinforcement strength can be optimized by allowing for short-term and long-term loads when designing an embankment over soft soil or a widened embankment. The optimization procedure has previously been used in some real-life projects, although there has been no written specification for it. Due to relatively large traffic loads of the Eurocode, it is no longer seen as sensible to assume that the full traffic load acts continuously on top of the embankment through the entire design life of the structure. The optimization is done by assuming that the reinforcement strength produced by traffic loads is short-term, and the reinforcement strength produced by embankment weight (and other dead loads) is long-term. The different characteristic reinforcement strengths are defined separately, assuming that the influence time of traffic load is at minimum one year. Characteristic strength of the reinforcement is defined by adding up the characteristic reinforcement strengths produced by long- and short-term loads as shown in Equation 3. =

+

,

,

(3)

where Tk,short = short-term (min. 1 year) characteristic strength caused by traffic loads only; Tk,long = long-term (design lifetime) characteristic strength caused by dead loads only. 3.2 Steel reinforcements 3.2.1 Design strength Steel reinforcements have not been present in Finnish design guidelines for georeinforcements before publishing of GES 2012. The design strength of steel reinforcements is defined based on the cross-sectional area of the tensile elements and tensile strength of the material. The cross-sectional area is determined allowing for corrosion that happens during the design lifetime. The design yield strength of the steel is calculated according to Equation 4. =

=

.

(4)

where fyd = design yield strength of the steel; fyk = characteristic yield strength of the steel; s = partial safety factor of reinforcement steel of concrete (1.15). The condition presented in Equation 5 must hold true in all cross-sections (SFS-EN 1993). 1.0

,

(5)

where NEd = tensile force at a steel element in the reinforcement; Nt,Rd = design tensile strength at a steel element in the reinforcement The design strength fd of the georeinforced structure is usually defined as tensile strength per 1 meter wide reinforcement strip. The design tensile strength of a steel reinforcement’s tensile element is determined with Equation 6. The design strength of the steel reinforcement per 1 meter wide reinforcement strip is determined with Equation 7. ,

=

=

,

,

=

.

.

,

(6) (7)

where Nt,Rd = design tensile strength of the tensile element of reinforcement; AN,netto = cross-sectional area of the tensile element of steel reinforcement at the end of its design life; M2 = partial safety factor for the cross section strength concerning tensile failure (1.25); fd = design strength of the steel reinforcement; n = number of tensile elements per one meter wide reinforcement strip; RFID = material factor of mechanical damage during construction. 3.2.2 Corrosion design of unprotected steel reinforcements In normal circumstances the steel reinforcements' corrosion is usually taken into consideration by overdimensioning with the so called corrosion allowance. It means increasing the thickness of the tensile elements enough to allow for corrosion during the design lifetime. The corrosion design in GES 2012 is done according to Finnish Transport Agency guidelines (2012b). 3.2.3 Corrosion design of protected steel reinforcements Protection methods against corrosion are cathodic protection and both organic and unorganic coatings. Protection methods can only be used if the method also withstands construction of the fill layer against the reinforcements without damages. (Finnish Transport Agency 2012b) Coating design is based on the reinforcement service life. Instead of evaluating the service life of reinforcement’s cross-section, the service life of the coating is evaluated. If the design shows that the

service life of the coating is exceeded, corrosion allowance must also be defined for the reinforcement. 4 WIDENING OF A ROAD EMBANKMENT 4.1 Benefits of using georeinforcements in widening structures Widening of a road embankment is usually done when improvement work is done for the old structure. The aim is usually to either enable more traffic or to improve road safety by widening or adding traffic lanes. Compared to conventional widening structures, using georeinforcements in road widening structures is clearly more beneficial in the following cases (Uotinen 1996): There is no separate dry crust layer in the subsoil (or the dry crust is thin and/or damaged and the dry crust’s strength is not remarkably higher than the strength of the soil below). The slope is steepened in pursuance of the widening, for instance due to a lack of space. When used together with lightweight aggregate, georeinforcements can reduce lateral strains 20…30 % compared to a conventional widening structure constructed with lightweight aggregate. As there are no internationally known specific design methods for georeinforced widened road embankments, the design methods used in GES 2012 are applied from one or several design methods of other georeinforced structures. The widening can be done either as a wide structure with a gentle slope, or with a steep slope that resembles retaining wall – type solution. 4.2 Embankment widening with a gentle slope – design principles In an embankment widening with a gentle slope, the reinforcement absorbs horizontal stresses caused by the weight of the embankment and reduces horizona)

tal displacements of the widening. The design is mainly done as in a normal reinforced embankment. However, instead of considering only the sliding of the slope, also two other cases must be examined: a) widening slides along the reinforcement and b) widening slides with the reinforcement away from the old embankment. (Forsman & Uotinen 1999) In the first case, sliding resistance between the reinforcement and embankment fill can be increased by turning the reinforcement into pouch-like structure. The widened part does not necessarily slide entirely (slide surface 1 in Fig. 5a), but may slide as shown in Figure 5a by the slide surface 2 for example due to cracking. In the second case the widening tries to slide with the reinforcement, and the sliding surface forms at the reinforcement/subsoil and reinforcement/old embankment interfaces (Fig. 5b). If the anchoring depth is small or the soil on top of the reinforcement has cracks, the soil layer over the anchoring length Lb may slide along the reinforcement. If there is not enough slide resistance, the anchoring length and strength must be increased for instance by turning the reinforcement into the embankment or attaching the reinforcement to the old embankment. 4.3 Embankment widening with a steep slope – design principles The design method for georeinforced widening of a road embankment with a steep slope (Fig. 6), that is presented in GES 2012, is a combination of two design cases; georeinforced embankment over soft soil and georeinforced retaining wall or steep slope. If the subsoil can be assumed to be firm or the widening is situated on top of ground reinforcement or foundation, design can be done based on the retaining wall design method with some modifications. In the case of soft subsoil, design should be done combining the design methods of both embankment over soft soil and retaining wall.

b)

Figure 5. Mechanisms of sliding failure: a) widened part slides along the reinforcement, b) widened part slides together with the reinforcement. (Finnish Transport Agency 2012a)

Figure 6. Principle of the georeinforced widened road embankmeng with a steep slope. (Finnish Transport Agency 2012a)

When widening of a road embankment with a steep slope is done using retaining wall design method, the external stability calculations include bearing capacity and sliding capacity calculations, wherein the traffic load used on top of embankment is a continuous uniform load (Fig. 3) The internal stability design must be done separately with both a continuous uniform load (analysis 1) and tire loads (analysis 2) as presented in Figures 7 and 8, respectively. In analysis 1, the traffic load influences on the top of the entire superstructure. The uniform load is taken into consideration while calculating the force in the reinforcement (the load reduces stability). However, while calculating the anchoring capacity, the uniform load is not considered in the analysis (the load would increase stability). In analysis 2, the traffic load is composed of two point loads (both of which are half of truck load, calculated from load diagram LM1 in Figure 1) near old embankment

the edge of the road. (Finnish Transport Agency 2011b). In analysis 2, if the widened part is narrow and the inner point load is situated on top of the old embankment at the anchoring zone of the reinforcements, the outer point load is considered to only increase loading, whereas the inner point load is considered to only increase anchoring capacity (based on the fact that two tire loads always affect simultaneously – the inner tire adds capacity needed to anchor the loads produced in the reinforcement by the outer tire). If the inner point load is situated on top of the reinforcements, but outside of the anchoring zone, it increases the load on the reinforcements and does not increase anchoring capacity. In the internal stability analysis the slope must be modeled as vertical, so the design can be done using “tie back wedge” –method like in retaining wall design. The point loads are modeled as vertical strip loads. Figure 8b shows that the uniform load produces small design strengths at the top of the embankment, increasing linearly towards the bottom. In turn, point loads produce high design strengths at the top of the embankment, reducing towards the bottom. Because of this, the design strength and design length must be chosen layer by layer from the analyses 1 or 2, depending on which one produces greater values. At the end of the internal stability design, the reinforcement lengths at the slope area (excluded from the internal stability analysis) must be added to the design lengths gained from the analysis above.

widening

q* = 81 kPa georeinforcements active zone

slope of the old embankment

pas s ive zone

steep slope is modeled as vertical in the internal stability analyses

boundary between active and passive zones *

the traffic load increases load in the reinforcements, but it is not taken into consideration while calculating the anchoring capacity.

Figure 7. Design principle of narrow embankment widening with a steep slope, when both the embankment and widening are founded on firm soil. Analysis 1, uniform continuous traffic load on top of the embankment. (Finnish Transport Agency 2012a)

a)

b) old embankment

widening 2m

125 kN/m = Q2 * **

Effect of the loading type to design strength 1m fd

Q1 * * = 125 kN/m

point load

georeinforcements

h

active zone

slope of the old embankment

uniform load

pas s ive zone

boundary between active and passive zones ** ***

point load increases load in the reinforcement point load increases achoring capacity

Figure 8. Design principle of narrow embankment widening with a steep slope, when both the embankment and widening are founded on firm soil. a) Analysis 2, point loads produced by a vehicle on top of the embankment, b) Change of the design strength with increasing depth in the analyses 1 and 2. (Finnish Transport Agency 2012a)

5 EFFECT OF EUROCODE TO THE DESIGN OF GEOREINFORCED STEEP SLOPES In Finnish guidelines the design of georeinforced steep slopes has been based on the design method presented by Jewell (1991). Application of Eurocode in this design method has brought some changes to the practices. Due to the large traffic loads in Eurocode (LM1), steep slopes with traffic load on top of the embankment cannot be designed with Jewell's method; for example the 81 kN/m2 load would cause a 4 m distortion to the effective embankment height, which is calculated with Equation 8. ´=

+

(8)

where H’ = effective embankment height; H = embankment height; qG = uniformly distributed dead load; qQ = uniformly distributed live load; G = partial safety factor for dead loads; Q = partial safety factor for live lo ads; 1 = total unit weight of the embankment. For the designer to understand when the use of the design method for steep slopes is applicable, a set of ground principles for steep slope and retaining wall design was established, as presented in Table 2. The method to be used depends on the height of the embankment and whether or not a traffic load exists.

Table 2. Design method for georeinforced retaining wall and steep slopes in different situations. (Finnish Transport Agency 2012a) Traffic Height, H Design method load Steep no*