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The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 THE USE OF PILED EMBANKMENTS FOR ROAD CONSTRUCTION ON SOFT SOIL BY USING EBGEO (2010) METHOD Slamet Widodo Abdelazim Ibrahim Technische Universität Bergakademie Freiberg Technische Universität Bergakademie Freiberg Gustav-Zeuner-Straße 1, 09599 Freiberg (Sachsen), Gustav-Zeuner-Straße 1, 09599 Freiberg (Sachsen), Germany and Tanjungpura University, Indonesia Germany and Al Neelain University, Sudan slamet@engineer.com or abdelazimi@yahoo.com slamet.widodo@student.tu-freiberg.de Abstract Thickness of pavements normally vary from 0.25 to 0.75 m. Therefore, road construction is low embankment. Sometimes we find deep soft soil when designing this infrastructures. Besides geosynthetics as a reinforcement, piles also be installed together to support it. When we meet soft soil not so deep and it is possible to build road embankment over end-bearing piles. In this paper the results will be analyzed using EBGEO-2010 method. The findings are stress concentration ratio increase with increasing height of embankment. Higher embankment and surcharge load will make arise for tensile force in geosynthetic resulted from vertical loading and also horizontal outward thrust. Keywords: End-bearing piles, soil arching, stress concentration ratio, soft soil, EBGEO 2010 INTRODUCTION There are two limiting factors relating to the construction of embankments over soft soil. Firstly, the presence of the soft soil restricts the geometry. Geosynthetic reinforcement at the base of embankment increases stability, but only within certain limits. These coincide with the ultimate bearing capacity of soil foundation. Secondly, soft foundation soils are compressible and relatively large consolidation settlements can occur. Various technique exist to increase the effective shear strength for ground improvement can be applied to build roadway over soft soil, such as replacement with better material, vertical drain, chemical stabilization, preloading and piled embankments. The use of piles to support embankment is a popular technique. The technique which the best enables embankments to be constructed to unrestricted heights, at any construction rate, with subsequent controlled post-construction settlements is piling. LITERATURE RIVIEW Soft Soil Almost thirty percent of land area in Indonesia is soft soil consisting of peat and/or soft clay (Satibi,2009; Puslitbang,2001). For conventional pavement design, bearing capacity of subgrade is at least 3 % of CBR value and when below this value some efforts must be done to overcome the problem. Empirical equation between CBR value and undrained The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 shear strength (Su) is that the rate 1 % of CBR is equal to 30 kPa. Soil consistency (Terzaghi&Peck,1967) is shown in Table 1 below Table 1 Soil consistency Consistency Su (kPa) Very soft < 12 Soft 12 - < 24 Medium 2 4 - <4 8 Stiff 4 8 - <9 6 Very stiff 9 6 - <1 9 2 Firm > 192 Piled Embankments For piled embankment it is normally assumed that all of the loading will be transferred through the piles down to a firm stratum. Over the last decade considerable use has been made of geosynthetics to support the base of embankments constructed over piled foundations. Figure 1 Geosynthetic reinforced piles supported embankments Transference of vertical stress For the design purpose it is assumed the total load of embankment will be carried solel y by the piles, with the foundation soil not contributing any load. Thus, assuming the piles are laid out in square grid, the maximum allowable spacing of the piles can be determined using relationship presented in Figure 2. Figure 2 Vertical stresses acting on pile caps caused soil arching (EBGEO,2010) Maximum pile spacing, s = [Fp / ( e. H) ]1/2 (1) The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 where : Fp is the allowable load capacity of the pile e = ( + q/H) is the effective unit weight of embankment fill H is the height of the embankment. Vertical stress at the lowest arching shells or at the surface of the subsoil ' zo is : (2) where : (2.a) (2.b) (2.c) (2.d) 'k is characteristic value of the internal angle of friction (embankment) 3 k is characteristic unit weight of embankment (kN/m ) p k is characteristic value of the live load (kN/m2) h g is vault height (m) : if h > s/2 --------> hg = s/2 h < s/2 --------> hg = h The subscript k denotes the characteristic value and d is the diameter of the pile cap. In 1 /2 case where the pile is not circular, d is taken as (4 Ac/ ) with Ac is the the pile cap 2 area. Kp is the passive lateral earth pressure, which is equal to tan (45+ k/2) and hg is the arching height. The characteristic effective pressure acting on top of pile cap is determined as follows : 'c,k = (( emb,k . h + qk ) - 'zo). AE /Ac + 'zo (3) Where AE is the area of one cell pile embankment. Geosynthetic Reinforcement The determination of overall working tensile properties of the geosynthetic reinforcement required for design can be divided into two parts. The first involves the determination of working tensile properties required to transfer the vertical loading of the embankment fill (and surcharge) onto the piles. The second part involves the determination of working The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 tensile to resist the horizontal outward thrust of embankment. Figure 3 shows distribution of load acting on geoynthetics with rectangular grids. ALx = ½ (sx. sy) – ½ d2.atn (sy/sx). /180 Fx,k = ALx . zo,k 2 ALy = ½ (sx. sy) – ½ d .atn (sx/sy). /180 Fyk = ALy . zo,k Figure 3 Vertical distributed load acting geosynthetic reinforcement (EBGEO,2010) To determine the tensile load on geosynthetic layer, the vertical force F as result of vertical pressure on the subsoil zo,k acting on influenced area AL , is calculated. The vertical force F is considered to act on a strip geosynthetic with a width of bErs and length of LW as shown in Figure 3 above. Here, bErs is equal to 0.5 d ( and Lw is equal to (s - bErs ). Once the maximum strain , max , is determined, the characteristic tensile load Trpk required in the geosynthetic to resist the vertical force can be determined. Trp,k = max . J (4) EBGEO 2004 gives chart to choose strain based on subsoil stiffness and triangular force acting on geotextile span. The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 Jk - tensile stiffness of geogrid kN/m) LW - clear distance = s – d [m] ks,k - modulus of subgrade [m] ks,k = Es,k / tW bErs - width of support [m] bErs = 1/2 .d . Figure 4 Chart to determine strain of geotextile (EBGEO,2010) The method provides a chart for practical design which strain or extension between 5 and 6 % for end of design life and maximum extension or maximum strain 6 % can be taken as design value. Geotextile also resists the horizontal outward thrust of embankment. This tensile load in geotextile as shown in Figure 5. Figure 5 Horizontal outward thrust resisted by geotextile (EBGEO,2010) Ek = [ 0.5. k. (h-z) + pk ] .(h-z). Kagh (5) Where Kagh is the active lateral earth pressure coefficient according to DIN 4085, which is defined as : Kagh = [ cos ' / (1+As) ]2 (6) where : As = [ (sin( ' + s,k).sin ' ) / cos s,k) ] 1/2 (6.a) The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 INPUT PARAMETERS Some input parameters such as H (height of embankment fill), s (piles spacing), a (width of pile cap), (unit weight of embankment material), (internal friction angle), q (surcharge load) have to be determined to make calculation. Magnitudes of some input parameters are H=0.30 to 1.5 m, =18 kN/m3 , vary from 25o to 40o and q (surcharge loads) vary from 0 kPa to 80 kPa. Moreover, maximum strain of geosynthetic ( max) taken 6 % and two different of elastic moduli of geosynthetics are 270 and 1500 kN/m. Partial factors are taken equal to one for analysis. Here, there are 2 (two) cases for geometry that will be addressed and arranged using square pattern, namely : Case 1 : a = 20 cm (circular pile cap) and sx or sy = 50 cm , (coverage ratio of 12.56 %) Case 2 : a = 30 cm (circular pile cap) and sx or sy = 127 cm, (coverage ratio of 4.38 %) Stress Concentration Ratio For q (surcharge load) varies from 0 and 80 kPa and stress concentration ratio (SCR) which ratio of stress on pile caps after arching of soil to overburden stress ( 'zs / 'zo) as shown in Table 2. Table 2 SCR on height of embankment with q= 0 kPa and 80 kPa and various internal friction angles for both cases Case 1 Case 2 H (m) Stress Concentration Ratio (SCR) Stress Concentration Ratio (SCR) Surcharge Load Internal Friction Angle Surcharge Load Internal Friction Angle 0 0 0 0 q=0 q = 80 q = 25 q = 30 q = 35 q = 40 q=0 q = 80 q = 250 q = 30 0 q = 35 0 q = 40 0 0. 30 2.90 2.90 2.45 2.90 3.43 4.04 1.56 1.56 1.41 1.56 1.76 2.00 0. 40 3.61 3.61 3.04 2.61 4.23 4.91 1.97 1.97 1.72 1.97 2.30 2.72 0. 45 3.81 3.81 3.22 3.58 4.45 5.13 2.21 2.21 1.89 2.21 2.61 3.13 0. 60 4.23 4.23 3.59 4.23 4.90 5.57 3.03 3.03 2.50 3.03 3.68 4.51 0. 75 4.48 4.48 3.81 4.48 5.16 5.83 3.95 3.95 3.20 3.95 4.87 6.02 0.90 4.65 4.65 3.96 4.65 5.34 6.00 4.92 4.92 3.94 4.92 6.10 7.54 1.20 4.86 4.86 4.14 4.86 5.56 6.22 6.22 6.22 4.99 6.22 7.65 9.30 1.50 4.98 4.98 4.25 4.98 5.69 6.35 7.00 7.00 5.62 7.00 8.57 10.35 Tensile Forces acting on geosynthetics. Tensile force occurs in geosynthetic from vertical loading and horizontal outward thrust resulted from different of elasticity modulus and surcharge loading as shown in Table 3 and Table 4 for both cases. The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 Table 3 Tensile forces in geosynthetic case 1 for = 30O H P R P , J=270 kN/m P R P , J=1500 kN/m Ek (m) q= 0 q= 20 q= 40 q= 0 q= 20 q= 40 q= 0 q= 20 q= 40 0.30 4.32 11.07 16.20 9.75 22.50 31.50 0.21 1.75 3.30 0.40 5.40 12.15 - 12.00 23.25 33.00 0.37 2.43 4.49 0.45 5.67 12.69 - 13.00 24.00 34.50 0.47 2.79 5.10 0.60 6.21 13.23 - 14.25 25.50 36.00 0.83 3.92 7.01 0.75 7.56 14.58 - 16.50 27.00 38.25 1.30 5.16 9.02 0.90 8.91 15.39 - 18.75 29.00 39.00 1.88 6.51 11.14 1.20 10.80 16.20 - 22.50 31.50 39.75 3.34 9.51 15.69 1.50 13.50 - - 24.00 33.00 40.50 5.21 12.93 20.65 Table 4 Tensile forces in geosynthetic case 2 for = 30O H P R P , J=270 kN/m P R P , J=1500 kN/m Ek (m) q= 0 q= 20 q= 40 q= 0 q= 20 q= 40 q= 0 q= 20 q= 40 0.30 16.20 - 16.20 25.50 79.50 31.50 0.27 2.27 3.30 0.40 - - - 31.50 87.00 33.00 0.48 3.15 4.49 0.45 - - - 36.00 88.50 34.50 0.61 3.61 5.10 0.60 - - - 42.00 89.25 36.00 1.08 5.08 7.01 0.75 - - - 49.50 90.00 38.25 1.69 6.69 9.02 0.90 - - - 54.00 - 39.00 2.43 8.43 11.14 1.20 - - - 64.50 - 39.75 4.32 12.32 15.69 1.50 - - - 70.50 - 40.50 6.75 16.75 20.65 DISCUSSION FOR ANALYSIS RESULTS Stress Concentration Ratio (SCR) From Table 2. we can see that SCR increase with increasing height of embankment, but the value is independent with increasing q (surcharge load). These values increase with the increasing of internal friction angle. 7 .0 0 St r ess Co n cen t r at io n Rat io 6 .0 0 5 .0 0 4 .0 0 SCR, p h i =2 5 3 .0 0 SCR, p h i =3 0 2 .0 0 SCR, p h i =3 5 1 .0 0 SCR, p h i =4 0 0 .0 0 0 .2 0 0 .4 0 0 .6 0 0 .8 0 1 .0 0 1 .2 0 1 .4 0 1 .6 0 H eigh t o f em ban k m en t ( m ) Figure 6 SCR values on variation of height of embankment and internal friction angle at case 1 with coverage ratio of 12.56 %. The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 1 2 .0 0 St r ess Co n cen t r at io n Rat io 1 0 .0 0 8 .0 0 6 .0 0 SCR, p h i =2 5 4 .0 0 SCR, p h i =3 0 SCR, p h i =3 5 2 .0 0 SCR, p h i =4 0 0 .0 0 0 .2 0 0 .4 0 0 .6 0 0 .8 0 1 .0 0 1 .2 0 1 .4 0 1 .6 0 H eigh t o f em ban k m en t ( m ) Figure 7 SCR values on variation of height of embankment and internal friction angle at case 2 with coverage ratio of 4.38 %. Geosynthetic Reinforcement Geosynthetic reinforcement resists tensile forces resulted from vertical stress (PRP) and horizontal outward thrust (Ek) as depicted in Figure 8 and Figure 9. based on Table 3 and Table 4. 3 5 .0 0 3 0 .0 0 T en sile f o r ce ( k N/m ) 2 5 .0 0 2 0 .0 0 P r p , J=2 7 0 , q=0 1 5 .0 0 P r p , J=2 7 0 , q=2 0 P r p , J=1 5 0 0 , q=0 1 0 .0 0 P r p , J=1 5 0 0 , q=2 0 5 .0 0 E k , q=0 0 .0 0 E k , q=2 0 0 .2 0 0 .4 0 0 .6 0 0 .8 0 1 .0 0 1 .2 0 1 .4 0 1 .6 0 T h ick n ess o f em ban k m en t ( m ) Figure 8 Tensile force from vertical load and horizontal thrust at case 1 for = 30o 1 0 0 .0 0 8 0 .0 0 P r p , J=2 7 0 , q=0 T en sile f o r ce ( k N/m ) P r p , J=2 7 0 , q=2 0 6 0 .0 0 P r p , J=1 5 0 0 , q=0 P r p , J=1 5 0 0 , q=2 0 4 0 .0 0 E k , q=0 2 0 .0 0 E k , q=2 0 0 .0 0 0 .2 0 0 .4 0 0 .6 0 0 .8 0 1 .0 0 1 .2 0 1 .4 0 1 .6 0 T h ick n ess o f em ban k m en t ( m ) Figure 9 Tensile force from vertical load and horizontal thrust at case 2 for = 30o The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 For geosynthetic with elastic modulus 270 kN/m and surcharge 0 kPa can support only 0,3 m of embankment thickness, but by adding surcharge load which strain in geosynthetic becomes higher than at maximum value of 6 % whereas using geosynthetic with J=1500 kN/m gives strain slightly lower than maximum strain. Geosynthetic with higher elastic modulus will receive higher tensile force. In addition to tensile forces resulted from vertical loading and horizontal outward thrust increase with increasing height of embankment and additional load. CONCLUSIONS It is worth to note that stress concentration ratio (SCR) is influenced by height of embankment but surcharge load does not affect these values A assumption that higher coverage ratio gives higher stress concentration ratio is not always true but pile spacing and height of embankment have to be considered as well. In the paper it took two cases namely case 1 for dense square grid with coverage ratio 12.56 % and other case (case 2) for rare square grid with coverage ratio 4.38 %. Actually that the definition for stress concentration ratio (SCR) in this paper is more appropriate as competency ratio or another definition as column stress ratio (CSR). Properties of soil such as the higher of internal friction angle will give the higher of stress concentration ratio. According to this method that after reaching a critical height of embankment which is more or less an half of diagonal spacing ,Sd/2, that height of embankment and surcharge load still influence the tensile force of geosynthetic caused by vertical loading and lateral thrust. In addition, application of higher stiffness of geosynthetic in the field will provide higher tensile force of geosynthetic and higher stiffness is needed for wider clear distance between piles. ACKNOWLEDGEMENT We would like to thank to Prof. H. Klapperich, Dr. Tandor Tamaskovics and my colleague Sebastian Althoff for valuable advice in geosynthetic. REFERENCES British Standard, BS8006 1995. Code of practice for strengthened/reinforced soils and other fills. EBGEO Draft 2004. Bewehrte Erdkörpern auf punkt-oder linienformigen Traggliedern, DGGT e.V stand Juli 2004. EBGEO 2010. Berechnung von Erdkörpern mit Bewehrungen aus Geokunststoffen, DGGT e.V Eekelen, S.v.,Bezuijen,A. 2008. Piled embankments, Considering the basic starting Points of the British standard , Deltares. Eekelen, S.v., Bezuijen,A.,Duijnen,P., Jansen,H.L. 2009. Piled Embankments using Geosynthetic Reinforcement in the Netherlands: design, monitoring & evaluation, Proceeding of 17th International Conference on Soil Mechanics and Geotechnical Engineering. The 14th FSTPT International Symposium, Pekanbaru, 11-12 November 2011 Eekelen,S. V., Bezuijen, A., Alexiew, D. 2008. Piled Embankments in the Netherlands, A Full- Scale Test, Comparing 2 Years of Measurements with Design Calculations, EuroGeo4 Paper Number 264. Kempfert, H.G., et al.Gobel,et al. 2004. German Recommendations for Reinforced Embankments on piles-similar elements,.DGGT. Prelovsky, B. et al. 1985. The Development of Piled Embankments Techniques over 25 years. Satibi,S.,(2009), The Numerical Analysis and Design Criteria of Embankments on Floating Piles, PhD Thesis, Stuttgart University. Vollmert, L. Schwerdt, S. 2010. Erste Erfahrungen mit der Anwendung, Bautex 2010 Puslitbang, 2001. Panduan Geoteknik-4, Desain dan Konstruksi, Puslitbang Prasarana Transportasi, WSP International.

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