PILE DESIGN METHOD FOR IMPROVED GROUND USING THE VACUUM CONSOLIDATION METHOD

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1 PILE DESIGN METHOD FOR IMPROVED GROUND USING THE VACUUM CONSOLIDATION METHOD K Tomisawa, Civil Engineering Research of Hokkaido, Japan S Nishimoto, Civil Engineering Research of Hokkaido, Japan Abstract This paper presents a study of a design method for improving the ground around piles using the vacuum consolidation method, one of various ground improvement methods, and using the increased ground strength to reflect onto the horizontal resistance of piles. This is a rational design method that can reduce construction costs for site conditions with soft ground, where horizontal resistance determines pile specifications. In this design method, the range of influence of the horizontal resistance, or the range of ground improvement requiring the vacuum consolidation method, is set from 1/β (the characteristic length of piles: β= 4 (k D)/(4EI)) to the range of passive earth pressure,θ=45+φ/2. The horizontal subgrade reaction of piles is determined by converting the shear strength C of ground, increased by consolidation, to the modulus of deformation E. The validity of the design method was empirically verified by applying it to an actual site and conducting horizontal loading tests on actual piles. The seismic behavior of the piles in the improved ground was examined using the conventional earthquake-resistant design method. Keywords: Vacuum consolidation method, Lateral resistance of pile, Load test, Aseismic design 1.Introduction When conducting ground improvement by the surcharge, deep mixing, sand compaction or other methods for earth structures such as embankments, slip stability and settlement are designed by reflecting the ground strength after improvement. In designing piles to be constructed in improved ground, it is usually the case that only the original, natural ground strength before soil improvement is used, as there are not many case examples in which the area affected by horizontal resistance and subgrade reaction characteristics after soil improvement have been verified through confirmation tests. These design methods, however, are not reasonable because they are based on ground properties that are different from actual characteristics, and the ground strength may be underestimated depending on the specific site conditions. The authors have studied design methods for improving the soft ground around piles using various methods and reflecting the increased shear strength of ground on the horizontal resistance piles 1), 2). These are rational design methods that can reduce construction costs for sites having soft ground, where pile specifications are determined by horizontal resistance. This paper presents a rational design method for improving the ground around bridge foundation piles using the vacuum consolidation method, which has been employed frequently in recent years as a type of surcharge method for soft ground. In this method, horizontal subgrade reaction of the piles is determined

2 at the time of design based on the improved ground strength after consolidation. The range of influence of horizontal resistance of piles, or the necessary area of soil improvement, was determined based on a three-dimensional cuneiform sphere conforming to the conventional design method. The validity of the design method was empirically verified by applying it to an actual site and conducting a horizontal pileloading test. The seismic performance of the piles in the improved ground was verified using the seismic intensity method and the seismic horizontal strength method, conforming to the conventional design method. 2. Basic ideas of the design method 2-1 Range of influence of horizontal resistance of piles In the case of horizontal resistance of piles, the limit equilibrium state will be maintained because the front ground will be compressed in the horizontal direction and horizontal earth pressure will therefore increase, if the maximum value of horizontal subgrade reaction is treated as the limit resistance proportional to the force of action in the same way as in the limit subgrade reaction design of rigid foundations. Thus, the range of horizontal resistance of front ground when horizontal force is applied to piles can be considered as a passive earth pressure area 3), which can be represented as the failure angle of soil. It is known that ground properties within the range of passive earth pressure in front of the Fig. 1 Three-dimensional range of influence of characteristics length 1/β controls the horizontal horizontal resistance of piles behavior in the linear area ofsemi-infinite long piles in soft ground where the force of action is applied to pile heads 4). Therefore, in this study, the range of influence of front ground horizontal resistance of piles in improved ground was set as the area raised to the working gradient θ = (45 +φ/2) (φ: angle of shear resistance of soil) of passive earth pressure from the depth of 1/β. It was also assumed that the two-dimensional range of influence was a fan shaped area θ=(45 +φ/2) spreading away from the direction of the force, in the same way as the horizontal subgrade reaction in the front. Based on these findings, horizontal subgrade reaction in the proposed method was established as a three-dimensional, four-sided square area, which was synthesized in two directions, parallel and perpendicular to the axis against the horizontal force shown in Fig. 1. The range of soil improvement was also based on this three-dimensional, four-sided domain. However, in the surcharge method, taking the increased ground strength after consideration into account, it is necessary to include the entire consolidated layer 1/βor deeper in the range of influence in the depth direction in order to ensure slip stability and residual settlement. 2-2 Method of setting design subgrade reaction In preload, vacuum consolidation and other surcharge methods, shear strength of the ground C is calculated by Eq. (1) 5). C=Co+ C=Co+m P U (1) Where, C is shear strength of the consolidated ground (kn/m 2 ), Co is undrained strength of the original ground (kn/m 2 ), C is shear strength of the ground increased by consolidation (kn/m 2 ), m is the increasing rate of strength, P is increased stress in the ground (kn/m 2 ) and U is degree of consolidation (%). Based on this concept and by considering C by consolidation as the equivalent ratio with the modulus of deformation E increased by soil improvement, the modulus of deformation E of the entire

3 improved ground can be calculated as E=Eo+ E by converting Eq. (1). Rational design methods regarding horizontal resistance of piles installed in the ground improved using the vacuum consolidation method When calculating the increased shear strength of soft ground C, however, there are still many unknown factors that must be clarified in future studies concerning the increasing rate of the modulus of deformation E of consolidated ground, although the increasing rate of strength 5), 6) m=c/p (general value of peaty ground: m=0.45) has almost been organized by consolidation load and type of ground. It is therefore desirable to reflect the modulus of increased deformation E on pile design by conducting experimental construction and directly measuring E after consolidation improvement and C, which is considered to be the equivalent ratio by the horizontal loading test in pressuremeter test in borehole and shearing test. Once the modulus of deformation of the ground after consolidation E is determined, the coefficient of horizontal subgrade reaction K for horizontal resistance design of piles can be calculated using Eq. (2) 4). K=(α E/0.3) 4 (D/β) (1/0.3) -3/4 (2) Where, K is the coefficient of horizontal subgrade reaction of piles (kn/m 3 ), E is the modulus of deformation of improved ground (kn/m 2 ), α is the estimated coefficient of horizontal subgrade reaction, D is the pile diameter (m), β is the characteristic value 4 (KD)/4EyI (m -1 ), E is the Young s modulus (kn/m 2 ), I is the geometrical moment of inertia. By following the above procedure, it becomes possible to design piles reflecting the increased strength after consolidation of soft ground. 3. Practical bridge design method The method of improving soft ground around bridge foundation piles and reflecting the increased shear strength after consolidation on horizontal resistance was applied on site for abutments A-1 and A-2 of the Shinkushirogawa Bridge in Kushiro, which is in the jurisdiction of the Hokkaido Regional Development Bureau. As the method for improvement of soft ground, the vacuum consolidation method was used in conjunction with embankment construction 7). Vacuum consolidation is a method for converging settlement and increasing the strength of ground in a short period of time by eliminating trapped air and water in soft ground using embankment loading with vacuum pumps and vertical drains. It has been used for many construction works in recent years. Figure 2 shows the foundation piles of abutment A-1 and the diagram of the vacuum consolidation method. Because soft Quaternary diluvium silt is distributed up to Fig. 2 Abutment foundation piles/working diagram of the depth of 40 to 50 m in the soil boring log vacuum consolidation method of the site, the bearing stratum was found in the sand stratum 60 m or deeper. As a result, it is necessary to use very long steel piles (pile diameter:φ600 mm, pile thickness: t=13, pile length: L=60 m) as bearing piles. The length of vertical drains for the vacuum consolidation method was up to 31.2 m under the ground based on the design settlement. The area of installation of vertical drains at the back of the abutment was

4 Table 1 Design subgrade reaction of piles in improved ground set as a range to ensure the slip stability rate of Fs=1.5, and the area in front of the piles was set as the area raised to the working gradient θ = (45 + φ/ 2) of passive earth pressure from the depth of 1/β, in accordance with the proposed design method. The height of the embankment for loading was set at 10 m based on the result of consolidation calculations, and the top of the slope was built as the drain edge in front of the piles. Calculated settlement was S 3.0 m and corresponded well with the field observation value. Steel abutment piles were constructed by the driving method following the removal of the embankment after convergence of consolidation. Design horizontal subgrade reaction of piles, which takes the increased strength of ground by the vacuum consolidation method into account, was calculated using the increasing rate of shear strength in relation to the original ground, based on the results of experimental construction of the vacuum consolidation method conducted in advance. A survey prior to the construction revealed that the average shear strength of original ground was Co=8.5 kn/m 2 in the silt layer in the 1/β section (Dep 6 m), which was the depth involved with the horizontal resistance of piles. As a result of the post-testing survey conducted after convergence of consolidation by the vacuum consolidation method, however, the shear strength of the same ground was C=Co+ C=45.2 kn/m 2. The coefficient of design horizontal subgrade reaction of piles K1 taking the increase in ground strength was therefore set at K1=40,000 kn/m 3 based on the increasing rate of shear strength (5 times 45.2/8.5) (Table 1). As a result, because pile specifications of the abutment foundation in question were determined mainly by horizontal resistance, the number of lines of abutment piles could be reduced from five (5 lines 7 piles = 35 piles) by using the Ko value of unimproved original ground to four (4 lines 7 piles = 28 piles) by using the Kl value of increased subgrade reaction. The reduction in construction costs would thus be approximately 20% (estimated construction cost: \90 million), even if the cost for the vacuum consolidation method is included. The use of this method may be effective under certain site conditions, when pile foundation design is determined by horizontal resistance, as in the case of soft ground. 4. Verification by on-site tests 4-1 Horizontal pile-loading test For verification of the validity of this method, an on-site horizontal pile-loading test was conducted to confirm the coefficient of design horizontal subgrade reaction k1. The horizontal loading test method consisted of multi-cycle static horizontal loading with the standard pile displacement of up to 15 mm in accordance with the specifications of the Japan Geotechnical Society, Method of horizontal loading test of piles and instruction manual 8), and load was applied by the load control method using a hydraulic jack. Figures 3 and 4 illustrate the relationship between the horizontal load H and displacement y and the bending stress of piles by loading stage, respectively, which were obtained as a result of the Fig. 3 Relationship between horizontal load H and displacement y in horizontal loading test

5 horizontal loading test. According to Fig. 3, pile displacement had a quadric curve relationship with horizontal load resulting from the nonlinearity caused by strain dependence, and was determined to be elastic behavior. Also, because bending stress of piles shown in Fig. 4 was free from the pile head, the maximum valueσmax 60 N/mm 2 was observed at the depths of 3 to 4 m, and its distribution was mostly concentrated at 1/β. 4-2 Evaluation of subgrade reaction The measured test value K2, as against the coefficient of design horizontal subgrade reaction Kl, was calculated backward by the basic equation of the elastic ground reaction method 4) shown in Eq. (3). Fig. 4 Bending stress of piles in horizontal loading test y=((1+βh) )H/3EIβ 3 (3) Where, h is the loading height (m) of the horizontal load. Figure 5 shows the relationship between the pile displacement y and coefficient of measured horizontal subgrade reaction K2, which was obtained as a result of test calculation. As the measured value K2 decreased together with the pile displacement, K2 43,000 kn/m 3, which was equivalent to the standard displacement yo=6 mm (pile diameter: 1%), was obtained (Table 2). This result is a value that can ensure the design value K1 within a non-excessive range. Fig. 5 Measured coefficient of horizontal subgrade r eaction k2 It was therefore considered that stability of the abutment foundations of this site was guaranteed, and that the validity and usefulness of this design method, in which shear strength increased by soil improvement is treated as the equivalent ratio of subgrade reaction of piles, were verified. When using this method, it is considered desirable to conduct direct verification of design subgrade reaction of piles through an in-situ test in the same way as the test conducted at this site. Table 2 Subgrade reaction measured in a horizontal pile-loading test 5. Verification of seismic performance Because the coefficient of measured horizontal subgrade resistance (K2 = 43,000 kn/m 3 ) obtained in the horizontal loading test slightly exceeded the coefficient of design horizontal subgrade reaction (K1 = 40,000 kn/m 3 ), seismic performance of abutment foundations based on feedback from measured values was reconfirmed by the seismic intensity and seismic horizontal strength methods 9), conforming to the conventional design method. The seismic intensity method is a common aseismic design method for verification of seismic performance by converting the action applied to structural foundations and ground

6 from the effects of earthquakes into static load using seismic intensity. Conversely, the seismic horizontal strength method is a method for verification of seismic performance taking the seismic horizontal strength, deformability and energy absorption of the plastic zone of structural foundations into account. Table 3 Seismic intensity method Table 3 describes the calculation results by the seismic intensity method, and Table 4 shows those of the seismic horizontal strength method. As a result of test calculation, there was no difference between the result of the seismic intensity method and the current design method. In the seismic horizontal strength method, allowable values were generally satisfied, although the yield αand maximum pile bending moments tended to diverge slightly. Table 4 Seismic horizontal strength method It was therefore thought that the seismic performance of the method of improving soft ground around abutment foundation piles at the site was verified. In the future, it will be necessary to continue evaluation of the seismic performance of the method as required. 6. Conclusions In this study, the following knowledge was gained concerning a design method, in which ground around bridge foundation piles is improved, and the increased shear strength is reflected as horizontal resistance of piles: (1) The method used to reflect the increased shear strength resulting from soil improvement around piles as horizontal resistance is considered to be a rational method for reducing construction costs under certain site conditions, such as in the case of soft ground where pile design is determined by horizontal resistance. (2) The range of influence of horizontal resistance of piles, or the range of soil improvement, can be explained as a three-dimensional, four-sided domain set from the depth of 1/β to an area raised to the working gradient of passive earth pressure θ=(45 + /2), based on the conventional design method. (3) When determining the horizontal subgrade reaction of piles from increased shear strength, it is possible to set C by consolidation as the equivalent ratio with the modulus of deformation E increased by soil improvement. It is, however, desirable to determine design subgrade reaction from the results of the ground survey by conducting experimental construction in advance. (4) The coefficient of design subgrade reaction must be verified by conducting an on-site horizontal loading test. At this site, the measured value K2 corresponded with the design value Kl, and the validity of this method was empirically verified. (5) Changes in subgrade reaction affect the seismic performance of foundation piles. Seismic behavior must be verified through the seismic intensity and seismic horizontal strength methods for level 1 and 2 earthquake activity as necessary. 7. Afterward It is considered that the viability of this method, in which ground around piles is improved and increased shear strength is reflected as horizontal resistance, was essentially verified through a series of tests in this study. It is desirable to use this method as a rational design method for reduction of

7 construction costs depending on site conditions. The authors intend to complement this design method as a more rational method through accumulation of further data from the on-site horizontal loading test and FEM analysis in the future. References 1) Koichi TOMISAWA and Jun ichi NISHIKAWA: Evaluation of horizontal resistance of foundation piles in composite ground, Collection Papers from 35th Research Presentation of the Japan Geotechnical Society, pp , June ) Koichi TOMISAWA and Jun ichi NISHIKAWA: A Study of a Practical Design Method of Composite Ground Piles, 57th Annual Academic Report Session of the Japan Society of Civil Engineers, July ) Koichi AKAI: Soil Mechanics, pp , ) Japan Road Association: Specification for Highway Bridges IV substructures, Mar ) Japan Road Association: Guidelines for Soft Ground Measures, Nov ) Civil Engineering Research Institute of Hokkaido: Manual for Peaty Soft Ground Measures, Mar ) Vacuum Consolidation Society: N&H forced consolidation dehydration method improved vacuum consolidation method, Oct ) Japanese Geotechnical Society: Method of horizontal loading test of piles and instruction manual, Oct ) Japan Road Association: Specification for Highway Bridges V aseismic design, Mar. 7, 2002