Pile Foundation Bearing Capacity Characteristics of Volcanic Ash in Hokkaido, Japan

Transcription

1 Pile Foundation Bearing Capacity Characteristics of Volcanic Ash in Hokkaido, Japan Kouiti TOMISAWA, Jun ichi NISHIKAWA Civil Engineering Research Institute of Hokkaido(CERI), Sapporo,Japan Abstract Since volcanic ash has both friability and bulk compressibility and the foundation strength is reduced by pile construction, no definitive pile foundation design method for volcanic ash has yet been established in Japan. Vertical load tests of piles and cone penetration tests were therefore conducted for volcanic ash distributed in Hokkaido, Japan, to verify the pile support mechanism in volcanic ash groung. In this study, characteristics of vertical bearing capacity were examined based on the results of field tests with an emphasis on the engineering properties of volcanic ash and pile construction methods. Results from the study revealed a correlation between the necessity of reducing the skin friction of driven and cast-in-place piles in designs in volcanic ash ground and the value of cone penetration resistance. Keywords: volcanic ash, pile, load test, bearing capacity, cone penetration test 1. Introduction The bearing capacity formula for pile foundations in volcanic ash ground is usually designed 1) based on sandy ground because volcanic ash has a certain shear strength. Volcanic ash, however, is said to be friable. It has been stated in many field reports that it is difficult to ensure the designed skin friction and end-bearing capacity of piles due to disturbance of the ground at the time 2),3) of pile construction, and no pile foundation design or construction management methods have yet been established for volcanic ash ground. Vertical load tests of steel pipe and cast-in-place piles were thus conducted for Shikotsu, Mashu and Komagatake volcanic ash distributed in Hokkaido, Japan, to verify the development mechanism of pile bearing capacity in volcanic ash ground. Electric cone penetration tests (CPT) were also carried out to evaluate the skin friction of piles through field investigations. In this study, the characteristics of vertical bearing capacity of pile foundations were examined with a focus on the 4), 5) workability of piles and engineering properties of volcanic ash based on the results of field tests to establish a reasonable design method for pile foundations in volcanic ash ground. 2. Volcanic ash Vertical load tests for piles were conducted to confirm the bearing capacity in volcanic ash ground in Hokkaido, Japan. Tests were carried out at four sites: Kotobuki viaduct with steel piles of 500 mm, L = 23 m (Site A) and Tetsuhoku Bridge with cast-in-place piles of 1,200 mm, L = 18.5 m (Site B) in Central Hokkaido, Shintomi Bridge with cast-in-place piles of 1,000 mm, L = 8m -1-

2 Fig. 1 Volcanic ash distribution in Hokkaido 6) (SiteC)inEasternHokkaido,andHakodateICBridgewithcast-in-placepilesof 1,200mm, L =14m (Site D) in Southern Hokkaido (See Fig. 1: Volcanic ash distribution in Hokkaido). Volcanic ash at each site is divided into Shikotsu pumice flow sediment (Spfl., Sites A and B), 6) Mashu pumice flow sediment (Mafl, Site C) and Komagatake volcanic sediment (Ko, Site D). Volcanic ash from these sites was generally classified as coarse-grained volcanic ash. As shown in the soil test results (Table 1), volcanic ash from all four sites had a relatively high shear strength in ground. According to field penetration test results, coarse-grained volcanic ash in Hokkaido generally has a low resistance to dynamic forces because particle breakage is likely to occur, and a high resistance to static forces due to the unevenness of grains. Table 1 Basic physical propenties of Volcanic ash -2-

3 3. Vertical load test 3.1 End bearing capacity of piles (qd) Vertical load tests of piles at each site were conducted using a multi cycle reaction load method, which consisted of column devices and used surrounding piles as a reaction force, in 7) accordance with the criteria of the Japanese Geotechnical Society. Each test pile was equipped with strain gauges placed at regular intervals in the depth direction of the piles to verify development of skin friction and end bearing capacity of piles at the time of the load test. Fig. 2 Geologic log at each site-relationship between the N value and the skin friction of the pile -3-

4 As a result of vertical loading tests, the ultimate bearing capacity (Ru) measured for all test piles satisfied the required design bearing capacity, and the goal of the field verification test was 2 accomplished. This was achieved because the design end bearing capacity (qd = 3,000 kn/m ) was satisfied for cast-in-place pipes, and an end bearing capacity far exceeding the relationship between the pile setting depth and the N value, or the relationship of the qd at the time of design/n - setting depth in the bearing layer/pile diameter, was secured for steel pipe piles. 3.2 Skin friction of the pile (f) Figure 2 shows the skin friction (f) of test piles at four sites obtained from vertical load test results, as contrasted with the N value in the depth direction. It can be seen that, for steel pipe piles at Site A, almost no skin friction was observed in the section where the N value was 30 or lower, and that a relatively large skin friction (f) that satisfied the calculated design value for sandy ground 2 f = 2N (kn/ m ) developed in the section where the N value was 30 or higher. At Sites B and D with cast-in-place piles, on the other hand, the results nearly corresponded with the relationship of f = 5N 2 (kn/ m ), which was the design value for sandy ground. For cast-in-place piles at Site C with confined water, however, the skin friction of piles was only 60% of the design value in the layer with N values of 30 or higher. This result was thought to be due to the difference in construction methods for steel pipe piles, which are referred to as non-displacement piles, and cast-in-place piles, which are referred to as displacement piles. It was presumed that, especially in the construction of steel pipe piles, vitreous material in volcanic ash was broken at the time of piling and development of skin friction of piles deteriorated considerably in sections with a low relative density. It was also assumed that, when confined water exists as in the case of Site C, the development of skin friction of piles was affected by the generation of water flows between the piles and volcanic ash soil and other factors even after pile construction. 3.3 Boring in the steel pipe piles For steel pipe piles at Site A, boring in pipes was conducted for open-end vertical load test piles and hammer-set piles with special cross-rib processing to verify the end blockage effect and ground strength in pipes before carrying out the vertical load test. Figure 3 shows the results of boring in pipes of open-end and cross-end piles. While end blockage was evident in the bearing layer at approximately 20 m or deeper, the ground strength of soil in pipes was generally low in cross-end rib piles and the strength tended to decrease remarkably at the finishing point of piling in the bearing layer. This phenomenon most likely occurred because the shear strength of friable volcanic ash in pipes was reduced by breakage caused by piling and, as a result, sufficient soil resistance or end bearing capacity of the pipes was not maintained. Fig. 3 Boring in steel pipe piles at Site A -4-

5 4. CPT test 4.1 Foundation strength (qt) CPT is a test method for measuring the point resistance (qt), skin friction (fs) and pore water pressure (w) as sequential ground data by penetrating the ground with a cone that has a built-in center at its top at a speed of 2 cm/s using a 200-kN penetrator. CPT tests were conducted at pile load test positions of Sites A, B and C. The point resistance 2 (qt) showed a relatively high correlation of qt 1,000 N (kn/ m ) to the N value at all sites. The CPT test was therefore considered to be somewhat effective as a simple test method that enables the direct measurement of point resistance (qt), skin friction (f) and pore water pressure (w) in volcanic ash ground. 4.2 Skin friction (fs) Figure 4 shows the skin friction (fs) in the depth direction at Sites A, B and C, obtained as a result of CPT tests. According to the figure, the skin friction (fs) obtained by cone penetration developed in proportion to the N value of volcanic ash shown in Fig. 2, and these measurement values roughly corresponded with the design values based on sandy ground. In comparison with the skin friction (f) measured using the vertical load test, which is also shown in the figure, values approximately equal to the lower limit of fs were found by utilizing the CPT test for cast-in-place piles at Site B, in the same manner as in the correlation chart with the N value shown in Fig. 2. For steel pipe piles at Site A and cast-in-place piles at Site C with confined water, however, the values were obviously smaller than the fs value found using CPT tests, before the N value became approximately 30. Fig. 4 Skin friction (fs) determined by CPT test -5-

6 4.3 Cone repetition method In addition to ordinary CPT tests, penetration was repeated in this study under the assumption that disturbance of ground occurred at a certain depth, which is shown in Fig. 4 (referred to as the repetition method, as opposed to the uniaxial method that is usually used), to verify the energy dispersion to the ground at the time of foundation pile construction and the decrease in skin friction due to the particle breakage peculiar to volcanic ash at Sites A, B and C. The repetition stroke was maintained at 5 cm, taking into account the influence of porous wall peeling. Results showed that the skin friction (fs) determined by the CPT test at Site A eventually converged at around 1/100 after approximately 20 repetitions. The converged skin friction (fs') value at Site A roughly corresponded with the skin friction in the volcanic ash layer with an N value of 30 or lower obtained using the vertical load test. It was therefore considered explainable as a phenomenon of decreased skin friction caused by the placement of steel pipe piles. Figure 5 shows the relationship between the ratio Fig. 5 Relationship between fs /fs of cone penetration of fs converged by the repetition test and point resistance qt for steel pipe piles at Site A method at Site A to fs obtained by utilizing the uniaxial method and the point resistance (qt) qc As a result, the exponential function formula of fs /fs = e was found. This formula shows the reduction in the decreasing rate of skin friction (fs /fs) by the repetition method with an increase in qt. It is therefore considered to represent the friability of volcanic ash due to the steel pipe pile construction at N values of 30 or lower, in the same way as the development of skin friction obtained through the vertical load test. Figure 6 shows the relationship between the skin friction at the first stage of the CPT repetition method (fs1) and the result of vertical load tests (f) at Sites B and C with cast-in-place piles. As can be seen in the figure, the relationship between fs1 and the load test value (f) was regular although data were insufficient, and it will be necessary to evaluate the influence of confined water at Site C in the future. In assuming that the decreasing rate of skin friction at the first stage of repetition was at the same level as the Fig. 6 Relationship between the skin friction at the disturbance of volcanic ash at the time first stage of repetition and that of the load test at of cast-in-place pile construction, it was sites with cast-in-place piles -6-

7 presumed that the circulation effect in sandy ground from the point of stress to surrounding areas 8) expressed by the cast-in-place pile bearing capacity formula of Myhof, which is the bearing capacity theory of Prandtl s system, appeared in the state where surrounding soil had not peeled off from the tip of the cone in the CPT test. At Sites A and B, recovery of the converged fs over time was not observed even when the repetition method was used again 12 hours after the repetition test was conducted at a certain depth. In the future it will be possible to establish design methods for the skin friction of piles according to the type of volcanic ash by accumulating basic data using the above method and verifying the correlation between the vertical load test and cone test values. 5. Conclusion (1) Vertical load test showed that it is possible to secure the required end bearing capacity (qd) of steel pipe and cast-in-place piles through appropriate setting of the bearing layer even in friable volcanic ash ground. During this test, especially in the case of steel pipe piles, it was considered to be difficult to display the end blockage effect due to the breakage of soil in pipes when special processing had been applied at the tips of piles. (2) Development of skin friction (f) of piles in volcanic ash was determined to be a result of the difference in workability of steel pipe piles (displacement piles) and cast-in-place piles (non-displacement piles). (3) The CPT test, which enables direct measurements of point resistance (qt), skin friction (f) and pore water pressure (w) in volcanic ash soil was found to be effective as a simple test method. (4) Because almost no skin friction of piles developed when the N value was 30 or lower for steel pipe piles in volcanic ash ground according to the results of vertical load tests, it was presumed that the development of skin friction was considerably reduced through breakage of vitreous material of volcanic ash due to pile driving. For steel pipe piles, an exponential function formula qc of (skin friction by repetition method fs )/(skin friction by uniaxial method fs) = e was also obtained using the CPT test. (5) While skin friction of piles based on sandy ground was mostly maintained for cast-in-place piles in volcanic ash according to the results of vertical load tests, only 60% of the design value was achieved at the site affected by confined water. The value of one stage of CPT repetition (fs1), which was thought to have a decreasing rate at the same level as disturbance of the ground at the time of cast-in-place pile construction, showed a relatively favorable relationship with the result of the vertical load test (f). 6. Afterwords This study made clear the necessity to conduct evaluation by reducing skin friction from the design ground constants for different types of piles. Other results related to foundation pile design for volcanic ash ground were also obtained. To establish feasible design and construction methods for foundation piles in volcanic ash ground in the future, it will be necessary to clarify the engineering properties of volcanic ash distributed widely in Hokkaido, as well as the development mechanism of the bearing capacity of foundation piles. -7-

8 References 1) Japan Road Association: Specification for Highway Bridges Instruction Manual IV - substructure, pp , Mar ) Tomisawa and Nishikawa: Bearing capacity characteristics of driving steel-pipe-pile in volcanic ash ground, Collection of lectures at the 41st technical research presentation of the Hokkaido branch of the Japanese Geotechnical Society, pp , Feb ) Kimiaki Akai, Yuichi Tsujimoto, Shozo Sakuma and Takeshi Hanzawa: Bearing capacity mechanism of steel pipe piles in the Shikotsu volcanic ash layer, Soil and Foundation, Vol. 132, No. 3, pp , July ) Ryosuke Kitamura: Topic on Crushable Siol and Crushable Ground in Geotechnial Engineering, Japanese Geotechnical Society, Soil and Foundation, Vol. 48, No. 1, pp. 3-6, Oct ) Seiichi Miura, Yoshikazu Yagi, Hiroyuki Tanaka and Tsuyoshi Asonuma: Mechanical Behavior of Crushable Volcanic Coarse-grained Soil in Hokkaido and its Evaluation, Japanese Geotechnical Society, Soil and Foundation, Vol. 48, No. 10, pp , Oct ) Committee for Engineering Classification of Volcanic Ash : Properties and utilization of volcanic ash in Hokkaido, Hokkaido branch of the Japanese Geotechnical Society, Oct ) Japanese Geotechnical Society: Instruction manual for vertical load test method for piles, June ) Meyhof G.G.: The Ultimate Bearing Capacity of Foundation, Geotechnique, Vol. 2, No. 4,