RAPID PLATE LOAD TESTS ON BEARING STRATUM OF A BUILDING FOUNDATION

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1 152 RAPID PLATE LOAD TESTS ON BEARING STRATUM OF A BUILDING FOUNDATION H. NEMOTO and H. SAKIHAMA ANDO Corporation, Ooi-chuo, Fujimino City, Saitama, Japan fvgv182@mb.infoweb.ne.jp Y. NAKASHIMA and K. MATSUZAWA Marubeni Construction Material Lease Co., Ltd., Shibakoen, Minato-ku, Tokyo, Japan k_matsuzawa@maruken-lease.co.jp T. MATSUMOTO Department of Civil Engineering, Kanazawa University, Kakuma-machi, Kanazawa, Japan matsumot@t.kanazawa-u.ac.jp SUMMARY: A 14-storey office building with a basement floor was constructed in Sendai, Japan, in 26. A diluvial gravel layer exists at the site to depths of 7 to 8 m underlain by a very hard rock to depths more than 3 m. Hence, a raft foundation at a depth of 5 m was adopted in the design stage to carry the weight of the building. The measured SPT N-values at the depth of 5 m varied widely from 24 to 6. Therefore, it was necessary to confirm the distribution of bearing capacities over the construction area densely. For this purpose, rapid plate load tests using the Spring Hammer devices were carried out. It was confirmed that the bearing capacities at all the locations tested exceed the required value of 9 kpa, and that the bearing stratum had enough stiffness to suppress settlements of the foundation below the allowable level. Keywords: load-displacement, raft foundation, rapid plate load test, spring hammer device. INTRODUCTION It is important to design buildings with due consideration for their performance. In the framework of performance-based design in Japan, foundations of a building are designed considering settlements as well as the bearing capacity to support the weight of the building. To perform such design, it is important to investigate the distribution of bearing characteristics such as bearing capacity and stiffness of the bearing stratum below the foundation, because the bearing characteristics may vary by location even in a narrow construction area. Foundations: Proceedings of the Second BGA International Conference on Foundations, ICOF28. Brown M. J., Bransby M. F., Brennan A. J. and Knappett J. A. (Editors). IHS BRE Press, 28. EP93, ISBN

2 1798 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto A 14-storey office building with a basement floor was constructed in Sendai, Japan, in 26. A diluvial gravel layer exists at the site to depths of 7 to 8 m underlain by a very hard rock extending to depths more than 3 m. Hence, a raft foundation at a depth of 5 m was adopted at the design stage to carry the weight of the building. The measured SPT N-values at the depth of 5 m varied widely from 24 to 6. Therefore, it was necessary to confirm the distribution of bearing capacities over the construction area. It was thought to be useful to carry out many plate load tests to ascertain the variation of the bearing stratum. However, it was difficult to carry out many static plate load tests in the construction site, because of high cost and time. Therefore, rapid plate load test using a Spring Hammer device (SH test, hereafter) were employed, because the SH test does not need a reaction system and requires a short time for preparation and testing 1,2. Use of the SH test method allowed us to carry out many plate load tests (15 locations) in a short time period. In addition, conventional static plate load tests were carried out at four locations on the site, in order to validate the SH test results. In this paper, outline of the construction site including the building and the ground conditions, the SH test method, and the results of the SH tests and the static plate load tests are presented. It will be shown that the results from both test methods are comparable, and that bearing capacities and coefficients of vertical subgrade reaction at all the locations tested exceed the required values adopted in the design. OUTLINE OF THE BUILDING AND THE GROUND Figure 1 shows the plan and side views of the building foundation. The raft foundation with a thickness of 2. m has an almost rectangular area, 4.7 m by 22.4 m. The building is a base-isolated structure, where anti-seismic laminated rubber is intercalated between the raft and the base of the building. Borehole investigations including measurements of SPT N-values were carried out at three locations as indicated in the Figure 1. Figure 2 shows the profiles of the soil layers and measured SPT N-values. A diluvial gravel layer exists at the site to depths of 8 to 9 m from G.L. at Bor. A (Tokyo Pail, T.P m) underlain by hard rocks to depths more than 3 m. The ground was excavated to T.P m or T.P m, and the raft foundation was founded at that lovel. The SPT N-values at the raft base level varies widely from 24 to 6 by location, because the bearing stratum contains gravels and cobbles as shown in Figure 3. Therefore, it was necessary to investigate the distribution of bearing characteristics at the raft base level, in order to confirm the validity of the soil parameters adopted at the design stage. The design maximum contact pressure at the foundation base was estimated at 3 kpa. According to Japanese building codes, a bearing capacity greater than 9 kpa is needed, adopting a safety factor of 3. Allowable maximum settlement of the foundation was set at 3 mm, because this value may not cause harmful cracks of the raft foundation. In order to satisfy this criterion, it was estimated empirically that coefficient of the vertical subgrade reaction, k v, estimated from the plate load test using a rigid circular plate having a diameter, D, of.3 m should exceed 22 MPa/m. This value was used to design the raft foundation.

3 Rapid plate load tests on bearing stratum of a building foundation 1799 Raft Fig. 1: Plan and side views of the building foundation. Fig. 2: Profiles of soil layers and SPT N-values at the construction site.

18 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto Fig. 3: Situation of the ground at the raft base level.

between a load cell on the pile top and a falling mass in order to prolong loading duration. In the SH device, spring unit consisted of several coned disc springs placed on rigid plate or pile top.

5). Velocity and displacement of the plate are obtained by single or double integration of the measured acceleration with respect to time, respectively.

4 18 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto Fig. 3: Situation of the ground at the raft base level. RAPID PLATE LOAD TEST METHOD SH device and test procedure Loading mechanism of the SH device is similar to those of the Dynatest 3 and the Pseudo-static load test 4, where springs are intercalated between a load cell on the pile top and a falling mass in order to prolong loading duration. In the SH device, spring unit consisted of several coned disc springs placed on rigid plate or pile top. Figure 4 illustrates the SH device with the data acquisition system in case of plate load test. Load on a plate and acceleration of the plate are measured by a load cell and accelerometers (see Fig. 5). Velocity and displacement of the plate are obtained by single or double integration of the measured acceleration with respect to time, respectively. Displacement can be measured directly using laser or optical displacement meters in some situations. On the building construction site, two types of the SH devices, portable type (Fig. 6, DSH-2) and machine-mounted type (Fig. 7, SH-7), were used. Table 1 summarises the specifications of DSH-2 and SH-7. Although the maximum load capacity of DSH-2 is limited to 2 kn, the device has the advantage of portability. Therefore, it is easier to carry out the tests at many points efficiently with DSH-2. Load cell Acceleromete Fig. 4: Loading and data acquisition system in the SH rapid load test method. Fig. 5: Load cell and accelerometers set on the plate

5 Rapid plate load tests on bearing stratum of a building foundation 181 Fig. 6: DSH-2 Spring Hammer test device for small plate (D=.3 m). Fig. 7: SH-7 Spring Hammer device for large plate (D=.6 m). Table 1. Specifications of Spring Hammer test devices. Device name DSH-2 SH-7 Device type Portable tripod Machine mounted Max. load 2 kn (standard use) 1 kn Hammer mass 2 kg 15 kg Max. fall height of hammer 2 m 3 m Spring value 5125 kn/m (variable) 87 kn/m

6 182 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto Non-linear damping interpretation method One of advantages of the rapid load test is that simplified interpretation methods can be used to derive a static load-displacement relationship from the measured dynamic signals. The non-linear damping interpretation method (Matsumoto et al) 5 was used in the SH rapid load tests. Figure 8 shows the modelling of plate and soil during rapid plate load testing. The plate is assumed as a rigid mass having mass of M p, and the soil is modelled by a spring and a dashpot. This type of modelling has been advocated by Middendorp et al. 6 and Kusakabe and Matsumoto 7. They assume non-linear spring and linear damping. On the other hand, in the non-linear damping interpretation method, both spring and damper are treated as non-linear. As shown in Figure 9, applied load, spring, K (F w ) F rapid Plate mass, M p, with additional soil mass, M s dashpot, C (F v ) Fig. 8: Modelling of plate and soil during rapid loading. F rapid, is corrected for the inertia of the plate and additional soil mass, to obtain the soil resistance, F soil. The additional soil mass, M s, can be estimated as follows following Randolph and Deeks ν M = 2D ρ (1) s s ( 1 ν ) where ν and ρ s are Poisson's ratio and density of the soil, and D is the plate diameter. Figure 1 shows the procedure of the non-linear damping interpretation. The soil resistance, F soil, is the sum of the spring resistance (static resistance), F w, and the dashpot resistance, F v. At the first step (i = 1), the initial stiffness, K(1), is calculated by the initial static load, F w (1), divided by the initial displacement, w(1). At the next step (at i+1), the soil spring, K(i+1) is assumed to be equal to K(i). Hence, the static resistance, F w (i+1), at i+1 can be calculated. The damping coefficient, C(i+1), can be found from the difference of F soil and F w divided by velocity, v(i+1). Fig. 9: Correction of inertia to obtain soil resistance, F soi Fig. 1: Non-linear damping interpretation.

Finally, the whole static load-displacement relationship, F w vs w, is constructed. Detail of this interpretation is mentioned in another paper in this proceeding 1.

7 Rapid plate load tests on bearing stratum of a building foundation 183 At the following step i+2, C(i+2) is assumed to be equal to C(i+1). Therefore, the values of F w (i+2) and K(i+2) can be determined. By repeating these procedures, the values of K and C for following steps are alternately updated consecutively. Finally, the whole static load-displacement relationship, F w vs w, is constructed. Detail of this interpretation is mentioned in another paper in this proceeding 1. TEST DESCRIPTION Figure 11 shows the locations of the plate load tests at the site. A total of 19 tests (15 SH tests and 4 static plate load tests) were carried out as listed in Table 2. A plate having a diameter, D, of.3 m was used at 13 locations, and a plate having D =.6 m was used at the other locations. In the SH tests, DSH-2 device was used for testing the plate of D =.3 m, whereas SH-7 device was used for testing the plate of D =.6 m. Static plate load tests were carried out at 4 locations in the site. A rigid plate of D =.3 m was tested at 3 locations while a plate of D =.6 m was tested at S4. Figure 12 shows a static plate load test using reaction beams. It took 2 days to perform each static load test including preparation and testing periods. Fig. 11: Locations of plate loading tests. Fig. 12: Loading system of static plate load test.

8 184 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto Table 2. Test conditions of SH and static plate load tests, and brief test results. Test conditions Brief test results Test No. Plate Diameter D (m) Hammer mass M H (kg) Max. fall height h (cm) Planned max. load (kpa) Yield stress (kpa) Bearing resistance at w = 3 mm (kpa) Initial stiffness in first loading k v (MPa/m) R R > R R > R R > R R > R R > R > R >15 66 R > R > R > S > S > S S TEST RESULTS Rapid plate load tests Figure 13 shows examples of dynamic signals measured in the first rapid load test on Plate R3 having a diameter of.6 m. The loading duration was about 5 ms. Figure 14 shows the results of the non-linear damping interpretation. A total of 5 tests were conducted on Plate R3, as shown in Figure 15. As mentioned before, the required value of the vertical subgrade reaction, k v, was 22 MPa/m and the allowable settlement of the foundation was set at 3 mm. The value of k v was estimated from the first loading, although that obtained from the following tests becomes larger. It is efficient to suppress the contact pressure of the raft below the yield stress, p y, in order to minimise the foundation settlements. The yield stress, p y, was defined as indicated in Figure 15. The bearing resistance at w = 3 mm, k v and p y are listed in Table 2.

9 Rapid plate load tests on bearing stratum of a building foundation 185 Force (kn) R3 blow Time (ms) (a) Pile head force Velocity (m/s) R3 blow Time (sec) Acceleration (m/s 2 ) R3 blow Time (sec) (b) Acceleration (c) Velocity (d) Displacement Fig. 13: Examples of measured test signals of Plate R3 (D=.6m) Stress (kpa) D =.6 m M H = 1.5 ton h =.5 m R3 blow 1 p rapid p soil p static R3 blow Time (sec) R3 p y p static (kpa) k v 1 h =.5 m h = 1. m h = 1.5 m h = 2. m h = 3. m D =.6m Fig. 14: Example of derived load-displacement curve of Plate R3. Fig. 15: Load-settlement relations of Plate R3. Figures 16 to 18 are the corresponding results from the rapid load test on Plate R4 with D =.3 m. The signals in the 4th test are shown. The yield stress from this test series is comparable to that of Plate R3 with D =.6 m, not showing so-called size-effects of plate diameter. In the rapid load tests on Plate R4, the final displacement of the plate was 9 mm which was less than the allowable settlement of 3 mm. In cases where the final displacement of the plate did not reach 3 mm, the stress at the final displacement is listed in Table 2. Static plate load tests Figure 19 shows the load-displacement curves of Plates S1 to S4 obtained from the static load tests. Each load was maintained for 3 min except for the test on Plate S2. Continuous loading was adopted for Plate S2 in which a loading rate of 113 kpa/min was used. The bearing resistance at w = 3 mm, k v and p y from the static plate load tests are also listed in Table 2.

10 186 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto 12 1 R4 blow Time (ms) (a) Pile head force Force (kn) R4 blow Time (ms) (c) Velocity Velocity (m/s) Acceleration (m/s 2 ) R4 blow Time (ms) (b) Acceleration R4 blow Time (ms) (d) Displacement Fig. 16: Examples of measured test signals of plate R4 (D=.3m) Stress (kpa) D =.3 m M H =.2 ton h =.98 m R4 blow 4 p rapid p soil p static Fig. 17: Example of derived load-displacement curve of Plate R R4 p static (kpa) p y h =.25 m h =.5 m h =.75 m h =.98 m D =.3m Fig. 18: Load-settlement relations of Plate R4. Comparison of SH and static plate load test results The load-displacement curves obtained from all the tests are compared in Figure 2. Variations of the bearing characteristics over the site obtained from the rapid load tests and the static load tests are similar. As can be seen from Figure 2 and Table 2, it was confirmed from the plate load tests that the ground over the foundation area has the bearing capacity and subgrade reaction coefficient exceeding the values adopted in the design stage. In all the tests, the yield stress, p y, exceeds 5 kpa that is sufficiently larger than the design contact pressure of 3 kpa. It is expected that the settlement of the foundation will be small enough, even if the size-effects exists.

11 Rapid plate load tests on bearing stratum of a building foundation 187 p (kpa) p static (kpa) Allowable settlement S1(D=3mm) S2(D=3mm) S3(D=3mm) S4(D=6mm) RLT SLT D =.6m D =.3m D =.3m D =.6m Fig. 19: Results of static plate load tests. Fig. 2: Load-displacement relationship obtained from rapid and static load tests. CONCLUDING REMARKS A number of plate load tests were carried out on a building construction site to estimate variability of bearing characteristics of the bearing stratum for the raft foundation. Rapid plate load tests using the SH device and static plate load tests were used. A non-linear damping interpretation method was used to derive static response of the plate in rapid plate load test. Variability of the bearing stratum obtained from the rapid load tests and the static load tests were comparable. It was confirmed from all the tests that bearing capacities and coefficients of vertical subgrade reaction at all the locations tested exceed the required values adopted in the design stage. Construction of the building proceeded as designed after the tests. Fifteen rapid load tests using the SH devices were done in 3 days, whereas it took 8 days to perform 4 static load tests. This fact encourages the use of the rapid plate load test method on sites to reduce delay to construction works. REFERENCES 1. Matsuzawa K, Nakashima Y, and Matsumoto T. Spring hammer rapid load test method and its validations. Proc. of 2nd Int. Conf. on Foundations, Dundee, Scotland, 28 (to be presented) 2. Matsuzawa K, Sakihama H, Nemoto H and Matsumoto T. Size and loading rate effects observed in plate load tests on a fill. Proc. of 1th Int. Conf. on Piling and Deep Foundations, Amsterdam, 26, pp Gonin H G C and Leonard M S M. Theory and performance of a new dynamic method of pile testing. Proc. of 2nd Int. Conf. of Application of Stress-Wave Theory to Piles, Stockholm, 1984, pp Schellingerhout A J G, Revoort E. Pseudo static pile load tester. Proc. of 5th Int. Conf. on Application of Stress-Wave Theory to Piles, Orland, 1996, pp

12 188 Nemoto, Sakihama, Nakashima, Matsuzawa and Matsumoto 5. Matsumoto T, Tsuzuki M and Michi Y. Comparative study of static loading test and statnamic on a steel pipe pile driven in a soft rock. Proc. of 5th Int. conf. and Exhibition on Piling and Deep Foundations, Bruges, Belgium, 1994, pp Middendorp P, Bermingham P and Kuiper B. Stanamic loading testing of foundation piles. Proc. of 3rd Int. Conf. on Application of Stress-Wave Theory to Piles, The Hague, Netherlands, 1992, pp Kusakabe O and Matsumoto T. Statnamic tests of Shonan test program with review of signal interpretation. Proc. 1st Int. Statnamic Seminar, Vancouver, Canada, 1995, pp Randolph M F and Deeks A J. Dynamic and static soil models for axial pile response. Proc. 4th Int. Conf. on Appl. of Stress-Wave Theory to Piles, The Hague, 1992, pp

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