Case History of Cyclic Statnamic Testing in the Netherlands

Transcription

1 Case History of Cyclic Statnamic Testing in the Netherlands P.Middendorp, Profound, The Netherlands R.J. van Foeken TNO Building and Construction Research, Rijswijk, Netherlands ABSTRACT: The Dutch company Jacbo has applied a new type of auger pile, the Jac-O pile, in the Netherlands. The Jac-O pile is based on an auger system that pushes most of the soil away (50 to 70%) instead of removing the soil. The Dutch Geotechnical Code only considers piles with soil removal and piles without soil removal, so standard design rules were not applicable. To allow the application of the Jac-O pile, a control method based on CPT testing was introduced. CPT tests performed before pile installation were compared with CPT tests performed after pile installation. On the Arnhem site, the CPT results before and after pile installation showed reduction in cone resistance values. The bearing capacity of the piles was checked by Statnamic cyclic load testing. 1 INTRODUCTION The Dutch company Jacbo introduced a new type of auger pile, the Jac-O pile, in the Netherlands. The tip of the auger consists of a conical core that pushes most of the soil away (50 to 70%) instead of removing the soil (Fig. 1). The advantages of such an auger system are: less soil spill on building site stiffer pile response compared with standard auger piles high production rate (50 piles per day 12m, ø 400 mm). In the Dutch Geotechnical Code for pile design (Ref. 1), only two types of piles are considered: 1 - piles with soil removal 2 - piles without soil removal The pile design in the Netherlands is strongly based on CPT testing before the installation of the piles. The Jac-O pile does not belong to one of the design types and the design rules are not directly applicable. To get the Jac-O pile approved, control CPT's have to be performed after installation of the piles. The control CPT's are compared with CPT results made for pile installation and the piles are accepted when the differences are minor. 2 THE DUTCH GEOTECHNICAL CODE For the Dutch Geotechnical Code the following condition has to be satisfied:

2 F r;max F 0,2Deq - F shaft;l (Eq. 1) In which, F r;max = maximum pile load F 0,2Deq = the load on the pile top during a load test, corresponding with a pile top displacement of 0.20D eq (piles with soil removal) D eq = the equivalent pile diameter 0.2D eq = 0.2 x 360 = 72 mm F shaft;l = the load by shaft friction which is not contributing to the bearing capacity of the pile. F shaft;l or negative shaft friction was considered to be zero for this site. So Eq.1 becomes: F r;max F 0,2Deq (Eq. 2) It is also required by the code that: F r;max,rep = ξ. F r;max (Eq. 3) In which ξ = factor for the load transfer type of the structure F r;max,rep = representative value for the maximum pile load So ξ = 1.0 for this case. F r;max = F r;max,rep (Eq. 4) It is also required by the code that with F r;max,d = F r;max,rep / γ m;b3 (Eq. 5) F r;max,d = design value for maximum pile load (allowable load) γ m;b3 = partial material (safety) factor For this case F r;max,d = 1600 kn γ m;b3 = 1.15 So the equations 3, 4 and 5 combined yield F r;max = F r;max,rep = F r;max,d. γ m;b3 = ,15 = 1840 kn (Eq. 6) and the requirement (Eq.2 ) becomes 1840 kn F 0,2Deq (capacity) (Eq.7) The piles were checked for this requirement.

3 3 CASE HISTORY ARNHEM Jac-O piles have been installed on a building site at Arnhem, the Netherlands and were checked by CPT testing after installation. The pile properties are presented in Table 1. Table 1. Pile Properties Pile type Omega pile, Jac-360-O Length [m] Diameter m Cross section m2 Pile mass 3664 kg Design Load 1600 kn Examples of CPT results taken before and after installation of pile 15 are presented in Fig.2. The control CPT results show a reduction in resistance and the allowable load is less than the design load (table 2). Fig 3. represents the CPT results of pile 6 taken before and after installation. In this case there is a considerable reduction in CPT resistance. Tabel 2. Pile capacities calculated from CPT data Before installation After installation Pile CPT Capacity CPT Capacity no no [kn] no [kn] P P Several piles on the building site showed similar reduction in CPT values and the client refused all piles. It was required to install driven cast in situ piles (non soil removing) as a replacement. The Jacbo piling contractor did not agree with this decision. He agreed that the capacity of the piles was reduced, but he was convinced that by considering the pile to be a soil removing pile, the remaining capacity was still enough to satisfy the requirements of the Dutch Geotechnical Code. In the Dutch code, the pile capacity is defined as the load at which a pile head displacement of 0.2 D eq for soil removing piles and 0.1 D eq for non-soil removing piles occurs (D eq = equivalent pile diameter). To prove the remaining capacities of the piles with reduced CPT values, statnamic load testing was performed (Fig. 4). A 3MN-statnamic device, with a hydraulic catch mechanism, was transported to the site. PMC from the UK performed the statnamic tests under the supervision of TNO. The piles were loaded by successive cycles to reach a total displacement of 0.2 D eq (72 mm). The three successive statnamic loading cycles for pile 6 are presented in Fig. 5. The measurements were analyzed with the Unloading Point Method (Middendorp 1992) and the resulting static curves were plotted as successive loading cycles (Fig. 6). The results from the CPT and statnamic tests are presented in Table 3.

4 Tabel 3. Pile capacities derived from CPT data and Statnamic Before installation After Installation Statnamic Pile CPT Capacity CPT Capacity Capacity (F 0,2Deq ) no no [kn] no [kn] [kn] P P It can be seen that the CPT results underestimate the capacity of the piles compared to the statnamic results. This information was used to convince the client that, although a reduction in CPT values had occurred, the piles were still able to satisfy the Dutch Geotechnical Code assuming that the piles were acting as soil-removing piles. 4 CONCLUSIONS The pile top displacement 0.2 D eq (72 mm) corresponding with the pile capacity could be reached by performing succesive cyclic statnamic load tests. Statnamic load testing was able to prove the remaining static capacity of piles after the occurrence of CPT values reduction. At the Arnhem, site the Dutch CPT design methods were underestimating pile capacity compared with the calculated static behaviour from statnamic load testing. 5 REFERENCES Middendorp, P, Bermingham P., Kuiper B., 1992, Statnamic load testing of foundation piles. 4th International Conference on Stress Waves, The Hague, Balkema Dutch Geotechnical Code, 1993, Geotechnics, Calculation method for bearing capacity of pile foundation, Compression piles, NEN 6743, Nederlandse Normalisatie Instituut, Delft

5 Figure 1, Jac-O auger. Before installation After installation Figure 2, CPT results of pile 15.

6 2nd Statnamic Seminat, Tokyo, 1998 Before installation Figure 3, CPT results of pile 5. Figure 4, Statnamic Testing with 3MN device at Arnhem After installation

7 Figure 5, three successive Statnamic Loading cycles Pile6 1st loading cycle 2nd loading cycle 0,20D eq F 0,2Deq 3rd loading cycle Figure 6. Calculated static load displacement diagram