Pile developments
7 A PRACTICAL EXAMPLE
The recently finished cable stayed “Jornalista Rob-erto Marinho” bridge in São Paulo, Brazil, is founded on approx. 0,40 m dia. rock socketed root piles, and on 0,90 m dia. rock socketed bored piles.
The main piers foundations are the bored piles and the access bridges are founded on the root piles.
The geotechnical project was not developed using a formal risk management plan, but all design deci-sions can be fitted into the framework outlined in item 3. This example intends to show that it is rela-tively easy to use geotechnical risk management and significant possible problems can be avoided.
Subsoil in the area of the bridge can be described as being:
• Superficial uncontrolled fill layer, few meters thick;
• Recent alluvial sandy and clayey deposits, up to 10 m thick;
• Residual soil (granitic-gneissic origin);
• Transition from weathered to sound granitic gneiss.
The five step procedure described in item 3 is reproduced below, and the associated design deci-sions are described:
Identify hazards and associated risks, through risk assessments, that impacts on a project´s outcome in terms of cost and programme, including those to third parties:
Possible hazards identified for the foundations of the bridge were:
• Pile integrity problems due to the fill and alluvial soils;
• Insufficient bearing capacity due to insufficient embedment into the residual soil/rock;
• Aggressive groundwater;
• Insufficient concrete strength;
• Location errors during construction.
Quantifying risks including their programme cost implications. To simplify this example the cost estima-tive is not presented in this item.
• Pile integrity problems due to the fill and alluvial soils—relatively low probability, if adequate con-struction technique is used. Consequences can be significant, if not prematurely detected.
• Insufficient bearing capacity due to insufficient embedment into the residual soil/rock—if ade-quately designed and soil investigation campaign is representative of subsoil profile, low probability.
Figure 5. Safety increase due to additional pile bearing capacity prediction/control methods. Safety Factor = Predicted Bearing Capacity /
Observed Bearing Capacity
Consequences can be significant, if not prema-turely detected.
• Aggressive groundwater—high probability due to environment. Consequences can be significant, if not prematurely detected.
• Insufficient concrete strength—low probability, because concrete will be prepared industrially and systematic control will be used. Consequences can be significant, if not prematurely detected.
• Location errors during construction—low prob-ability. Consequences can be significant, if not prematurely detected.
Identifying pro-active actions planned to eliminate or mitigate the risks.
• Pile integrity problems due to the fill and alluvial soils—pile constructive method should be chosen, so that this risk becomes insignificant.
• Insufficient bearing capacity due to insufficient embedment into the residual soil/rock—sufficient boreholes should be perforated to allow adequate pile length estimative. For design verification/
comparison between prediction and performance, load test should be performed. Additionally, pile perforation should be controlled, verifying boring difficulties that indicate the bedrock depth.
Figure 6 beneath presents the results of the load tests performed on 2 test piles—40 cm root piles, pile 1 9,4 m long and pile 2, 11,4 m long. These piles were installed prior to foundation construction. The load tests were limited to twice the maximum struc-tural load and performance was considered adequate.
Considering the observed behavior, pile design was considered adequate. No design optimization (i.e., proposition of shorter piles) was proposed. Possibly, the use of a Bayesian inference procedure could have lead to some optimization, but due to lack of time and willingness of the involved parties, the design as per-formed was considered adequate.
• Aggressive groundwater—due to the presence of aggressive groundwater, technological concrete
measures should be foreseen to protect concrete from corrosion.
• Insufficient concrete strength—a test program to evaluate concrete strength should be run during pile construction.
• Location errors during construction—topography should verify pile location prior to construction.
Identify the methods to be utilized for the control of the risks.
• Pile integrity problems due to the fill and alluvial soils—constructive method should be controlled.
The pile should be lined or supported by stabilizing fluid during excavation of the soil layers. For the root piles, grouting pressure should be kept high during concreting.
• Insufficient bearing capacity due to insufficient embedment into the residual soil/rock—Pile per-foration should be controlled, verifying boring dif-ficulties that indicate the bedrock depth, to allow comparison between soil profile estimative and actual profile. Complementary verification load tests should be performed.
Figure 7 presents the routine tests performed on 3 piles of the foundation.
Figure 7 shows that pile behavior can be consid-ered adequate and in accordance with the previously performed tests.
• Aggressive groundwater—concrete quality should be controlled.
• Insufficient concrete strength—concrete quality should be controlled during pile construction.
• Location errors during construction—topography should verify pile location prior to construction.
Allocating the risks to the various parties to the contract.
• Pile integrity problems due to the fill and alluvial soils—the foundation contractor is responsible for pile integrity.
• Insufficient bearing capacity due to insufficient embedment into the residual soil/rock—the owner
Figure 6. Load test results—prior to foundation construction.
Test Piles
0 500 1000 1500 2000 2500 3000 3500
Load (kN)
settlement (mm)
Load Test 1 Load Test 2
Figure 7. Routine control load test results.
Pile Construction control
0 500 1000 1500 2000 2500 3000
Load (KN)
settlement (mm)
Pile 1 Pile 2 Pile 3
is responsible for additional pile length, if bed-rock elevation is different from the elevation that resulted from the site investigation campaign.
• Aggressive groundwater—the concrete supplier is responsible for concrete quality.
• Insufficient concrete strength—the concrete sup-plier is responsible for concrete strength.
• Location errors during construction—the contrac-tor is responsible for adequate pile location.
The example above shows that, in a certain way, on this specific project, risk management was actu-ally performed. The positive results were only pos-sible due to the good relationship between owner, constructor and designer.
8 CONCLUDING REMARKS
Competitive markets are the driving forces for, on the on side, build safer and, on the other side, build cheaper.
These two apparently opposite paths can only be pursued improving knowledge about pile behavior and managing rationally risks.
Improvements of pile behavior prediction capabil-ity using the current common soil testing and pile prediction methods are difficult to achieve. The vari-ability of the different factors affecting these methods are probably restricting reliability improvements due to their intrinsic variability.
Possible ways to improve knowledge and, there-fore, reliability of piles, is the concurrent use of con-ventional bearing capacity prediction methods and an additional, independent, method, like, for example, measurement/specification of pile installation effort.
Rational risk management is, on the other hand, a valuable tool for all involved parties to analyze, miti-gate and chare risks, explicitly allocating responsi-bilities. Hopefully, this tool will be used in a way that all parties can decide when and where “cheaper” can mean “better”.
ACKNOWLEDGEMENTS
The support and possibility of presenting load test results of “Jornalista Roberto Marinho” bridge by Construtora OAS is gratefully acknowledged.
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© 2009 Taylor & Francis Group, London, ISBN 978-0-415-47556-3