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Piles – Part 3. Vertical (Tension) Bored Pile

posted Sep 15, 2021, 5:28 AM by jeffery jim   [ updated Sep 15, 2021, 6:52 AM ]

Piles (deep foundation) are normally designed to distribute load(s) from superstructure and its burden, stress and loading to ground in order to maintain its state of equilibrium which retains its stability through fixity with minimal or acceptable settlement that may not be detrimental to the structure. In such state of design, piles are designed to be in constant state of compression if there is no consideration on lateral force which may induce moment for rotation and/or shear off pile at the plastic hinge point. That is how piles are typically designed.

2. Nevertheless, in certain cases, piles are not designed to be under constant compression but to perform under tension and it is called tension pile. Such application is unusual and can only be found in special projects and applications. The design is usually related to buoyancy and/or water hydrostatic/pore pressure (may varies depending on ground water table/level) as the main force which causes uplifting apart from extreme moment rotation (from lateral forces too), wind or cyclone event, and seismic activity. In some instances, this can cause by less known natural phenomenon of swelling for certain types of expansive clays or clayey soils. Similarly, certain types of calcareous soil (sandy, although most of the cases would involve downdrag or negative skin friction) that will result the same apart from short piles on certain weathered rock. Therefore, it is important to understand the soil pedology or at least guided by soil taxonomy in order to include a calculated risk involving the site, planning for site investigation, and the structure which will be constructed.

3. In some common practices, certain configurations of piles are required to be functional as assigned, compression and/or tension based on assumed (and/or accidental) load direction as well as the setting with incline angle (raked pile) as a system or part of a system within a group of piles which provides stiffness to the design. The imperative consideration for a tension pile design is to always ensure it is workable in cyclic loads (compression and tension scenarios) which more often create design problem or complication if it is constructed without proper planning or simulation based on fieldtest during the site investigation phase.

4. Tension pile are primarily utilized in marine and oil & gas industry. Vertical tension piles are usually installed at dockyard which remain at rest when flooded with water and perform as demanded when vessel is placed as turned as drydock. Hydrostatic pressure and uplift happen when this infrastructure turns to a drydock without vessels as counterweight and produce compressive downward in putting up with load and propagate it to bearing. Likewise, some other applications would be raking piles as a measure to balance the lateral forces coming from fender piles for a typical harbour or jetty foundation during berthing of large vessels. Raking piles with both compression and tension members are introduced on superstructure which are exposed to slip and slide as well as rotation/bending especially those constructed on undulating surface or steep slope by enforcing stiffness to the ground. The ultimate application for tension pile would be the anchoring or the mooring system for oil rig platform at the ocean depending on the platform type.

5. For tension pile requirements in standard specifications, we shall take into account all relevant British and European standards as reference in the next few paragraphs discussing about design principles and literature. There are three main references which we shall use as guide rules in this discussion as this will guide the design of bored pile which is a replacement pile.
i. BS EN 1997 Part 1, Geotechnical Design (General Rules, Design Assisted by Fieldtesting) for both (1994 and 2004) and Malaysia Annex MS EN 1997:2012
ii. BS EN 1536, Execution of Special Geotechnical Works – Bored Pile (2000 and 2010)
iii. BS 8004, Code of Practice for Foundation (1985 and 2015)

6. Let us review the significant changes made in BS 8004 Code of Practice for Foundation based on two revisions; 1986 and 2015. BS 8004:2015 revision is still in place although BS EN 1997 or Eurocode 7 take precedence in modern design criteria. The profound changes made in BS 8004: 2015 is the integration of BS EN 1997-1 into the latest revision and hence, the former should be read in conjunction with the latter.

6A. Literature BS 8004 (1985 vs. 2015)
There is no detail on bored pile reinforcement except for execution works in BS 8004:1985 while BS 8004:2015 made recommendation to refer to BS EN 1536 as guideline as it provides larger coverage for execution works. Literature on tension pile is also limited as BS 8004:1985 make recommendation to design by considering all possible loadings to the surrounding ground and the reinforcement in the pile should be adequate to carry the tensile stress and considerations for cyclic loads. Additional recommendation made for bored pile where reinforcements should be carried down for the full length of the piles (which is mentioned in BS E 1536:2000 as well) and into the enlarged base if the tensile force is substantial. BS 8004:2015 makes reference to BS EN 1997 Part 1 for information on tensile resistance.

6B. BS EN 1997 Part 1 (1994 & 2004)
BS 8004:2015 improvises the previous revision by stipulating the sole reference for design to be made based on BS EN 1997:1994 Part 1. In this general rule section, the basic consideration of normal pile should be designed and cater tensile force acting on group of pile based on values derived for water pressure (upward and downward forces) and shear resistance at the boundary of the block of soil especially for cyclic condition as discussed earlier. No detailed reinforcement consideration is made, except that it is to be derived from calculation as suggested.

6C. BS EN 1536:2010
BS 8004:2015 redirects designer to BS EN 1536:2010 on materials and products for construction, execution works and tolerances as well as supervision and monitoring works. There are considerable small references made for design considerations with normative references to other parts of the Eurocode mainly BS EN 1991 and other parts which is/are relevant to the structure or application. Reinforcements was discussed but on superficial level with minimum percentage of longitudinal reinforcement area size ratio to nominal bored pile cross section area size, and some bar bending standard requirements on minimum spacing, reinforcement lapping length and maximum nominal size of aggregates for concrete and et cetera.

7. One design criterium which requires extra consideration when performing design analyses on software is the partial safety factors which are different in Eurocode when compared to our local Annex, MS EN 1997 Part 1. The tensile resistance partial safety factor based on BS EN 1997:2000 where γs;t can be as low as 1.40. However, MS EN 1997:2012 strictly enforced the partial safety factor to be 2.20 and as usual, it renders the commitment of the committee to promote higher safety factor just like other geotechnical applications. A short discussion with other professionals indicates a much higher safety of factor since in certain circumstances, pile under tension may permits momentarily robustness into serviceable limit state and fail in a shorter period of time or under low cycles in comparison to compression pile if design is not carefully considered.

8. Literature from specifications do not explicitly mention exact values and parameters as it only covers concepts for considerations. Hence, designers would have to reflect on their own solution which is site-specific. There are numerous studies have been conducted which can be good bases of consideration when planning for a bored pile for tension pile application.

9. Let us consider the design shape and for the ease of understanding, the barrette shapes shall be eliminated from this discussion and focus on bored piles with circular cross-section. Further to that, in order to extend high skin friction, steel casing shall not be included into the permanent design and shall be only use as temporary casing on stratum with loose soil and removed upon casting. The use of chisel is allowed and therefore the base of the shaft will not affect much settlement under cyclic load mainly under compression apart from using flight auger and drilling bucket. Enlarged base bored pile would be great to resist uplift however due to availability of such technology and smaller diameter bored pile (less than 1.5m), this will also be eliminated from the design criteria. These are some of the assumptions we are using when making recommendation.

10. Reinforcement is essential in this case where it supposed to be designed in order to cater the tensile capacity within the bored pile and other component such as the steel plate and components of the substructure member. Apart from increased steel reinforcements in comparison to conventional compression pile, it is crucial to ensure the spacing for reinforcement is design with ample spacing and pitch for both helicoidal, hoops and main reinforcements; unlike its conventional counterpart, the compression vertical pile. It is recommended to have larger bored pile with at least two to three percent for reinforcement/pile area size ratio. If the steel surface ratio is too low and insufficient to resist tensile force, steel tube as permanent casing should be considered when soil friction resistance is fairly high with proper rock socket (if it is not highly weathered rock or else further and comprehensive considerations should be made). High tensile capacity can be achieved if socketing zone and length is embedded on rock with high shear strength. The preferred length of rock socket is quite arbitrary and high safety factor should be made for chalk and marl type of rock.

11. For general consideration and comparison on skin friction value, the β value is important as it guides the theoretical outcome of possible skin friction based on McClelland (1974) formula. When the effective stress and perimeter surface area of the pile remains the same or constant, there will be almost significant change when the β value for compression pile is 0.15 to 0.35, while tension pile is only 0.10 to 0.25. This clearly renders the possible intention on why Malaysia Annex MS EN 1997 prescribes partial safety factor of 2.20 for tension pile when BS EN 1997 allows 1.40 - as a safety precaution to cover designers from erroneous calculation derived from overestimating the actual skin friction.

12. I hope this short note will give you some perspectives on tension vertical bored pile that maybe quite foreign to some of you in construction. Design considerations can be derived from site investigation and test pile. These may guide you in preliminary design and then refined design as the result from a test pile will allow you for some contingencies in capacity and to comply to design philosophy as per BS EN 1990 and BS EN 1997 Part 3 which highlights the concept on design assisted by testing.