Improvement of the bearing capacity of footings with micropiles | |
| Author | You, Guanlin |
| Call Number | AIT Diss. no.GE-03-01 |
| Subject(s) | Piling (Civil engineering) Concrete footings Reinforced concrete |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Civil Engineering |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. GE-03-01 |
| Abstract | It is found in this study that micropiles are effective to improve the bearing capacity of footings in different grounds under different loading conditions (vertical, inclined and lateral) from model and/or large-scaled field tests on footings, micropiles, and micropile foundations (or footings reinforced with a group of micropiles). The load bearing behavior of the micropile foundation is analyzed under the concept of the piled raft foundation, and Network Effect Index (R) is used to quantitatively evaluate the variation of bearing capacity in the micropile foundation with reference to a footing and a micropile group. Comparative analyses find that the bearing capacity in micropile foundation tests can be less than that in micropile tests in loose or soft grounds, so current design method may overestimate the bearing capacity of micropile foundations in soft grounds. Furthermore, applying prestress to micropile foundations is employed in this study to improve their performance in soft grounds. First, the load bearing behavior of the micropile foundation is systematically investigated from the data of model tests in sand obtained in Hokkaido University in 1998. The effect of relative density on the bearing capacity is significant in the model tests. The dilative behavior of dense sand may significantly enhance the improvement of bearing capacity, while the contraction of medium-dense and loose sands may cause a negative interaction resulting in the reduction in the bearing capacity. From the analyses, Network Effect Index is more than one after a small amount of settlement in the dilative dense sand, but it is less than one even at a very large settlement in loose/soft grounds. And it is also found that sufficient number of micropiles is necessary to secure the confining effect so as to mobilize a positive network effect in the micropile foundation, Skin friction of micropiles is the major component of the bearing capacity of micropiles as verified by the surface roughness and length of micropiles, and it affects the load bearing behavior of micropile foundations in the early loading stages. On the other hand, bending stiffness of micropiles is important to effectively confine the encased soil under large settlements, The bearing capacity and the coefficient of subgrade reaction are also affected by the battered angle of micropiles in micropile foundations. Under vertical loading conditions, the bearing capacity is high at small battered angles, but it decreases significantly at large battered angles due to the lateral loading behavior of the battered micropiles; and the coefficient of subgrade reaction decreases with the battered angle of micropiles. Under inclined loading conditions, the bearing capacity and the coefficient of subgrade reaction decrease with the load inclination; the vertical coefficient of subgrade reaction is peaked at the battered angle of 15º, while it is 45º for the horizontal coefficient in the micropile foundation. From the analyses of model prestress tests, it is found that the prestress is effective on the improvement of the bearing capacity of micropile foundations, especially in the early loading stages with a small displacement. Consequently, coefficients of subgrade reaction and network effect index increase with prestress. Large-scale field-tests were subsequently conducted in a cohesive soft ground to examine the effect of prestress. There are footing tests, single micropile loading tests, field prestress tests on singleton and grouped micropiles, and horizontal- and vertical-loading tests on prestressed and non-prestressed micropile foundations. The effect of prestress on the improvement of the bearing capacity is significant; the pullout resistance of a single prestressed micropile is about twice of a non-prestressed micropile; the improvement ratio of bearing capacity is 1.4 in the early loading stages in the prestressed micropile foundation. Network Effect Index is significantly increased in the prestressed micropile foundation. At the yielding point, Network Effect Index is 0.8 and 1.1 for the non-prestressed and prestressed micropile foundation, respectively. The coefficient of subgrade reaction is remarkably increased with prestress in the field tests. Prestress is also affecting the load distribution between the footing and micropiles in the micropile foundation. The footing contributes 16 to 32% of the bearing load in non-pressed micropile foundations in different grounds, and it appears the footing takes more portion of the applied load in soft/loose grounds. The base pressure of the footing increases after prestress due to the enhanced contact between the footing and subsoils, and the skin friction of micropiles may increase due to the densification of the encased soil and the increased confining effect after prestress. However, the effect of prestress on the load distribution is not consistent in all aspects in the model and field tests. The increment of the base pressure is dominantly increased while the increment of equivalent base pressure is insignificant in model prestress tests in sand, which may indicate the footing contributes the improvement of bearing capacity of the foundation. On the other hand, the increment of the base pressure is significant only at small settlements while the increment of the equivalent base pressure increases at all settlement levels in the field prestress test, indicating that the micropiles mainly contribute to the improvement of the bearing capacity. This difference may result from the complete different soil conditions in the model and field tests, the poor base contact in model tests, and the relaxation of prestress with time in field tests. |
| Year | 2003 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. GE-03-01 |
| Type | Dissertation |
| School | School of Civil Engineering |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Geotechnical Engineering (GE) |
| Chairperson(s) | Takemura, Jiro;Miura, Kinya; |
| Examination Committee(s) | Bergado, Dennes T.;Worsak Kanok-Nukulchai;Noppadol Piengwej;Matsumoto, Noppadol; |
| Scholarship Donor(s) | China Scholarship Council, Beijing China; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2003 |