الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 137 |  |  |
| | | from | 137 | | |
Abstract
In this thesis, two advanced constitutive models, the Soft Soil model (SSM), the Soft Soil Creep model (SSCM), have been tested and evaluated by simulating two field tests, test embankment constructed to failure on soft marine clay in Malaysia, and Haarajoki test embankment on soft clays with and without PVDs.
Malaysian test embankment has been taken as a reference to study the validation of the used constitutive models to simulate the short term loading i.e. the construction stage until failure. Soft Soil Creep model (SSCM) has proven to be quite accurate in predicting the settlement-heave profile of the tested embankment along its construction stages. On another hand, the Soft Soil model was quite accurate in predicting the settlement along the embankment profile for the very early stage but failed to predict the settlement-heave profile of the embankment at the intermediate and final stage. i.e. immediately after the failure. Consequently, the constitutive model, Soft Soil Creep model (SSCM), has been proven to have the capability of modeling an embankment of soft soil during its construction stage and could predict the failure of the embankment.
Haarajoki test embankment has been used to test the constitutive models on the long term with and without PVDs. i.e. predicting the settlement of the embankment for a long time after the construction of the embankment. The Soft Soil Creep model (SSCM) has been proven to give a quite accurate prediction for the settlement along the time. On another hand, the Soft Soil model (SSM) underestimates the settlement of the embankment.
Also, this case study has been utilized to validate a proper technique for 2D finite element modeling. In which, different techniques have been tested. This techniques proposed by (Hird, et al., 1992); (Indraratna & Redana, 1997); (Chai & Miura, 2001). The approaches proposed by (Hird, et al., 1992) and (Indraratna & Redana, 1997) are quite identical and pretty close to the field monitored data. This approaches slightly underestimates the settlement along the time, maybe a sensitivity analysis needs to be initiated to make the results of these two approaches complied with the field monitored data. While, (Chai & Miura, 2001) approach slightly overestimates the settlement along the time.
The Soft Soil Creep model shows good capability in predicting the settlement of the embankment either in the short term condition or in the long term condition, with or without the presence of PVDs.
Moreover, a parametric study has been initiated in order to find the optimum length of the PVDs. i.e. It is possible to leave a layer adjacent to the bottom drainage boundary without prefabricated vertical drain (PVD) improvement and achieve approximately the same degree of consolidation as a fully penetrated case. In this study, a case study of Haarajoki test embankment improved with PVDs was taken as a reference. The study shows that the effective depth of the PVDs is depending on PVDs spacing, as the spacing of the PVDs increases the optimum length will decreases. Owing to the fact that, the optimum length of the PVDs for spacing 1m is 11m and the optimum length of the PVDs for spacing 2m is 8m.
Also, the study of optimum length of PVDs has been applied on Port-Said soil profile with its extended clay up to 50m. First of all, the critical length of the embankment without any improvement has been tested and found to be 2.5m. this critical height has been taken as a reference to the successive studies.
Thereafter, the optimum length of the PVDs found to be 25 and 20 meters for PVDs spacing 1m and 2m, respectively.
Those studies of the optimum length of the PVDs show that the optimum length of the PVDs at it best results can’t exceed about 50% of the clay layer. Which means that the long PVDs is not effective in the extended clay layers. Also, show that PVDs with proper spacing and length have a huge effect on reducing consolidation time. On the other hand, Using PVDs may be more efficient at clay thicknesses less than that in the case studies. i.e. At Haarajoki soil profile, The time reduction to reach 90% consolidation using PVD length 11m and spacing 1m is 86.4%, which is a huge value, but still, the time taken to reach 90% consolidation is about 15.8 year which is not a great achievement. While at Port-Said case study, The time reduction to reach 90% consolidation using PVD length 25m and spacing 1m is 70.27% in 3.53 years which is better than Haarajoki’s case. But still, not the best result.
The PVDs have been proven to have a very slight impact on embankment’s stability during construction stages. This impact may increase as the construction time duration increases. The impact of PVDs on stability has been measured for a construction rate of 1m per every 6 days. Which is considered a fast construction rate. Also, the effect of replacement has been studied and proven to give a very positive impact on embankment’s stability during the construction stage, as the replacement thickness and width outside the edge of the embankment increases the factor of safety against side slope failure will increase. Immediately after construction, a height of 5m embankment could be reached only with replacement.
Moreover, a hybrid solution, as a combination of soil replacement and berm, has been studied and proven to give a huge positive effect on embankment’s stability. This hybrid solution is more complicated than the replacement only case. In which, the berm height is a very sensitive factor affecting the embankment’s stability. where the relationship between the berm’s height and overall stability is not a linear relationship. i.e. the overall stability usually increases then decreases as the berm height increases. While the increase of berm and replacement width will increase the stability until it won’t be effective.
Finally, a hybrid solution, as a combination of soil replacement, berm, and PVDs, should be utilized to construct a stable embankment during the construction, immediately after construction, final consolidation stages. Soil replacement and berm are responsible for embankment’s stability. While PVDs are mainly responsible for reducing consolidation time and may help at construction stage stability. But the construction rate has to be slow to give a significant effect.
Further research should be carried out to study the optimum length of PVDs for different single drained and double drained clay layer thicknesses. At Haarajoki case study, a 3D analysis should be carried out to study the embankment with its improved and unimproved zones together. A replacement with different shear strength parameters should be tested and study its effect of embankments stability. A study should be carried out to check the effect of larger berm heights. A parametric study should be initiated to study the effect of changing soil parameters