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PLAXIS Numerical Analysis

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Eng. Bushra

on 29 March 2014

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Transcript of PLAXIS Numerical Analysis

Terzaghi’s Theory of Consolidation:

When a load is applied over a saturated compressible soil at the ground surface:
Numerical Analysis of Road Embankment Consolidation Settlement via PLAXIS Software Program
CASE
STUDY

OBJECTIVES
PLAXIS Results
CONCLUSION
Consolidation
INTRODUCTION
Often, consolidation process of soft cohesive soil results in excessive long-term settlement problems.
Therefore, Ground Improvement works are needed to be performed to control the excessive settlement so as to prevent any future risks.
Ground Improvement
Through
Pre-loading
method:

The consolidation will occur before the construction of the proposed structure which then will be termed as Pre-Consolidation.
Ground improvement:

Is a technique that introduces soil’s treatment methods in order to achieve the required and suitable ground conditions for new construction projects.
One of the ground improvement techniques used specially for soft cohesive soil is
Pre-Loading
.
Preloading Technique:

Is performed through applying an extra temporary load, greater than the post-construction load, such that more settlement, than the expected post-construction settlement, is reached.
Preloading
Consequences of
Direct-Loading
and
Pre-loading
Consolidation process occurs very slowly due to the low permeability of soft soil that prevents the escape of pore water from the soil voids.
Using preloading alone as a ground improvement method increases the pore water pressure of a compressible soil however, the settlement can take significant time, as a consequence of slow water dissipation and the extreme drainage thickness.
PVD
Prefabricated Vertical Drain, PVD, is recommended to be used to accelerate the consolidation process.
Soil consolidation using PVD in addition to preloading can accelerate the settlement rate hence, reduces consolidation times from years to months.
PVD facilitates the dewatering process through allowing water to dissipate horizontally to the nearest PVD as well as vertically to the sand filter layers.
PVD is a prefabricated material consists of a plastic core covered by synthetic geotextile.
Are economical
PLAXIS Software Program
PLAXIS is the main geotechnical finite element software, specifically developed for the 2D and 3D geotechnical analysis of deformation, stability and groundwater flow.
Boubyan Island is largely formed from soft compressible deposits which are typically soft to very soft clay covering up to 26 m depth.

Ground improvement by Preloading with PVD was the chosen treatment method in Boubyan Island Seaport Project.
Boubyan Seaport Project Phase (1) involves the construction of a dual 3-lane carriageway highway and the construction of a parallel twin line railway. Both the highway and the railway run west to east across the island, leading to the proposed port location on the east coast of Boubyan Island.
Design Consideration of Bobyan Island Seaport Project
Designed Construction Stages
Stage I:

Involves the construction of 4.2m of embankment.
Then the soft clay is subjected to consolidation for a period as calculated based on time to 90% primary consolidation.
Stage II:

Involves the construction of 3m of surcharge fill.
Then, the soft clay is subjected to consolidation for a period as calculated based on time to 90% primary consolidation.
Design Expectations
The final primary settlement is
1.561m
The used value for the compression index (Cc) was
0.297
.
The expected 90% consolidation settlement value is
1.304m
which will be achieved after
368.4 days
using the PVD.

Provide higher permeability through artificial drainage paths,
And can easily be installed at close spacing.
The main objective of this study is to validate and calibrate the design results of the preloading method adopted in Boubyan Island's soft clay by applying numerical analysis via PLAXIS V8 software.
To emphasize the effectiveness of preloading and PVD methods.
Curves
Excess pore water pressure vs. Time
Trial (1)
Excess pore water pressure vs. Time
Trial (2)
Settlement Variation vs. Time
Trial (2)
PVD reduces the increase in excess pore water pressure by
55.2%
.

The use of PVD in Model (II) reduced the time required to reach minimum excess pore water pressure by
65.5%
.
Settlement Variation vs. Time
Trial (1)
Model (II)
417 Days
Model (I)
500 Days
Final Primary Settlement of Trial (1)
0.28m
Model (I)
400 Days
Model (II)
300 Days
Final Primary Settlement of Trial (2)
Model (I)
366 Days
Model (II)
361 Days
90% Consolidation Settlement
Model (I)
239 Days
Model (II)
225 Days
90% Consolidation Settlement
0.25m
0.25m
0.28m
Comparison
90%
Consolidation Settlement
Final Primary Settlement
Back Calculation
With an approximately more accurate Cc value estimation, PLAXIS settlement values will be within 18% and the time duration within 19.9% of the estimated one. Probably, the reason behind this small difference is owned to the assumed soil properties.
PLAXIS V8 software shows clearly the effectiveness of preloading plus PVD as a ground improvement method for soft clay soil through confirming the following:
The expected increase in the excess pore water pressure during the un-drained construction phases was reduced to an average of 43.45% due to PVD.
The waiting time during the drained periods to reach the minimum excess pore water pressure have been reduced by 67.6% owing to PVD.
PVD increases the overall settlement by an average of 13% and reduces the time interval of 90% consolidation by an average of 65%.
The manually calculated results as per the design report of MPW were too high compared to the numerical results due to the high Cc value which was estimated experimentally.
The study had shown that the PLAXIS software program is a feasible program that can be used to model consolidation with PVD.
Height of embankment is 4.2m with side slope of 1V:6H.

Height of a surcharge preloading is 3m.
Embankment Geometry
PVD reduces the increase in excess pore water pressure by
55.3%
.

The use of PVD in Model (II) reduced the time required to reach minimum excess pore water pressure by
69.6%
.
PLAXIS software gave settlement values that are less than the designed one by an average of 81.45%. Additionally, the time intervals obtained from PLAXIS program were also less than the designed values by an average of 32.45%.
Gratitude
Dr.Waleed Abullah
Dr. Thamer Alyaqoub
Eng. Anath
Dr. Mohammad Bou-Shahri
Dr.Mohamad Al-Khalidi
Full transcript