The joy of modelling practical deep excavations in soft clay using SIGMA/W

Deep excavation in Singapore is challenging as the local geology consists of thick layer of soft clay with occasional lenses of sand and peaty/organic soils coupled with a high ground water table.

Engineers need to predict the retaining wall lateral deflection and the adjacent ground vertical settlement, which has strict limits set by the building control authority. Detailed analysis with GeoStudio or other FEM program is essential for submission to authorities before construction. This presentation describes how GeoStudio was used to model deep excavation problems.

Overview

Video Transcript

[00:00:18.269]
<v ->Thank you, chairman, for the kind introduction.</v>

[00:00:21.470]
Thank you for giving me this opportunity

[00:00:23.850]
to share my experience with using GeoStudio Program.

[00:00:34.640]
I’m from National University of Singapore

[00:00:39.360]
and I would like to take this opportunity

[00:00:41.090]
to share my journey and joy of

[00:00:43.660]
modeling practical deep excavation

[00:00:45.750]
program in soft clay using SIGMA/W.

[00:00:51.290]
Singapore is actually a very highly urbanized city

[00:00:54.700]
with many high-rise building, come with basement

[00:00:58.810]
construction, as well as a massive

[00:01:01.790]
underground train network system.

[00:01:05.230]
6 million people is going to pack,

[00:01:07.710]
is packing into this small, tiny islands

[00:01:10.490]
of which a high-rise building is common.

[00:01:14.139]
Hence deep excavation work become a common

[00:01:17.220]
feature in Singapore geotechnical engineering work.

[00:01:23.060]
We have some challenges in carrying out

[00:01:26.210]
these deep excavation work in Singapore,

[00:01:28.670]
partly due to the local geology,

[00:01:31.120]
which consists of thick layers of softly clay

[00:01:34.220]
with occasional lenses of sand in between them,

[00:01:38.000]
especially in the downtown CBD area

[00:01:41.040]
where a lot of high-rise buildings are located.

[00:01:43.890]
Secondly, we also have very high ground water table,

[00:01:46.780]
typically about half a meter to about 1.5 meter

[00:01:49.900]
below existing ground level, hence,

[00:01:52.610]
any excavation would need to deal with

[00:01:55.760]
groundwater pumping as well as groundwater lowering.

[00:01:59.450]
Thirdly, most of these deep excavation work

[00:02:02.330]
carry out next to existing building,

[00:02:05.300]
hence, the impact of excavation

[00:02:07.227]
on the existing building become a critical issue

[00:02:10.110]
for engineer to reckon with, hence,

[00:02:13.120]
engineer need to predict the retaining wall

[00:02:15.740]
lateral movement, lateral deflection,

[00:02:18.160]
as well as ground vertical settlement

[00:02:20.880]
to the adjacent building, next to the excavation.

[00:02:25.120]
Both of these two parameters are well controlled

[00:02:28.720]
by building control authority, which set

[00:02:31.590]
a very strict allowable limit to both of them.

[00:02:35.220]
Hence detailed analysis using some sort of

[00:02:38.280]
finite element program is therefore a must

[00:02:41.030]
for any deep excavation work to be carried out

[00:02:43.970]
in Singapore, during or before construction,

[00:02:48.230]
as well as during the whole process of construction.

[00:02:51.450]
And I’ve done quite a number of job

[00:02:53.530]
using GeoStudio Program, particularly,

[00:02:56.220]
SIGMA/W, in all the excavation modeling.

[00:03:00.280]
So let me show you a few pictures

[00:03:02.120]
on the city landscape of Singapore,

[00:03:05.790]
which can clearly show some excavation work,

[00:03:08.093]
carried out next to an existing building,

[00:03:10.850]
as well as some MRT station next to downtown of Singapore.

[00:03:16.600]
And this picture, depeat, the clear

[00:03:19.940]
complex geological formation in Singapore,

[00:03:22.540]
again, as present in downtown area,

[00:03:24.800]
where you can see thick layer of soft clay,

[00:03:26.640]
is 30, 40 meter with some lenses of sand in-between.

[00:03:33.440]
This picture showed the crowded underground

[00:03:37.280]
in Singapore full of basement construction,

[00:03:41.040]
some existing tunnels for utility, for water,

[00:03:44.080]
for sewer as well as mass rapid transit,

[00:03:47.970]
existing as well as a lot of future

[00:03:50.850]
planned tunnel that we’re going to work with.

[00:03:55.380]
This just one example in downtown Singapore

[00:03:59.010]
next to the existing Downtown University,

[00:04:01.810]
we have a, we call Bencoolen Street station,

[00:04:04.970]
of which you can see four, almost up to

[00:04:07.100]
four levels excavation is to be constructed

[00:04:10.440]
or has been constructed to the depth of

[00:04:12.780]
almost 43 meter, right?

[00:04:15.520]
Cut through layers of soft clay, sand lenses,

[00:04:18.340]
et cetera, et cetera, with the strict,

[00:04:20.880]
very stringent defamation requirement of the retaining wall.

[00:04:26.030]
This picture, depicted two concerns,

[00:04:27.750]
I’ve just mentioned, one is the lateral defamation.

[00:04:32.290]
And number two of course is a settlement

[00:04:35.040]
next to excavation without proper

[00:04:38.320]
numerical modeling and control,

[00:04:40.350]
you’ll find that both or either of this

[00:04:42.880]
parameter would not be acceptable

[00:04:45.040]
by the authority and create problem

[00:04:47.420]
to the adjacent building.

[00:04:49.410]
Hence, FEM modeling is a must and SIGMA/W

[00:04:54.150]
actually fit into this very well.

[00:04:57.580]
Some key consideration needed in conducting

[00:05:00.340]
this finite element modeling for deep

[00:05:02.360]
excavation problems, at least seven of them,

[00:05:06.040]
eight of them, to share with the audience here.

[00:05:08.480]
Number one, the FEM modeling should be able

[00:05:11.060]
to model the soft clay behavior adequately.

[00:05:13.950]
Since the thick layer of soft clay,

[00:05:15.390]
it become a critical issue.

[00:05:17.390]
Number two, we have to able to conduct

[00:05:19.720]
a coupled consolidation type of analysis.

[00:05:22.380]
As excavation work may carry out over many months

[00:05:27.370]
of which poor water pressure, this pressure

[00:05:29.750]
will take place especially because we have the,

[00:05:32.950]
we have the constantly pumped on water

[00:05:34.760]
within excavation site.

[00:05:37.010]
Point number three is, we have to analyze

[00:05:39.180]
both total stress as well as effective stress

[00:05:41.710]
analysis for long-term stability.

[00:05:44.860]
Number four, able to model the realistic drainage

[00:05:47.730]
condition of the retaining wall.

[00:05:50.090]
No retaining walls are perfectly watertight

[00:05:53.760]
in a practical sense, hence, a slight leakage

[00:05:58.357]
in retaining wall may occur or has

[00:06:01.700]
occurred in a number of cases,

[00:06:03.310]
especially when the sand lenses is in in-between

[00:06:05.400]
two soft clay, hence, able to model this

[00:06:08.600]
real risky, very important in finite element modeling.

[00:06:11.530]
Number five, a couple with that is to model

[00:06:14.540]
the interface property between the retaining wall,

[00:06:16.920]
which is cheaper wall or diaphragm wall,

[00:06:19.800]
as well as adjacent interface.

[00:06:22.500]
Number six, able to conduct sensitivity study easily.

[00:06:26.690]
Number seven, able to model the complex

[00:06:29.280]
excavation stages, which include obviously

[00:06:32.150]
in-situ stages, with initial stress

[00:06:35.460]
followed by insertion of retaining structures,

[00:06:38.160]
followed by excavation, inserting,

[00:06:40.520]
excavation again, inserting, excavation,

[00:06:42.810]
inserting, et cetera, et cetera,

[00:06:44.070]
in between you also do groundwater lowering.

[00:06:46.710]
So these complex excavation stages must be able

[00:06:50.050]
to clearly include that in your final modeling

[00:06:53.160]
and hopefully easily edit it as you move along.

[00:06:57.960]
Of course, finally is able to analyze

[00:07:00.090]
one strut failure for scenario

[00:07:02.590]
for contingency analysis.

[00:07:05.950]
And over the years I’ve used SIGMA/W,

[00:07:09.140]
to perform, and I found that all the seven,

[00:07:12.109]
eight points stated above are well,

[00:07:14.730]
are easily handled by SIGMA/W

[00:07:17.350]
very nicely and very neatly.

[00:07:19.720]
Then let us, let me share some example

[00:07:21.880]
of this beautiful features of SIGMA/W

[00:07:23.950]
in solving this particular deep excavation,

[00:07:26.350]
hopefully we’ll make this challenging

[00:07:28.310]
technical challenge become a very joyful experience.

[00:07:33.700]
Number one, able to model soft clay behavior

[00:07:36.670]
adequately, SIGMA/W, equipped with many common

[00:07:41.360]
soil models and the latest version of SIGMA/W

[00:07:44.486]
which is 2021, has some new model called

[00:07:47.900]
Mohr-Coulomb Hardening or Softening model

[00:07:51.430]
that is very useful for soft clay.

[00:07:53.490]
And we have been using this lately

[00:07:55.700]
and found it very useful.

[00:07:58.570]
Number two, the special feature called

[00:08:01.170]
Analysis Tree in SIGMA/W, in fact that appear

[00:08:04.390]
in all the GeoStudio Programs, are very

[00:08:06.900]
useful, especially for complex problem like

[00:08:09.840]
deep excavation and dimension that include

[00:08:12.110]
many, many stages from initial stressors

[00:08:14.740]
to surcharging, to excavation, to strut,

[00:08:17.330]
et cetera, et cetera, which very useful,

[00:08:19.460]
and it’s very clear, for example,

[00:08:21.540]
what I illustrated here, is the Initial Stress Stage.

[00:08:24.780]
You can choose Gravity Activation turn-on stage

[00:08:27.830]
or Ko Procedure state, for this particular steps,

[00:08:30.690]
which is very clear and very easy to adjust.

[00:08:35.160]
Of course, coupled with this, is a nice

[00:08:37.781]
graph, graphics, user interface, GUI,

[00:08:41.130]
which indicate very clearly, for example,

[00:08:43.960]
in this particular slide, I’m showing you,

[00:08:46.410]
when you do the In-situ stress analysis, the,

[00:08:50.193]
the soil element will be shown with a hatched,

[00:08:54.560]
meaning that is, the unit weight is applying,

[00:08:58.010]
gravity turn-ons is applying, while as in other

[00:09:00.590]
masses, you will not be hatched, right?

[00:09:03.410]
Point number three is able to model

[00:09:05.710]
the coupled consolidation process.

[00:09:07.880]
What I described here is to explain that

[00:09:11.150]
during the excavation work in soft clay

[00:09:13.610]
involve unloading of soil, as well as groundwater lowering.

[00:09:18.160]
These two all costs us a reduction on changes

[00:09:21.280]
of poor water pressure during this process.

[00:09:23.500]
Hence, you need to have a coupled consolidation.

[00:09:26.610]
And SIGMA, traditionally coupled consolidation

[00:09:29.260]
can be solved by Biot type of analysis.

[00:09:31.600]
And I think SIGMA/W, very nicely, cleverly.

[00:09:36.130]
Coupled, the stress strain relationship

[00:09:39.200]
with the continuity equations, and of course,

[00:09:41.657]
the continued equation here is typical

[00:09:44.700]
of just your type of a familiar program

[00:09:47.900]
that incorporate the unsaturated or

[00:09:50.760]
partially saturated, soil mechanics,

[00:09:53.360]
which make this program very interesting.

[00:09:59.180]
To select this coupled consolidation,

[00:10:02.000]
actually is quite easy, you just choose

[00:10:04.160]
under the analysis type here, choosing

[00:10:06.400]
an acid, a consolidation type, with this option,

[00:10:10.210]
the flow equation and stress-free equation

[00:10:13.180]
will be automatically coupled in the course of analysis.

[00:10:17.650]
Beautiful feature number four,

[00:10:19.430]
is the interaction of soil and retaining wall.

[00:10:22.190]
Can we easily molded with many types

[00:10:24.820]
of interface element, as well as shown

[00:10:27.140]
on the diagram here, you can indicate,

[00:10:29.080]
the blue lines here indicate the interface

[00:10:31.190]
on two sides of the retaining wall

[00:10:33.720]
and select the interface element here,

[00:10:35.970]
and there are whole range of models for you to choose.

[00:10:42.320]
Not only the soil models, in type of strength

[00:10:45.030]
that can be chosen, many models available,

[00:10:47.560]
but also the hydraulic condition

[00:10:49.760]
for this interface element, which indicate

[00:10:53.110]
the leakage features if there is

[00:10:57.890]
of the retaining structures.

[00:10:59.320]
So hence you can choose a hydraulic feature here

[00:11:02.140]
and you can choose either you deal with saturated

[00:11:05.440]
or unsaturated, of course, if it’s a deep excavation,

[00:11:08.540]
most of the time will be saturated,

[00:11:10.110]
but near to the top, it can also be

[00:11:11.870]
a little bit unsaturated.

[00:11:15.340]
Beautiful feature number six is actually,

[00:11:17.300]
you can define all sort of function,

[00:11:20.670]
in type of stress function, in term of

[00:11:23.440]
all the hydraulic conductive function.

[00:11:24.892]
using a beautiful feature here that can define

[00:11:29.330]
all the function instead of a constant value,

[00:11:32.260]
with respect to a stress level, with respect to time steps.

[00:11:38.760]
And beautiful feature number seven,

[00:11:40.950]
is a very beautiful and very engineering styled

[00:11:44.445]
of input and output, which I personally find it

[00:11:47.510]
very useful for engineers like us,

[00:11:50.820]
which you can see what we input with a very clear

[00:11:54.740]
graphical interface and beautiful color combination.

[00:11:59.060]
Internal input, as well as a result,

[00:12:00.920]
this slide showing the results they can see,

[00:12:03.030]
you can see a deformed shape very easily, all right,

[00:12:06.930]
by looking at the, the movement of each

[00:12:08.930]
and every note, or you can move by even

[00:12:12.230]
including the color of the material layer

[00:12:15.270]
into the, into the graphical interface.

[00:12:18.160]
You can see clearly which portion

[00:12:19.890]
of the retaining wall has been deformed more severely.

[00:12:22.900]
And of course, this picture illustrate

[00:12:24.930]
the easiness in selecting the groundwater table

[00:12:29.820]
as the excavation process progresses step by step.

[00:12:35.630]
And this slide show one of the typical

[00:12:38.480]
or important result that engineer

[00:12:40.540]
always have to look at it, which is

[00:12:42.030]
a lateral defamation of retaining wall

[00:12:43.910]
at different time, all right,

[00:12:45.650]
versus as well as the bending moment

[00:12:47.340]
of the retaining wall at different time

[00:12:49.450]
and very engineering style of analysis of

[00:12:52.970]
results that make engineer interpretation,

[00:12:55.320]
as well as your report submission, very easy and nice.

[00:13:00.330]
And of course this result is what the engineer

[00:13:02.610]
we’re dealing with, and I hope this result

[00:13:05.020]
will also be useful for people who wanting

[00:13:07.610]
to analyze similar type of problem in the region.

[00:13:11.090]
So in summary, I’d like to say that

[00:13:13.270]
there are many advantages and I highlight

[00:13:15.740]
at least five of them here for by using SIGMA/W

[00:13:19.650]
for modeling deep excavation problem,

[00:13:22.280]
especially in soft clay area.

[00:13:25.070]
Number one, well coupling of seepage flow

[00:13:27.610]
and stress-deformation formulation

[00:13:29.610]
that make the whole analysis technically sound.

[00:13:32.390]
Number two, the availability of many suitable

[00:13:36.470]
soil models and number three, ease of setting up

[00:13:39.940]
the complex stages of excavation,

[00:13:42.670]
including changing water table step-by-step

[00:13:46.430]
that is very important and very useful.

[00:13:49.600]
Beautiful feature number four,

[00:13:52.230]
good interface element allow the realistic

[00:13:55.690]
modeling of the stressors, stress transfer

[00:13:59.840]
as well as the drainage condition

[00:14:01.870]
at the retaining structures.

[00:14:04.560]
Last but not least, they’re well-designed

[00:14:06.810]
and very user-friendly GeoStudio

[00:14:08.910]
type of graphical user interface

[00:14:11.420]
will make your modeling talent,

[00:14:14.090]
become a joyful experience.

[00:14:16.450]
And I hope this sharing will help

[00:14:19.730]
engineers designers in the region

[00:14:22.190]
or around the world, that have similar

[00:14:23.840]
type of problem, deep excavation in soft clay,

[00:14:26.690]
as well as other geo-technical problem.

[00:14:28.570]
We’ll find SIGMA/W experience useful.

[00:14:32.120]
With that, I thank you.