some geotechnical observations for deep excavations … sharp - some geotechnical... · some...
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Some Geotechnical Observations for
Deep Excavations in Weak Rock
Peter Sharp
Mott MacDonald
• Stratigraphy and Strength of Weak
Rock Formations
• Barziman Formation in Dubai
• Dammam Formation in Doha
• In Situ Stress and Stiffness
• Ground Movement Prediction
Scope
Excavation Experiences
Name Location Type Depth (m) Rock Formations
Arabian Canal Trial
Site – 2006 to 2009
Inland Dubai 2Mm3 Open
excavation
35 Gayathri and Barziman Formations
(siliceous and carbonate type
sandstones, siltstones, and mudstones)
Mall of Emirates –
Current
Coastal Dubai Barrette
Foundations
10 Barziman / Gayathri Formation –
Sandstone
Deep shaft – Current Inland Abu Dhabi Deep shaft 30 Fars Formation carbonate mudstones
and gypsum.
Central Doha Sites –
Current
Doha 1
Doha 2
Open excavation
and D wall
support
Currently 5 to 20 Dammam Formation Simisma
limestone, Midra Shale, and
Eocene - Rus Formation - chalky
limestones and siltstones
New Doha
International Airport –
2010 - 2011
Doha coast Permanent D
wall
20 Dammam Formation
Simsima limestone, Midra Shale
Key themes for deep foundation
engineering
• Geological stratigraphy and strength stratigraphy
• In situ stress
• Rock stiffness
• Raft – pile interaction
• Ground movement prediction
• Groundwater is not covered in this presentation
Stratigraphy
Arabian Canal
& MOE– clear
stratigraphy?
Doha
weathering
profile ?
Geological Stratigraphy – Arabian Canal Gamma - gamma ( bulk density) profiling
1 km
Rock Strength and Dry Density
Arabian Canal
Doha
1.0
10.0
100.0
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
UC
S (
MP
a)
Dry density (kg/m3)
RF
MS
SL
Doha
Sites
Rock Strength from Instrumented Drilling
Doha
• Origins in Offshore oil drilling – check on efficiency of drilling
operations relative to rock strength
• Specific Energy = work done to drill unit volume of rock (MJ/m3
or MNm/m3) – Teale 1965
• Monitor bit pull down pressure, rate of penetration, applied
torque, and rpm
• Good summary of technique by Okuchaba – MSc dissertation,
Texas A and M University, 2008
• SE = (vertical work + rotational work + hammer work)/unit
volume
• SE approximately equates to Unconfined Compressive
Strength
Cyclic Parameters - Arabian Canal (1)
Gamma – Gamma profiling of bulk density
02468101214161820
05101520253035404550
Specific Energy (MJ/m3)
Gamma – Gamma wireline logging for
Cyclic Parameters - Arabian Canal (2) 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
05101520253035404550
Sp
ec
ific E
nerg
y (M
J/m
3) S
eis
mic
Ve
loc
ity (m
/s)
Rock Strength – Arabian Canal
Arabian Canal Doha
1
10
05101520253035404550
Specific Energy (MJ/m3)
-30
-20
-10 0
10
20
30
40
0.1
110
100
Un
co
nfin
ed
Co
mp
ress
ive
Stre
ng
th (M
Pa
)Reduced Level (mDMD)
Carb
onate
Unit: C
last D
om
inate
d C
onglo
mera
te
Carb
onate
Unit: M
atrix D
om
inate
d C
onglo
mera
te
Carb
onate
Unit: N
on C
onglo
meritic
Upper C
lastic
Gro
up: C
onglo
mera
te
Upper C
lastic
Gro
up: N
on C
onglo
meritic
Sp
ec
ific E
ne
rgy (M
J/m
3) an
d
UC
S (M
Pa)
Cyclic Parameters - Doha 1 ?
-30
-25
-20
-15
-10
-5
0
5
1000 2000 3000 4000
Ultrasonic Shear Wave Velocity ( m/s)
-30
-25
-20
-15
-10
-5
0
5
1.4 1.9 2.4
Dry Density (Mg/m3)
-30
-25
-20
-15
-10
-5
0
5
0 1 10 100
UCS (MN/m2)
Cyclic Parameters – Doha 2?
0
5
10
15
20
25
1 10 100
Dep
th (
m)
Unconfined Compressiive Strength (MPa) and Specific Energy (MJ/m3)
UCS
80 per. Mov.Avg. (SpecificEnergy(MJ/m3))
Instrumented
drilling
comprising
open hole
drilling with
down the
hole hammer
-5
0
5
10
15
20
1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
Dry Density (Mg/m3)
In situ stress
Doha and Abu Dhabi
-70
-60
-50
-40
-30
-20
-10
0
10
0.0 1.0 2.0 3.0 4.0
Ele
vati
on
(m
QN
HD
)
Ko, K
Simsima Limestone
Midra Shale
Rus Formation
Estimated Ko Profile
K profile from D wallinstallation - Plaxisoutput Ko - Hydrofracture AbuDhabi
Elevated Ko from:
• Overburden removal
• Sea level fluctuations
• Urban groundwater
level rise
• Tectonic stress
Significant Ko reduction
due to D Wall installation
KNDIA = 0.36
In situ horizontal stiffness – Doha 1
10
100
1000
10000
1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01
Yo
un
g's
Mo
du
lus (
MP
a)
Shear Strain (%)
Pressuremeter Young's Modulus (ul-rl)
RUS (HPD) RUS (Lab) MS (HPD) MS (Lab) SL (HPD) SL (Lab)
NDIA
Crosshole
Downhole
Ultransonic
Logging
Doha
Pressuremeter
Doha UCS
tests
In situ horizontal stiffness – Doha 1
10
100
1000
10000
1.E-02 1.E-01 1.E+00 1.E+01
Yo
un
g's
Mo
du
lus (
MP
a)
Shear Strain (%)
Pressuremeter Young's Modulus (ul-rl)
RUS (HPD) RUS (Lab)MS (HPD) MS (Lab)SL (HPD) SL (Lab) 10
100
1000
10000
0.01 0.1 1 10
Yo
un
g's
Mo
du
lus (
MP
a)
Shear Strain (%)
Initial Load
Reload 1
Reload 2
MOE Doha 1
Raft – Pile Load Interaction
Rock Stiffness
(GPa)
Pile Stiffness
(GPa)
% Load Share
Rock Piles
5 30 to 50 90 10
1 30 to 50 70 30
0.5 30 50 50
Doha 2 Site: 2 m thick raft underpinned by 1.5 m
diameter piles on 6 m x 9 m grid carrying
compression load within SF capacity of piles
Ground Movement Prediction (1) • Lack of case history data for comparable
rock types
• NDIA station box – 1.5 m wide diaphragm
wall, L= 25 m, D = 20 within Simsima
Limestone.
• Propped top down construction
• Initial settlement from diaphragm wall
installation of up to 6 mm (4 m offset from
walls).
• Settlement behaviour behind D wall is
trough like.
• PLAXIS back analysis – linear isotropic
stiffness models do not work.
• Link ground movement to lateral wall
movement
Settlement Trough
Ground Movement Prediction (2)
New Doha International Airport
0.00
0.20
0.40
0.60
0.80
1.00
0 5 10 15 20 25 30 35 40 45
Sv/S
hm
ax
Distance from Wall (m)
RATIO OF Sv/Shmax- 2011 - Measurements Vs Plaxis Model
MeasuredPoints-South
MeasuredPoints- North
MeasuredPoints NearBox CornersPlaxis
Ground Movement Prediction (3)
Simple Comparison Model
0.00
0.10
0.20
0.30
0.40
0.50
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Sv/S
hm
ax
X/Z
RATIO OF Sv/Shmax Vs X/Z - Plaxis Linear and Non-Linear Models
Non Linear Parameters
Linear Parameters
Findings • Stratigraphy is important but strength stratigraphy may not match. Expect cyclic variation
in density, strength and stiffness with depth.
• Adopt wider use of instrumented drilling as a strength profiling tool – may prove important
for anchored wall design. Increased value for money and reduced turnaround time for SI.
• Expect strength to correlate closely with in situ dry density and specific energy– profile
density by down-hole geophysical methods and determine m/c profile carefully.
• Expect elevated Ko values but also large reductions in Ko due to installation effects.
• Expect non linear stiffness behaviour - an important aspect in resolving pile – raft load
share interaction. Design territory will be very small strain in the foundations.
• Wall / behind wall movement predictions require non linear elastic models and/or an
empirical basis of trough settlement form linked to wall lateral displacement.
Ground Movement Prediction (4)
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
0.080
0.090
0.100
0 5 10 15 20 25 30 35 40 45 50
dv/Z
Distance from Wall (m)
RATIO OF dv/Z- 2011 Measurements - Vs Plaxis Model
MeasuredPoints - South
MeasuredPoints - North
MeasuredPoints Near BoxCorners
PLaxi
Measured Points - Lower