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