Engineering Successful Dewatering in Arid Urban Areas: Abu ... · Engineering Successful Dewatering...
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Abu Dhabi City CenterRecently, the Abu Dhabi government
developed a geodatabase by digitizing,
processing and qualifying an extensive
amount of existing information (i.e., more
than 26,000 borehole logs). Because of the
evaluation and processing of the geodata-
base, detailed maps of geotechnical and
hydrogeological surface features were
generated using 3D modeling. Ultimately,
the geodatabase was used to develop risk
maps indicating geohazards, such as
sabhka (or salt flats), uncontrolled fill and
cavities. This information identifies the
location and extent of the areas vulnerable
to surface subsidence.
Dewatering can change pore water
pressure, which, in turn, alters the effective
vertical stress in a soil mass and can affect
groundwater chemistry. Changes in pore
water pressure and groundwater chemistry
can significantly affect soil and rock. The
potential effects of dewatering include:
• Change in effective stresses and shear
strengths in soils
• Increase in seepage velocities and pressures
• Erosion or transport of soil particles and
piping
SPECIAL :ISSUE
DEWATERING AND GROUNDWATER CONTROL
FEATURE ARTICLE
DEEP FOUNDATIONS • MAY/JUNE 2017 • 99
As the capital of the United Arab Emirates
(UAE), Abu Dhabi is one of the world’s
fastest-developing cities. To sustain its
significant growth rate, Abu Dhabi must
surmount challenging engineering problems
in the most efficient manner. One of the
challenges facing Abu Dhabi is ground
settlement caused by poor groundwater
control practices in inherently difficult
subsurface conditions. For construction
projects in Abu Dhabi located near the
Arabian Gulf, groundwater is frequently
encountered and must be accounted for and
addressed during design and construction.
Engineering Successful Dewatering in Arid Urban Areas: Abu Dhabi Dewatering Guidelines
AUTHORSM. Melih Demirkan and Juan J. Gutierrez, RIZZO Associates, and Raghav Ramanathan, Langan Engineering & Environmental Services
Ground failure and settlement problems in Abu Dhabi City
Abu Dhabi City Center
100 • DEEP FOUNDATIONS • MAY/JUNE 2017 DEEP FOUNDATIONS • MAY/JUNE 2017 • 101
water leve l s can be drawn down
considerably and, thus, can create relatively
steep hydraulic gradients, increase
groundwater seepage velocities, and
increase hydraulic uplift pressures.
As a part of the preconstruction
planning for a dewatering project, potential
permeable zones should be identified
based on borehole logging, monitoring of
piezometers and wells, and geophysical
measurements. Depending on the project
Tool drops and loss of water circulation
have also been described in the calcarenite
layers. Water loss is also commonly
associated with highly-permeable soils
(e.g., silty sand with gravel/shells, and
gravels). Settlement has been observed in
certain areas after the water level drops due
to the intense dewatering occurring during
construction activities, especially in those
areas in which more permeable soils (e.g.,
gravel, sandy gravel and sand bars) are
present. Dissolution of salt crystals in the
fill material also contributes to settlement
in some areas of the ADM.
Analyzing and interpreting potenti-
ometric surfaces indicated that ground-
water elevations in the western and
northcentral regions of Abu Dhabi are
relatively flat, with elevations ranging from
approximately El. -49 ft to El. -33 ft MSL
(El. -15 m to El. -10 m MSL). The shallow
groundwater system is recharged primarily
from the ground surface via precipitation,
irrigation and stormwater runoff detention
ponds. A portion of the groundwater flow is
discharged horizontally into the Arabian
Gulf. However, a large portion of ground-
water discharge can be attributed to
evaporation to the atmosphere, especially in
sabkhas and low elevation areas, where the
surface of the groundwater table is very
close to the ground surface. According to
available literature, a water budget was
estimated for the UAE coastal sabkhas
based on data collected at two locations
along the UAE coast. The calculations
revealed that the average annual precipita-
tion for Abu Dhabi is approximately 3.5 in
(90 mm), and the average annual recharge
to the sabkha groundwater is approximately
1.75 in (45 mm). The unconfined aquifer
ranges about from 0 ft to 79 ft (0 m to 24 m)
in thickness. In general, greater saturated
thicknesses occur in the southcentral region
of Abu Dhabi, within the aforementioned
core geotechnical hazard area.
Approximately 1,000 different hy-
draulic conductivity field tests performed in
the Abu Dhabi area were collected from
various engineering reports. This data
revealed that sands and gravels in the
overburden and sandstones, calcarenites,
and conglomerates in the bedrock forma-
tions generally have the highest hydraulic
conductivity values. Lower values of
hydraulic conductivity were generally
found in the finer grained sediments,
gypsum, claystone, and mudstones.
However, there is considerable variation
and overlap within and between lithologic
types. Natural hydraulic gradients within
the surficial aquifer are generally low.
However, when large-scale construction
dewatering programs are implemented,
• Settlement
• Collapse of subsurface cavities or voids
• Transport of groundwater containing
contaminants
• Dissolution of soluble materials, such
as rock salt or gypsum
These aforementioned effects can signifi-
cantly affect the area being dewatered and
the structures located within the zone of
dewatering influence. Several cases of
ground failure and extensive settlements in
Abu Dhabi have been attributed to nearby
uncontrolled dewatering activities or poor
construction practices. Ground failures and
associated settlements that exceed about 5 ft
(1.5 m) have occurred, especially in some
residential sections of Abu Dhabi. To
minimize potential subsidence/collapse
problems related to dewatering, construc-
tion companies and dewatering contractors
need to be informed about the risks
associated with dewatering and possible
mitigation measures to reduce risks.
U n d e r t h e o w n e r s h i p o f t h e
Municipality of Abu Dhabi City (ADM), a
set of guidelines was developed that
standardize dewatering practices to
s u p p o r t t h e d e w a t e r i n g p e r m i t
applications. These dewatering guidelines
provide various dewatering project
examples (suggested complete approach)
to support dewatering applications in the
city of Abu Dhabi. Special emphasis is
given to control and to eliminate potential
subsidence/collapse problems related to
dewatering projects in the ADM area. The
guidelines are designed to support
designers, contractors, consultants and
third-party reviewers regarding the
technical aspects of the dewatering project
according to three main phases in a
dewatering project: (1) field investigation,
(2) dewatering design and (3) monitoring
program. The requirements addressed in
these guidelines are scalable to the
magnitude of the dewatering project.
Subsurface ConditionsSince much of the coastal strip has been
reclaimed or developed, most surfaces in
the ADM are covered with a variable
amount of made ground. In the coastal
strip, the made ground is often composed
o f ca rbona te s and dredged f rom
neighboring lagoons. Typically, Quaternary
sediments, which are deposited in the
coastal areas, underlie the made ground.
Landward of the coastal deposits is mostly
aeolian sand, and fluvial sand and gravel
deposits. In much of the ADM area, there
has been extensive carbonate-evaporitic
sabkha development, which consists of
loose, silty, fine carbonate sands, and where
cementation increases with depth. Sabkha
is characteristically found in low-lying
areas, which are prone to flooding during
high spring tides. Stratigraphically, below
the Quaternary sediments are rock layers,
which are composed of sandstones in the
southern areas; dolomitic conglomerates,
sandstones and siltstones in the northern
areas; and dolomites and limestones along
with evaporitic mudstone and siltstone in
the eastern portions of Abu Dhabi. The top
of bedrock elevations within the ADM area
typically range from about El. -66 ft to El.
-328 ft (El. -20 m to El. -100 m) mean sea
level (MSL).
During various geotechnical investi-
gations in Abu Dhabi, voids have been
identified by tool drops in fractured
calcareous mudstones and siltstones with
gypsum inclusions, calcarenite and sands,
between or above massive gypsum layers,
and, in what is referred to as, the
weathered/fractured top of rock. Intensive
dewatering has also been interpreted as
having the potential for either increasing
the size of pre-existing voids in the
subsurface or creating voids through the
removal of the fine particles commonly
found in the voids. Irrigation of gardens
and inland farmland exacerbates the
p rob l em by inc rea s ing the l oca l
groundwater head. This situation, in
conjunction with construction-related
dewatering within the urban area, is
changing the hydraulic gradient, thereby
creating one of the key triggers for
sinkhole development. Sabhka distribution in Abu Dhabi
Thickness of overburden (unconsolidated sediments and fill)
100 • DEEP FOUNDATIONS • MAY/JUNE 2017 DEEP FOUNDATIONS • MAY/JUNE 2017 • 101
water leve l s can be drawn down
considerably and, thus, can create relatively
steep hydraulic gradients, increase
groundwater seepage velocities, and
increase hydraulic uplift pressures.
As a part of the preconstruction
planning for a dewatering project, potential
permeable zones should be identified
based on borehole logging, monitoring of
piezometers and wells, and geophysical
measurements. Depending on the project
Tool drops and loss of water circulation
have also been described in the calcarenite
layers. Water loss is also commonly
associated with highly-permeable soils
(e.g., silty sand with gravel/shells, and
gravels). Settlement has been observed in
certain areas after the water level drops due
to the intense dewatering occurring during
construction activities, especially in those
areas in which more permeable soils (e.g.,
gravel, sandy gravel and sand bars) are
present. Dissolution of salt crystals in the
fill material also contributes to settlement
in some areas of the ADM.
Analyzing and interpreting potenti-
ometric surfaces indicated that ground-
water elevations in the western and
northcentral regions of Abu Dhabi are
relatively flat, with elevations ranging from
approximately El. -49 ft to El. -33 ft MSL
(El. -15 m to El. -10 m MSL). The shallow
groundwater system is recharged primarily
from the ground surface via precipitation,
irrigation and stormwater runoff detention
ponds. A portion of the groundwater flow is
discharged horizontally into the Arabian
Gulf. However, a large portion of ground-
water discharge can be attributed to
evaporation to the atmosphere, especially in
sabkhas and low elevation areas, where the
surface of the groundwater table is very
close to the ground surface. According to
available literature, a water budget was
estimated for the UAE coastal sabkhas
based on data collected at two locations
along the UAE coast. The calculations
revealed that the average annual precipita-
tion for Abu Dhabi is approximately 3.5 in
(90 mm), and the average annual recharge
to the sabkha groundwater is approximately
1.75 in (45 mm). The unconfined aquifer
ranges about from 0 ft to 79 ft (0 m to 24 m)
in thickness. In general, greater saturated
thicknesses occur in the southcentral region
of Abu Dhabi, within the aforementioned
core geotechnical hazard area.
Approximately 1,000 different hy-
draulic conductivity field tests performed in
the Abu Dhabi area were collected from
various engineering reports. This data
revealed that sands and gravels in the
overburden and sandstones, calcarenites,
and conglomerates in the bedrock forma-
tions generally have the highest hydraulic
conductivity values. Lower values of
hydraulic conductivity were generally
found in the finer grained sediments,
gypsum, claystone, and mudstones.
However, there is considerable variation
and overlap within and between lithologic
types. Natural hydraulic gradients within
the surficial aquifer are generally low.
However, when large-scale construction
dewatering programs are implemented,
• Settlement
• Collapse of subsurface cavities or voids
• Transport of groundwater containing
contaminants
• Dissolution of soluble materials, such
as rock salt or gypsum
These aforementioned effects can signifi-
cantly affect the area being dewatered and
the structures located within the zone of
dewatering influence. Several cases of
ground failure and extensive settlements in
Abu Dhabi have been attributed to nearby
uncontrolled dewatering activities or poor
construction practices. Ground failures and
associated settlements that exceed about 5 ft
(1.5 m) have occurred, especially in some
residential sections of Abu Dhabi. To
minimize potential subsidence/collapse
problems related to dewatering, construc-
tion companies and dewatering contractors
need to be informed about the risks
associated with dewatering and possible
mitigation measures to reduce risks.
U n d e r t h e o w n e r s h i p o f t h e
Municipality of Abu Dhabi City (ADM), a
set of guidelines was developed that
standardize dewatering practices to
s u p p o r t t h e d e w a t e r i n g p e r m i t
applications. These dewatering guidelines
provide various dewatering project
examples (suggested complete approach)
to support dewatering applications in the
city of Abu Dhabi. Special emphasis is
given to control and to eliminate potential
subsidence/collapse problems related to
dewatering projects in the ADM area. The
guidelines are designed to support
designers, contractors, consultants and
third-party reviewers regarding the
technical aspects of the dewatering project
according to three main phases in a
dewatering project: (1) field investigation,
(2) dewatering design and (3) monitoring
program. The requirements addressed in
these guidelines are scalable to the
magnitude of the dewatering project.
Subsurface ConditionsSince much of the coastal strip has been
reclaimed or developed, most surfaces in
the ADM are covered with a variable
amount of made ground. In the coastal
strip, the made ground is often composed
o f ca rbona te s and dredged f rom
neighboring lagoons. Typically, Quaternary
sediments, which are deposited in the
coastal areas, underlie the made ground.
Landward of the coastal deposits is mostly
aeolian sand, and fluvial sand and gravel
deposits. In much of the ADM area, there
has been extensive carbonate-evaporitic
sabkha development, which consists of
loose, silty, fine carbonate sands, and where
cementation increases with depth. Sabkha
is characteristically found in low-lying
areas, which are prone to flooding during
high spring tides. Stratigraphically, below
the Quaternary sediments are rock layers,
which are composed of sandstones in the
southern areas; dolomitic conglomerates,
sandstones and siltstones in the northern
areas; and dolomites and limestones along
with evaporitic mudstone and siltstone in
the eastern portions of Abu Dhabi. The top
of bedrock elevations within the ADM area
typically range from about El. -66 ft to El.
-328 ft (El. -20 m to El. -100 m) mean sea
level (MSL).
During various geotechnical investi-
gations in Abu Dhabi, voids have been
identified by tool drops in fractured
calcareous mudstones and siltstones with
gypsum inclusions, calcarenite and sands,
between or above massive gypsum layers,
and, in what is referred to as, the
weathered/fractured top of rock. Intensive
dewatering has also been interpreted as
having the potential for either increasing
the size of pre-existing voids in the
subsurface or creating voids through the
removal of the fine particles commonly
found in the voids. Irrigation of gardens
and inland farmland exacerbates the
p rob l em by inc rea s ing the l oca l
groundwater head. This situation, in
conjunction with construction-related
dewatering within the urban area, is
changing the hydraulic gradient, thereby
creating one of the key triggers for
sinkhole development. Sabhka distribution in Abu Dhabi
Thickness of overburden (unconsolidated sediments and fill)
requirements, additional field tests (e.g.,
borehole seepage and pumping tests) may
be performed to evaluate the quantity of the
water likely to be encountered during
dewatering operations. In the analysis of
any dewatering system, the source of
seepage must be determined, and the
boundaries and seepage flow charac-
teristics of geologic and soil formations at
and adjacent to the site must be generalized
into a form that can be analyzed. For
example, the source of seepage can be
modeled as a line or a circle; the aquifer as a
homogeneous, isotropic formation of
uniform thickness; and the dewatering
system as one or two parallel lines or circles
of wells or well points.
According to the recommendations in
the dewatering practices guidelines, the
scope of the geotechnical investigation and
laboratory testing for a project should
include an evaluation of potential zones of
cavity collapse. The formation of cavities
involves natural processes of erosion or
gradual removal of slightly soluble bedrock
(e.g., limestone, gypsum or rock salt) by the
percolation of water, collapse of a cave roof
or lowering of the groundwater table. The
geotechnical investigation should identify
the existence of the collapsible or soluble
materials. If such materials exist, which is
likely in Abu Dhabi, the dewatering system
and excavation should be designed to
prevent the collapse of potential cavities,
which could be influenced by the
dewatering operations.
There is a potential risk that dewatering
may result in settlement. Compressible
soils and loose granular soils have the
potential to compress and consolidate
when the groundwater table is lowered and
the effective stress is increased. In such
cases, settlement analyses are necessary to
predict negative effects on nearby
structures and then to plan measures to
mitigate such effects. The dewatering
guidelines provide the recommended
102 • DEEP FOUNDATIONS • MAY/JUNE 2017
Key - dewatering design scope
Hazard zones and dewatering design types
scope of dewatering analysis and design
based on the level of hazard of the project,
the proximity of sensitive structures to the
dewatering site, the depth of excavation,
and the type of dewatering and excavation
techniques selected.
R a p i d d r a w d o w n o r f r e q u e n t
fluctuation of the groundwater level or loss
of fine-grained soil due to dewatering
causes serious settlement and even
structural damage to nearby structures.
These issues are amplified if natural ground
has soluble zones (e.g., halite or salt layers)
or if manmade fill is not placed and
compacted properly. Consequently, the
ADM scrutinizes each dewatering permit
application, and, therefore, adequate time
should be reserved in the construction
schedule for dewatering permit application
and approval.
The ADM has a state-of-the-art,
internet-based dewatering permit appli-
cation system as a part of its construction
permitting system. The dewatering applica-
tions are divided into two main categories:
(1) building projects of any size and (2)
infrastructure projects, such as roads,
pipelines, trenches and any kind of
excavation that will require discharge of
water, which may change the groundwater
regime. It is not uncommon that the
installation of instrumentation to monitor
discharge of the water may be requested
during the review process.
AcknowledgementsThe authors appreciate the support of the
ADM Town Planning Sector and the ADM
Spatial Data Division, and the cooperation
of all agencies that provided assistance and
contributed data to complete this study.
M. Melih Demirkan, Ph.D., P.E., is
principal of geodynamics at RIZZO Associates,
which provides site characterization for new
and existing nuclear power plants, engineering
analysis for dam and water resources
structures, and regional risk assessments for
geohazards.
Juan Gutierrez, Ph.D., P.E., is a senior
engineer with RIZZO Associates. His experi-
ence includes soil-structure interaction, soil
retaining structures and slope stability, water
seepage, rock mechanics, and numerical
methods applied to geotechnical problems
along with various software programs.
Raghav S. Ramanathan is senior staff
engineer with Langan Engineering & Environ-
mental Services. His experience includes
geohazard mapping, GIS, data and database
management, and nuclear power plant site
assessments including settlement, slope
stability and liquefaction analysis.
DEEP FOUNDATIONS • MAY/JUNE 2017 • 103
HazardZone
Proximity of
Structures
Excavation/ Dewatering
Type Excavation
Depth
Pumping CapacityAnalysis
SettlementAnalysis
Third Party
Review
C
1,2
i Shallow a I
i, ii, iiiMedium a IDeep c I X
3
i Shallow a I
i, ii, iiiMedium a IDeep b I
B
1,2
i Shallow a I
i, ii, iiiMedium b I XDeep c II X
3
i Shallow a I
i, ii, iiiMedium a IDeep b II
A
1,2
iShallow
c I X ii, iii c I X
i, ii, iiiMedium c II X Deep c II X
3
iShallow
a Iii, iii a I
i Medium
b I X ii, iii b I X
i Deep
c II X ii, iii c II X
Geologic/Hydrogeologic Hazard Zone
A High Potential
B Medium Potential
C Low Potential
Proximity of Structures
1 Sensitive or Large Structures Nearby
2 Structures Could be Impacted by Project
3 No structures that could be impacted
Excavation Depth
Shallow 0-3 m
Medium 3 m-10 m
Deep >10 m
Excavation/Dewatering Type
i Open Cut (Sumps and Open Pumping)
ii Cutoff Structure
iii Wells and Ejectors
Pumping Capacity Analysis
a Analytical Solution
b Flow Net
c Numerical Analysis
Settlement Analysis I Hand Calculation
II Numerical Analysis
Third-Party Review X Third-Party Review Required
Third-Party Review Not Required
requirements, additional field tests (e.g.,
borehole seepage and pumping tests) may
be performed to evaluate the quantity of the
water likely to be encountered during
dewatering operations. In the analysis of
any dewatering system, the source of
seepage must be determined, and the
boundaries and seepage flow charac-
teristics of geologic and soil formations at
and adjacent to the site must be generalized
into a form that can be analyzed. For
example, the source of seepage can be
modeled as a line or a circle; the aquifer as a
homogeneous, isotropic formation of
uniform thickness; and the dewatering
system as one or two parallel lines or circles
of wells or well points.
According to the recommendations in
the dewatering practices guidelines, the
scope of the geotechnical investigation and
laboratory testing for a project should
include an evaluation of potential zones of
cavity collapse. The formation of cavities
involves natural processes of erosion or
gradual removal of slightly soluble bedrock
(e.g., limestone, gypsum or rock salt) by the
percolation of water, collapse of a cave roof
or lowering of the groundwater table. The
geotechnical investigation should identify
the existence of the collapsible or soluble
materials. If such materials exist, which is
likely in Abu Dhabi, the dewatering system
and excavation should be designed to
prevent the collapse of potential cavities,
which could be influenced by the
dewatering operations.
There is a potential risk that dewatering
may result in settlement. Compressible
soils and loose granular soils have the
potential to compress and consolidate
when the groundwater table is lowered and
the effective stress is increased. In such
cases, settlement analyses are necessary to
predict negative effects on nearby
structures and then to plan measures to
mitigate such effects. The dewatering
guidelines provide the recommended
102 • DEEP FOUNDATIONS • MAY/JUNE 2017
Key - dewatering design scope
Hazard zones and dewatering design types
scope of dewatering analysis and design
based on the level of hazard of the project,
the proximity of sensitive structures to the
dewatering site, the depth of excavation,
and the type of dewatering and excavation
techniques selected.
R a p i d d r a w d o w n o r f r e q u e n t
fluctuation of the groundwater level or loss
of fine-grained soil due to dewatering
causes serious settlement and even
structural damage to nearby structures.
These issues are amplified if natural ground
has soluble zones (e.g., halite or salt layers)
or if manmade fill is not placed and
compacted properly. Consequently, the
ADM scrutinizes each dewatering permit
application, and, therefore, adequate time
should be reserved in the construction
schedule for dewatering permit application
and approval.
The ADM has a state-of-the-art,
internet-based dewatering permit appli-
cation system as a part of its construction
permitting system. The dewatering applica-
tions are divided into two main categories:
(1) building projects of any size and (2)
infrastructure projects, such as roads,
pipelines, trenches and any kind of
excavation that will require discharge of
water, which may change the groundwater
regime. It is not uncommon that the
installation of instrumentation to monitor
discharge of the water may be requested
during the review process.
AcknowledgementsThe authors appreciate the support of the
ADM Town Planning Sector and the ADM
Spatial Data Division, and the cooperation
of all agencies that provided assistance and
contributed data to complete this study.
M. Melih Demirkan, Ph.D., P.E., is
principal of geodynamics at RIZZO Associates,
which provides site characterization for new
and existing nuclear power plants, engineering
analysis for dam and water resources
structures, and regional risk assessments for
geohazards.
Juan Gutierrez, Ph.D., P.E., is a senior
engineer with RIZZO Associates. His experi-
ence includes soil-structure interaction, soil
retaining structures and slope stability, water
seepage, rock mechanics, and numerical
methods applied to geotechnical problems
along with various software programs.
Raghav S. Ramanathan is senior staff
engineer with Langan Engineering & Environ-
mental Services. His experience includes
geohazard mapping, GIS, data and database
management, and nuclear power plant site
assessments including settlement, slope
stability and liquefaction analysis.
DEEP FOUNDATIONS • MAY/JUNE 2017 • 103
HazardZone
Proximity of
Structures
Excavation/ Dewatering
Type Excavation
Depth
Pumping CapacityAnalysis
SettlementAnalysis
Third Party
Review
C
1,2
i Shallow a I
i, ii, iiiMedium a IDeep c I X
3
i Shallow a I
i, ii, iiiMedium a IDeep b I
B
1,2
i Shallow a I
i, ii, iiiMedium b I XDeep c II X
3
i Shallow a I
i, ii, iiiMedium a IDeep b II
A
1,2
iShallow
c I X ii, iii c I X
i, ii, iiiMedium c II X Deep c II X
3
iShallow
a Iii, iii a I
i Medium
b I X ii, iii b I X
i Deep
c II X ii, iii c II X
Geologic/Hydrogeologic Hazard Zone
A High Potential
B Medium Potential
C Low Potential
Proximity of Structures
1 Sensitive or Large Structures Nearby
2 Structures Could be Impacted by Project
3 No structures that could be impacted
Excavation Depth
Shallow 0-3 m
Medium 3 m-10 m
Deep >10 m
Excavation/Dewatering Type
i Open Cut (Sumps and Open Pumping)
ii Cutoff Structure
iii Wells and Ejectors
Pumping Capacity Analysis
a Analytical Solution
b Flow Net
c Numerical Analysis
Settlement Analysis I Hand Calculation
II Numerical Analysis
Third-Party Review X Third-Party Review Required
Third-Party Review Not Required