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