Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/261179216

NumericalModelingtoReduceBaghdadSoilSubsidenceandSettlementonlocationsofSewagesystems.AustralianJournalofCivilEngineering,Vol.6,No.1,2010.

ARTICLE·JUNE2010

DOWNLOADS

17

VIEWS

35

1AUTHOR:

AqeelAL-Adili

UniversityofTechnology,Iraq

33PUBLICATIONS9CITATIONS

SEEPROFILE

Availablefrom:AqeelAL-Adili

Retrievedon:14September2015

Numerical Modeling to Reduce Baghdad Soil

Subsidence and Settlement on locations of Sewage

systems

Al-ADILI, AQEEL1, N. SIVAKUGAN

2 , and GANG REN

3

1 Asst.prof, Department of Building and Construction Engineering, Uni. of Technology,

Baghdad, IRAQ. E-mail; aqeeladili@hotmail.com

2 Head, Civil & Environmental Engineering Department, James Cook University,

Townsville, Qld 4811, Australia, Telephone: +61 7 47814431, Telefax: +61 7 47816788,

Email: siva.sivakugan@jcu.edu.au

3 Lecturer, School of Civil, Environmental, and Chemical Engineering, RMIT University,

Melbourne, Vic.3001, Australia, Tel. +61 3 99252409, E-mail; gang.ren@rmit,edu.au

ABSTRACT

Sewage related subsidence is a serious problem in Baghdad, and it represents severe

damages and disturbances to services. The soils of Baghdad show a wide range of

variation in grain size distribution. The aim of this paper is to study strategies to prevent

and/or minimize soil subsidence phenomena affecting the sewerage and road systems in

Baghdad city. This was achieved through simulation of soil profiles with appropriate

properties to a depth up to 12 m at nine different locations by using PLAXIS, a finite

element 2-D modeling software. The subsidence and resulting settlements were reduced

by providing a geosynthetic-reinforced granular fill placed over a soft soil deposit, as

well as through grouting techniques. A simulation of these two types of ground

treatments suggests that the gorund improvement techniques have been quite effective in

reducing the settlement.

By using 1-layer of geotextile reinforcement with stiffness of 5 kN/m acceptable

and noticeable subsidence and settlement reduction of 12-80% were achieved for most of

the studied locations. Grouting was modeled as volume change in soft soils and weakness

cluster, and it showed reasonable results with settlement reduction from 0.5 mm to 0.1

mm at the 9 different locations.

2

It would appear that the particular simulation chosen for the calculation of this

problem model fulfils the two important treatment methods which are common with the

FE simulations.

Key words;

Soil subsidence, Grouting, PLAXIS modelling, Soil Reinforcement, Geotextile, finite

element, Interface, Slippage.

1. INTRODUCTION:

Baghdad city is located in the Mesopotamian alluvial plain. The general altitude ranges

between 30.5 and 34.85 m.a.s.l. The Tigris River divides the city into a right (Karkh)

and left (Risafa) sections. The area is characterized by arid to semi arid climate with dry

hot summers and cold winters; the mean annual rainfall is about 150 mm. The area is

underlain by quaternary alluvial deposits. The soil is relatively saline as a result of high

gypsum and salt contents of the parent formations from which the sediments were

derived and the dry climate and long periods of cultivation. Chlorides and sulfates are

the predominant types of salts.

The aim of the present work is to study the prevention and reduction of soil

subsidence phenomena affecting the sewerage and road systems in Baghdad city. This

was achieved through the study of soil sections and properties up to a depth up to 12 m at

nine different locations. It should be noted here that the city of Baghdad may be regarded

as an example of a modern city built on alluvial plain deposits and its subsidence

problems may be encountered in other cities with similar conditions.

2. SOIL PROPERTIES OF STUDY AREA

The soil of Baghdad showed a wide range of variation in grain size distribution. The top

soil generally consists of fill material with variable thickness ranging from 0.5 m to 4 m.

This is generally underlain by clay, silty clay or clayey silt, with sand lenses in places.

At depths of more than 7-8 m, the sediments are generally sandy (Al-Adili, 1998). This

study has shown that at 23 of the 33 locations studied (representing 70% of the cases),

subsidence took place within the silty clay and clayey silt layers; at 4 of the locations

(12.5%) subsidence took place within the silty sand layers. On the other hand, the

3

sewerage pipes of 69% of the areas which show no subsidence are laid within clayey

silty sand.

However, some of geotechnical properties of lean soils lead to subsidence due to low

cohesions and internal friction with high voids or pores, as well as low compressive

strength for carrying the loads coming from the sewage pipes and the self weight of the

soil.

3. SOIL SUBSIDENCE PHENOMENA IN BAGHDAD SOILS

Subsidence is a serious problem in Baghdad soils at various locations of sewage systems

and it has caused a wide range of damages and disturbance to services (Figure-1). Al-

Adili (1998) concluded in his research that the main reasons for subsidence in Baghdad

soil are one or more of the following: soil liquefaction, concrete pipes corrosion,

dissolution of gypseous soil, swelling of clay, presence of loose soil, and the break down

of joints of pipes within soft soil singularity or in combination. The soft soil within the

pipes depth (or beneath pipes) with the presence (and fluctuation) of ground water table

are considered in this paper, along with the infrastructures that were simulated here as

applied load.

The numerical modeling tool used in this research is the 2-D finite element

special purpose computer package PLAXIS-8 (Brinkgrev and Vermeer, 1998), which was

used to determine the displacements of surface soils, as well as the settlement of adjacent

foundation. To minimize the boundary effect, the vertical boundary at the far end, on the

right-hand side, is set 12 m away from the centre of loading (two and half of foundation

width), that are assumed to be free in the vertical direction and restricted in horizontal

direction. The bottom horizontal boundary is restricted in both the vertical and horizontal

directions against displacements.

The plane strain analysis based on Mohr–Coulomb material model was carried

out to simulate the behavior of soils as a continuum, with undrained conditions, to model

the soil conditions at field, and to assess the settlements, as well as, the elastic – plastic

deformation, without taking the decay of excess pore pressures with time into account.

Furthermore, ground water flow in this porous medium could be described by

Darcy’s law, assumed to be steady flow, and the interface elements are treated specially

4

in ground water calculations in PLAXIS ver-8. When the element is activated there is full

coupling of the pore pressure degrees of freedom.

Four different materials are involved in the analysis: soft soils, granular soil, fill

material, and geosynthetic reinforcements. However, all the materials are assumed to be

non linear. Figure-2, shows the typical model for the calculation which represents one of

the nine locations studied in this research (S1), which also happens to be the worst case.

Table-1 summarizes the details pertaining to the nine locations (studied locations) that

include the soil layer thinknesses, water table location, depth and diameter of the pipe,

and the horizontal distances between the centre lines of the footing and the pipe. Table-2

shows the soil properties that were derived from laboratory tests which were used as the

input parameters in the Plaxis models.

4. REDUCING SUBSIDENCE SIMULATION

There are different procedures to prevent or reduce soil subsidence and settlement of

adjacent foundation, depending on the causes of subsidence. Among these measures, soil

reinforcement (Geotextile) and grouting are two effective methods in reducing the ground

subsidence effects.

4.1. Influence of Geotextile (geogrid) on subsidence:

Geotextile has potential application beneath footings to improve bearing capacity by

spreading the loaded area thereby reducing the contact pressures within the soil. The

interaction of the fabric, the dimensions of which are large relative to soil grains, and soil

effectively increases the angle of internal friction and cohesion (Bowels, 1988). Soil may

be strengthened by reinforcement which is placed in the direction of tensile strain so that

deformation in the soil generates tensile force in reinforcement (Jewell, 1988). Some

studies on single layer reinforced system with finite element modeling to solve such

problems are also reported in the literature (Love et al., 1987; Poran et al., 1989; Yin,

1997, 2000). Yin (1997) idealized the soft soil by a series of springs in contrast to the

present approach where it is modeled as a 2-D continuum. In this approach the reinforced

fill soil bed is treated to be an elastic half-space. With reinforced soil structures, the loads

in the reinforcement are transferred to the surrounding soil by friction at the interface.

Soil reinforcement procedure has been used to simulate the reinforcement lying

5

mid-way between the foundation and the sewage pipes, to model the construction of

superstructure as built after the pipes network were made (Figure-2), in purpose of

reducing subsidence of the soil strata above pipes as well as settlement of foundation.

A single layer of geotextile has been used for all studied locations. The geosynthetic

reinforcement layer represented by stretched elastic membranes was assumed to be rough

enough to prevent slippage at the interface with soil.

4.2. Influence of Grouting on subsidence:

Grouting is a general term means inserting special materials (e.g. stabilizing agent) in soil

body to reduce permeability, voids and pores size, and increasing soil strength.

Simulation of soil grouting in this model have been carried out by increasing volumetric

strain for particular soil cluster. However, in the field there are different materials used in

grouting such as sand-cement, cement-bentonite, silica jel, and sodium silicate. Each type

is suitable for grouting in a specific soil type. Al-Adili (1998) studied some of those

materials either for treatment and prevents soil subsidence and/or reoccurrences of soil

subsidence by reducing the soil compressibility and increase soft soil strength. The

injected material tends to travel along more permeable layers or along planes of

weakness, often emerging at a considerable distance from point of injection. Thus,

precaution on sewers or subsurface structures in the vicinity of injected soil strata

locations, due to pressure of injection which may cause displacement. From the first

author’s experience, pressure during injection should not exceed 6 bar, especially in the

vicinity of substructures and not large enough to lift the ground surface.

Different grout percentages have been examined into soft soil cluster where the

pipes that sufffered subsidence lie in, Grouting in this model has been carried out to

simulate actual structures where the subsidence occurred adjacent to the sewerage pipe

locations.

5. DISCUSSION OF THE RESULTS :

5-1. Results for reinforcement of soil with geotextile;

Reinforcement was used in this research to provide short term stability over soft

foundation soils, to maintain equilibrium until consolidation can occur in the soft

foundation soils. Reinforcement has been simulated as a horizontal layer because the

6

tensile strain is typically developed in a horizontal direction when the major loading is

applied. The results showed different results of vertical displacement reduction from 1%

to 80% depending on many variables such as pipe diameter, pipe depth, soil types and

thickness of each stratum, horizontal distance from pipe to foundations, and geogrid

stiffness.

As a result of using geotextile, the total and vertical displacements have been

reduced both under the foundation and around the pipe in the reinforced soil (Figure-

3,A&B and Figure 4-A&B). With the presence of reinforcements, major parts of the

shear stress zones are taken up by the geosynthetic layers. Figure-5 showed that total

strain in soft and granular soil has lower magnitude when geotextile is present. In

contrast, the plastic locations in this model, which represent the failure points or regions

in soil clusters, were also reduced when geotextile is present (Figure-6). This figure also

showed that the tension cut-off points do not occur in reinforcement soil, especially

above pipe location. Among the benefits of using geotextile is reducing foundation

settlement, differential settlement and bending moments especially if the horizontal

distance of the foundations or structures from pipes location is increasing (Figure-7). The

benefits obtained from the reinforced soil results from the generation of frictional stresses

at soil-reinforcement interface. Due to these stresses the confining pressure increases

which restricts lateral strains in the soil. As a result, higher lateral stresses are induced in

the reinforced case as compared to the unreinforced case (Figure-8).

Geogrid with stiffness of 5 kN/m conduct the best results in reducing the soil

displacements for all locations (table-3). The model showed that for a particular

geotextile, settlement has been reduced with increase in the pipe diameter (Fig.9).

Moreover, this model revealed that there is a reverse relationship between the depth of

the sewerage pipe and settlement when using geotextile with stiffness 5 kN/m as shown

in Figure-10. From this model, no significant relationships relating geogrid depth and

ground water table depth with subsidence or settlement reduction were observed. This

could be attributed to fact that the geotextile is above the phreatic surface and the major

parts of the shear stresses are taken up by the geosynthetic layers (Deb et al., 2007). Thus,

the presence of the reinforcements causes a reduction in the outwardly acting shear

stresses leading to better performance of the foundation under the superimposed load.

7

5.2. Results of grouting in soft soil:

As with regards to grouting simulation, the preferable percentages of the injection

materials to increasing the volume are 0.65% to 0.87% with volume change of

0.960m3/m to 1.205m

3/m respectively, depending on different location properties such as;

pipe dimension, pipe depth, and horizontal distance between pipes and adjacent

foundation. Figure-11 shows the reduction in settlement with increasing volume change

until when the settlement of about 0.1mm is reached (for the typical location S1).

These percentages of grouting are sufficient to form a number of grout piers over

the weakness area, and the foundation then be in form of a stiff raft designed to span

across the piers. This procedure will ensure no lifting or pushing of the ground surface or

foundation, and reduce the cost since it is not necessary any more to fill completely all

cavities or voids beneath the substructures (Tomlinson, 1986). This model revealed also,

that permeable water bearing soils (in location S2 and S4) contain about 30% to 35%

void spaces, required to be treated using grout curtain of 2.25 to 2.5m thick. Table-4

shows the least settlement of each location (after grouting simulations) with preferable

(most suitable) volumes change.

6. CONCLUSIONS

The present study demonstrates a successful application of PLAXIS in analyzing the

response of a geosynthetic-reinforced granular fill placed over a soft soil deposit, as well

as volume change by grouting techniques. Modeling of those two types of ground

treatment for subsidence has provided suitable and rational results to prevent and/or

reduce this phenomenon of subsidence.

By using 1-layer geotextile reinforcement with stiffness of 5 kN/m, the finite

element model shows significant subsidence and settlement reduction for most of the

studied locations. The reductions in settlement are in the range of 12-80%, with

maximum reduction taking place at location S2. The percentage reduction of settlement

and subsidence depends on some parameters such as pipe depth, pipe diameter, and soil

cluster and properties. Geogrid stiffness of 5 kN/m provided the best results for soil

displacement reduction, and no significant relationships relating the geogrid depth and

W.T. depth with reduction in subsidence or settlement were observed.

8

One can consider some additional benefits from the application of a more

powerful finite element method, e.g. not only an overall distribution of the vertical

displacements but also of the horizontal displacements can be obtained (see Fig.8).

Simulation of volume change as grouting in soft soils and weakness clusters

shows a reasonable result for subsidence and settlement reduction from 0.1 mm up to 0.5

mm at the 9 different locations with grouting injection carried out in the vicinity of

substructures.

However, this study showed that the particular techniques adopted in the

simulations and calculations for this problem are adequate for modelling the two ground

improvement techniques, using geosynthetics and grouting.

REFERENCES

Al-Adili, A.SH.(1998). Geotechnical Evaluation of Baghdad soil Subsidence and Their

Treatments, Ph.D. thesis, Univ. of Baghdad , 142p.

Brinkgreve, R. B. T., Vermeer, P. A. (1998), ‘PLAXIS- Finite Element Code for

‘soil and Rocks Analysis’. Version 7 and 8,A. A. Balkema-Rotterdam-Brookfield

Bowles, J. (1988) , Foundation Analysis and Design , 4th.ed., MeGraw-Hill inc.,

Newyork , 659 p.

Deb, K., Sivakugan, N., Chandra, S., and Basudhar, P. K. (2007). Numerical analysis

of multi layer geosynthetic-reinforced granular bed over soft fill, Geotechnical

and Geological engineering , Springer ,The Netherlands,(in press).

Jewell,R.A.(1988). The Mechanics of Reinforced Embankment on Soft Soils, Geotextile

and Geomembranes ,Elsevier,Voi.7, No.3. pp 34-45.

Love, J.P., Burd, H.J., Milligan, G.W.E. and Houlsby, G.T. (1987). Analytical and model

studies of reinforcement of a layer of granular fill on soft clay subgrade,

Canadian Geotecnical Journal, 24, 611-622.

Poran, C.J., Herrmann, L.R. and Romstad, K.M. (1989). Finite element analysis of

footing on geogrid-reinforced soil. Proceeding of geosynthetics, USA,231-242.

Tomlinson, M. (1988) , Foundation Design and Construction, Pitman Publ., UK, 450 p.

Yin, J. H. (1997). Modeling geosynthetic-reinforced granular fills over soft soil,

Geosynthetics International, 4(2), 165-185.

9

Yin, J. H. (2000). Comparative modeling study on reinforced beam on elastic foundation,

J. Geotech. and Envir. Eng., ASCE, 126(3), 265-271.

Table-1: Description of each study location for subsidence model .

Location Soil profile Pipe

diameter

(m)

Pipe

depth

invert (m)

Distance between

foundation and pipe

(m)

W.T.

(m)

S1 Fill= 2m

Soft soil =7m

Sandy soil=3m

1.5 6.5 3 -2.5

S2 Fill=4m

Silty clay=5m

Silty sand=4m

3 12 5 -4

S3 Fill=1m

Silty clay=4m

Clayey silty sand=6m

1.4 8 7 -2.5

S4 Fill=1m

Silty clay=7m

Clayey silty sand=3m

1.6 9 8 -1.5

S5 Fill=2m

Clayey silt=2m

Sandy clayey silt=4m

1.6 6 7 -2

S6 Fill=3.5m

Clay=5m

Clayey sand=2.5m

2 7 5 -2.5

S7 Fill=1m

Clay=2.5m

Clayey silt=3.5m

Sandy silt=2

Clayey silty sand=3m

1.4 5 8 -2

S8 Fill=2m

Silty clay=3m

1.5 6.5 7 -2.25

10

Clayey sandy silt=3m

S9 Fill=2

Clayey silt=4m

Silty sandy clay=2m

Silty sand=3m

1.0 4 8 -2.5

Table-2 : Soil properties as input data in the typical model(lab test results) .

Parameters Granular soil Soft soil Fill material

Unit weight dry(kN/m3) 18 16 16

Unit weight saturated(kN/m3) 20 18 20

Permeability-h (m/day) 0.5 0.0001 1

Permeability-v(m/day) 0.5 0.0001 1

Young modulus(kPa) 10000 2000 4000

Poisson’s ratio 0.33 0.3 0.3

Cohesions (kN/m2) 1 5 1

(degree) Friction angle 30 25 30

Table-3: Settlement reducing with different geotextile stiffness

Geotextile stiffness kN/m Settlement (mm)

No geogrid used 38.93

5 18.85

50 19.7

1000 20.9

5000 22.2

Table-4: The least subsidence after grouting simulation due to volume change (increasing

volume).

Locations Least subsidence (mm) Volume change(m3/m)

S1 0.1 0.960

S2 0.5 1.205

S3 0.3 0.964

S4 0.4 1.002

S5 0.1 0.974

S6 0.2 1.205

S7 0.35 1.197

S8 0. 3 1.108

S9 0.2 0.960

11

Figure-1: Photos of some subsidence locations on the roads and sewage locations in

Baghdad city.

12

Figure-2:Typical Model of the soil section and sewerage pipe in PLAXIS

calculations(not to scale).

7 m

2 m

B = 5m

Foundation=15 kPa

Soft soil (clayey soil)

One-layers Geosynthetic reinforcements 1m below G.S.

L1=3m

Fill soil

3 m Granular soil

Load weight-=15 kN/m2

Sewerage pipe

W.T.

L2=12m

13

-A-

14

-B-

Figure-3: Total displacement due to pipe and structure exist for soil without

reinforcement (geotextile)(A) and with reinforcement (geotextile) (B).

15

-A- -B-

Figure-4: Vertical displacement for soil without geotextile (A) and with geotextile (B).

16

-A- -B-

Figure-5: Total strain for soil without geotextile(A) and with geotextile(B)

-A- -B-

Figure-6: Plastic point for soil without geotextile(A) and with geotextile(B) .

17

-A- -B-

Figure-7: Foundation settlement without soil reinforcement(A) and on soil reinforcement

(B).

18

-A-

.

19

-B-

Figure-8: horizontal displacements as shading intensity for subsidence model without

reinforcement (A) and with reinforcement (B)

y = 5E-10x3.2071

R2 = 0.9552

0

10

20

30

40

50

60

70

80

90

900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900 3100

Pipe diametre (mm)

Se

ttle

me

nt

red

uc

tio

n %

20

Figure-9: Settlement reduction percentage vs. pipe diameter by using geogrid stiffness

5kN/m.

y = 0.0273x3.2227

R2 = 0.9938

0

10

20

30

40

50

60

70

80

90

3 4 5 6 7 8 9 10 11 12 13

Pipe depth (m)

Se

ttle

me

nt

red

ucti

on

%

Figure- 10: Pipe depth vs. settlement reduction percentages by using geogrid stiffness

5kN/m

.

y = 25.761x2 - 50.208x + 25.516

R2 = 0.9344

0

5

10

15

20

25

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Volume chang (m3/m)

Se

ttle

me

nt

(mm

)

Figure-11: Settlement –volume change profile due to grouting simulation in location S1.