THE EFFECT OF GEOTECHNICAL PROPERTIES ON THE BEARING CAPACITY OF SELECTED SOILS IN AL - NAJAF GOVERNORATE-IRAQ

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 1, January 2017, pp. 222–233, Article ID: IJCIET_08_01_023

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THE EFFECT OF GEOTECHNICAL PROPERTIES ON

THE BEARING CAPACITY OF SELECTED SOILS IN

AL - NAJAF GOVERNORATE-IRAQ

Kamal R. Mauff, Muhsen O. Khalif, Rand S. Al-Salami and Amer A. Lefta

Department of Applied Geology, College of Science,

University of Babylon, Iraq

ABSTRACT

Study of the characteristics of the physical, chemical and engineering of the soil is considered

as an important matter in the processes of engineering projects (such as highways, dams, bridges,

etc..). Study was done at selected locations in the governorate of Al-Najaf by drilling three

boreholes with 10m depth, for disturbed (DS) and undisturbed (US) samples, to determine soil

characteristics, and the level of groundwater depth in the study area because of their effect on the

design of foundations. The laboratory and field tests showed that the soil is clayey high plasticity

(CH) in most of the study area, while the chemical analysis of the water in the boreholes has a high

concentration of SO4 (1031-1037) mg/l and PH values range from (7.7-8.0). The number of blows

in the standard penetration (SPT) test was between (58-86) blows. The depth of groundwater was

(0.5-0.9) m in the boreholes. The bearing capacity using the dynamic method was (21.45–31.35) T

/m² for all boreholes, while the bearing capacity using the static method for depths from (1-3) m

ranged from (9.82-14.20) T /m². The study concluded that this soil needs some engineering

treatments before establishing the engineering structures.

Key words: Soil Mechanics, Bearing Capacity, AL-Najaf-Iraq.

Cite this Article: Kamal R. Mauff, Muhsen O. Khalif, Rand S. Al-Salami and Amer A. Lefta. The

Effect of geotechnical Properties on the Bearing Capacity of Selected Soils in Al - Najaf

Governorate-Iraq. International Journal of Civil Engineering and Technology, 8(1), 2017, pp. 222–

233. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=1

_________________________________________________________________________________

1. INTRODUCTION

Soil geotechnical evaluation is very important and essential in civil engineering and dependent on the

physical, chemical and engineering characteristics employment for surface layers and subsurface that are

affected by the stresses from the loads imposed on it, (Terzaghi, 1996).Geotechnical study is expanding to

include exploring the site and study of ground water and its relationship with soil to understand soil

behavior in the case of certain structures on it.

Geotechnical assessment includes the study of geometric properties, the stress influenced on the soil as

consolidation, resistance and compressibility. The chemical contents of soil and water affect the physical

and engineering properties (Bowles, 1979).All geotechnical investigations can be obtained from field

works at the engineering structure in addition to test the samples to find suitable drilling method (Fang,

The Effect of geotechnical Properties on the Bearing Capacity of Selected Soils in Al - Najaf Governorate-Iraq


2005).The bearing capacity of soil is to select the extent of dynamic and static loads without failure. The

soil compressibility characteristics study is very important when establishing any engineering structure.

The bearing capacity of soil is calculated from the standard penetration test (SPT) and core penetration

test (CPT), or depend on engineering and physical properties obtained from test results for the samples in

the laboratory , (Terzaghi,1996).The bearing capacity depends on several factors such as soil quality,

cohesion soil (fine-grained), cohesionless soil (coarse grained), and groundwater in addition to soil

saturation condition, and wet and dry density (Freeze, 1979)By knowing the bearing capacity for the soil,

the type of foundations and its depths can be gussed, as well as improving soil properties to take the

necessary precautions to the damages from earthquakes (Santamarina, 2001).

2. SITE LOCATION AND DESCRIPTION

The site is located (163Km) southern of Baghdad is part of Quaternary sediments. The site in general is flat

area. The location of the boreholes was set-up by the client. The boreholes coordinates are as shown in

Table(1), and site plan for Six boreholes location is shown in Fig.(1) .

Table 1 boreholes coordination

Figure 1 The satellite picture of boreholes



3. GEOLOGY AND GEOMORPHOLOGY OF THE STUDY AREA

Quaternary old alluvium unit is observed at the area. Obtained units as the result of boreholes in the field

and laboratory tests are given below.

• greenish ,grayish , soft to medium to stiff , silty sandy clay , sandy silty clay (CL,CH)

• grayish , greenish , silty sand (SW,SP) with clayey and silty (SC,SM) loose to medium to dense to very

dense .

The reason of the variation at this unit’s thickness is variation of water movement directions at old

riverbeds. These types of riverbeds are called as Meanders.

The most obvious topographic indication for the presence of a growing subsurface anticline, in the

Mesopotamia Plain is that of Samarra subsurface anticline. The area involved is covered by Quaternary

sediments (Sissakian, 2000), but the presence of the subsurface anticline is proved by geophysical studies

(C.E.S.A., 1992 and Al-Kadhimi et al., 1996), besides the morphology of the area that indicates clearly a

double plunging anticline. The Tigris River has abandoned channels in different places within the

Mesopotamia Plain. The main one is between Al-Euphrates River and the current river channel (Sissakian,

2000). This abandoned channel is either the old course of the Tigris River or that of Al-Euphrates River.

The authors believe that the growing of the subsurface anticlines in the area was the main factor for

abandoning of the river its original channel. Many authors (Al-Sakini, 1993; Mello et al., 1999;

Bhattacharya et al., 2005 and Philip and Virdi, 2007) recorded such cases. The Euphrates River has also

abandoned its channel, between Al-Ashraf Al-Najaf and Al-Ashraf Al-Najaf cities; it is south of the

current river course (Sissakian, 2000). The authors believe that the main reason for abandoning of the

channel is the activity of the Abu Jir Fault Zone. The activity of this fault is proved by Fouad (2007).

4. METHOD OF STUDY

All tests are performed according to ASTM and B.S standards.

4.1. Field Work

4.1.1. Drilling and Sampling

Three boreholes have been bored during December -2016 by using mechanical machine type “Flight

Auger” drill method , The method of drilling was carried out according to the standard of the American

Society for Testing and Materials (ASTM D-1452 –D5783) which are used for taking the samples. The

depth of boring were selected by the Client to extend to underneath the zone of influence of significant

foundation pressure to materials that were relatively incompressible. The depth of boring was 20 m from

the existing Natural Ground Surface (N.G.S) . Three types of sample were taken;

4.1.2. In site Testing

4.1.2.1. In Situ Testing (SPT)

Standard penetration test (SPT) was carried out at various depths in all boreholes. The tests were

performed in accordance with ASTM D 1586-99.

4.1.2.2. Ground Water Table Observation

The underground water level (actually it is at the same the river water level) was measured at end of boring

at the time of sub-soil investigation (December, 2015) from the natural ground surface (+0.00m) shown in

Table (2). The specified depth was fixed after 24 hours of boring termination. However, the depth

December fluctuate during the seasons of the year.



Table 2 The underground water level

4.1.2.3. Permeability Test Results

The results of coefficient of permeability K= for layers it varies from (4.27x 10‾5 to 1.66 x10‾7) cm/sec.

The permeability of this soil is medium to good, and this is caused by variation of increasing the sand

percentages in the sub-soil.

4.2. Laboratory Testing

Routine geotechnical laboratory testing for determining physical, mechanical and chemical properties was

carried out on the soil samples obtained from the boreholes. All soil samples were tested at the soil

mechanics laboratory in the by Al-Mawal for investigation company. The actual test proposed for a

particular sample depends on the type of sample (DS, US and SS) and the nature of its material.

5. RESULTS AND CONCLUSION

By depending on US and BS international standards in conducting lab tests of sample study area for the

purpose of obtaining physical and chemical properties of the soil of study area as follows:

5.1. Physical Properties of Soil

5.1.1. Grain Size and Hydrometer Analysis

The grain size distribution curves of soil samples taken from the boreholes at site were determined using

sieve and hydrometer analysis.

According to Unified Soil Classification System (USCS), the site soil can in general, be classified as

poor to well graded sand (SP to SW) to poorly graded sand with silt (SP-SM) to silty sand (SM). On the

other hand, the cohesive soil pockets that encountered at different depths in some locations can be

classified as clayey silt of low to high plasticity (ML to MH).

5.1.2. Atterberg Limit and Soil Activity

With the plasticity index and liquid limit known the Casagrande plasticity chart shows the cohesive soil to

have wide range of plasticity CL (clays of medium plasticity ) and CH (clays of high plasticity) and OL or

ML (silts of medium or high compressibility and clay as shown in Table (3) . The average ratio of plasticity index to clay content equal (0.59) which release that this soil have poor

clay activity according to ASTM specifications, so this soil has low swelling tendency. The results

generally indicate that the value of moisture content is closer to the plastic limit than to the liquid limit.

This trend suggests that the cohesive less layer is loose dense and cohesive layer and cohesive layer is

consolidation. Linear shrinkage results are from 12.0 to 17.0 percent which indicate that the cohesive layer

might exhibit swelling and shrinkage potential.



Table 3 Plasticity Index and Liquid Limit PL=LL-PI

The Liquidity Index (LI) has been proposed as a measure of quantifying liquefaction problem. Values

of LI ≥ 1 are indicative of a liquefaction or quick potential. As long as most calculated values of LI shown

in Table 3 is less than and equal to one, so the samples have no liquefaction potential. Values of LI in

Table (3) with less than zero indicate also that the consistency of the soil is in a semi-solid or solid state,

while other values indicate that the soil is in a plastic state. The value of LI with less than or equal zero

indicates that the soil is over consolidated. Which is the ratio of Plasticity Index to clay content, is a

measure of degree to which soil will exhibit colloidal behavior?

Values of Activity (A) in Table (3) less than 0.75 are termed inactive clays. Normally active clays have

activities between 0.75-1.25. The samples with activity more than 1.25 are active clays. The test results

indicate that most of the soil samples have activity of less than 0.75. This means that the samples are of

inactive clay.

Liquid Limit and plasticity Index values put on Casagrande Chart table (4) to make material

classification as a result of Atterberg Limit Test and CH “medium-high plasticity clay” is determined from

table (5).

Table 4 Casagrande plasticity Chart



Table 5 Index test results and soil classification

5.2. Engineering Properties of Soil

5.2.1. Bearing Capacity

5.2.1.1. Calculating Bearing Capacity at shallow foundation by Dynamic Method.

The consistency of cohesive soils can be described qualitatively by terms such as very soft, soft, medium,

etc. The classification is based on the undrained shear strength (su) as Shown in Table (6) below. There is

an experimental correlation between the shear

Strength and N-values quoted by Terzaghi and Peck. This correlation should be used as a guide only,

and in situations where no enough data is available. Corrected for N- SPT test dependent on equation: - Nc

= 15 +0.5(Nm-15) where is:-

N= No. of blows for SPT, Nc= correct value for N ,Nm= measurement value for N, dependent on

formula( Terzaghi & Peck ,1967 and (Terzaghi (1967)) , have outlined the correlation of SPT-N value

(SPT blow count ) with undrained shear strength and consistency of clays as in table (6) .

Table 6 The correlation of SPT-N value with undrained shear strength and consistency of clays

Shear strength of cohesion less soils is usually described in terms of relative density. The relative

density is an index that quantifies the degree of packing between the loosest and densest state of coarse

grained soils. The denseness state of a cohesion less soil can be described as very loose, loose, medium-

dense, dense, and very dense. Some standards (like BS 5930) give the relationship shown in Table (7)

below, between N-values and the relative density of cohesion less soils.

Table 7 Below, between N-values and the relative density of cohesion less soils.



From the results of in-suite the allowable bearing capacity of the soil from N-SPT method for depth

from (1.5 m to 10.0m) is ranging from (23.10–31.35) T /m² for all boreholes While Shear Strength of

cohesion less soils ..Results are shown in Table (8) below:-

Table 8 Allowable bearing capacity of the soil from N-SPT method

5.2.1.2. Calculating the Bearing Capacity at shallow foundation by Static Method

Since damaging may result from foundation failure (collapse) as well as from excessive settlement, the

following criteria must always be used in evaluating the bearing capacity:-

• Adequate factor of safety against failure.

• Adequate margin against excessive settlement.

The bearing capacity could be evaluated from one of the following method.

1-The bearing capacity is calculated according to Terzaghi equation with modification suggested by

Meyerhof (1963)

qult= C Nc +q Nq +0.5BγNγ continuous footing

qult= 1.3CNc +q Nq +0.4γBNγ square footing

qult= 1.3CNc +q Nq +0.3γBNγ round footing

qult= C Nc Sc dc +q Nq Sq dq +0.5γ B Nγ Sγ dγ Meyerhof

Nc ,Nq, N γ Bearing capacity factor

Sc, Sq ,S γ Shape factors

dc , dq , d γ Depth factors

Sc =1 + Nq b / Nc L , Sq= 1 + B/L tanø , Sγ=1 - 0.4 B/L

dc= 1 + 0.4 Dƒ/B , dq= 1 + tanø(1-sinø)²D/B



2-Bearing capacity for foundation on undrained saturated clay for ø=0, so the general expression will be :

qult= C Nc + γ Dƒ (i.e. Nq=1, Nγ=0)

(Nc) rectangular = (1+ 0.2 B/L) (Nc) strip (Skempton Formula)

3-The net allowable bearing capacity of clay or plastic is approximately equal to the unconfined

compressive strength where

qult= C Nc + γ Dƒ for ø=0

The net ultimate bearing capacity (qult) is defined as the pressure that can be supported at the base of

the footing in excess of that at the same level due to the surrounding surcharge.

qult= qult- γ Dƒ = C Nc + γ Dƒ - γ Dƒ

qult= C Nc take F.O.S=3

qult= C Nc/3

C= q unconfined /2 , usually Nc≈6 , so

qall= q unconfined x 6 /2x3 , so

qall= qult/ Sf safety factor =3.0

Thus the allowable bearing capacity of clay or plastic silt is approximately equal to the unconfined

compressive strength.

Bearing capacity formulas considering -ground relation depending on ground types are given below.

5.2.1.2.1. According to Terzaghi

For foundation on CH ground, according to the Terzaghi’s general bearing capacity equation which will

provide the most consistent results;

qult = C N c + γ Df N q + 0.5 γ sub B N γ

For transition to safety bearing capacity this value is divided into 3 safety number; factor of safety (FS)

qall=qult/Fs T/m2 Fs=3 Table 9 Bearing capacity factors



In the calculate

Ones;

Foundation depth = 1.00 m

Foundation width = 2.00 m

SF= Safety factor = 3. 00

Nc, Nq & Nγ are representing the bearing capacities and given in Table (9) above according to the

internal friction angles.

The results of the unconfined, direct shear and triaxial tests are shown in Table (10). The results

indicate that the consistency of cohesive soil layer is very stiff and the relative density of cohesive less soil

layer is very dense. The allowable bearing capacity for foundation from Terzaghi equation are shown in

the table (11)

Table 10 Strength parameters (unconfined& triaxial test and direct shear test results with depth)

Table 11 The allowable bearing capacity for foundation from Terzaghi equation

Df= the depth of foundation (m ) allowable bearing capacity T/m²

(1.00) m (9.82) T/m²

(2.00) m (11.96) T/m²

(3.00) m (14.20) T/m²

5.3. Chemical test

5.3.1. Chemical Test of Soil

From the chemical tests of the soil samples were analyzed for sulphates, chloride content, organic matters

content, calcium carbonate, pH, TSS and gypsum content. The results are summarized in Table (12).

Table 12 Results of chemical analysis for soil



5.3.2. Chemical Test of Water

According to ASTM D-4750, chemical analysis for ground water is listed in Table (13).

Table 13 Chemical analysis for ground water

6. CONCLUSIONS

1-The study found that liquidity limit between (68-45) % and plasticity index its value ranges from (45-27)

%, and that means that the soil in the study area are high plasticity.

2-The study the soil classification by using grain size and hydrometer method according to Unified soil

classification found the soil is (CH), and some layer is (CL).

3-From the results of in-suite the allowable bearing capacity of the soil from N-SPT method for depth from

(1.5 m to 10.0m) is ranging from (23.10–31.35) T /m² for all boreholes Those values are good compared

with Mesopotamian region in Iraq.

4-The groundwater table is about 0.6 m.b.g.l below existing ground level .Water flow around the

foundations may cause scouring under the foundations so foundation type must be chose carefully.

Dewatering is required for the part of structure below the water table level.

5- The water is high alkalinity , medium to high in salts content and it has harmful amount of sulfates

(according to ASTM specifications). Precaution should be taken in concreting examining the tests results,

it can be seen that the range of sulphate (SO4) in water between (1030-1037) mg/l, while the range of

chloride content is between (213-220) mg/l [For water samples]. On the other hand it also noted that the

range of pH value for water samples were (7.7-8.0) It can be seen that the TDS of water samples is high

and varies from (1246-1253) mg/l.

6- The highest value of sulfate as SO3 % is in the range of (0.42-1.81%) for soil and the range of chloride

content is (0.026-0.051) %. Organic matter of soil samples is varied from 0.19 to 0.45 %. On other hand

the TSS and gypsum content were found to vary from 2.68 to 9.14 % and 1.10 to 3.92 %, respectively ,so

the suitable cement should be used in concrete that contact with soil .

7. RECOMMENDATIONS

• Study and observing the groundwater table in other seasons of the year to see decline and rise in the study

area also study the movement of groundwater and its direction.

• Study of the mineral elements for the study area in the water and soil with possibility of benefit from it as an

economic feasibility after additions.

• Work study compared with other areas of Najaf for comparison and definition.

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APPENDIX

Physical properties & field test of soil for. (BH.1)



