MOBILE ZONE: consolidation

The main aim of our project is to investigate the swelling behavior of expansive soil of Bannu Campus.

Expansive soil is a type of soil which greatly depends upon climate and moisture variation, this soil has got greater tendency to expand when moisture or water seepage through the soil increases and also has got tendency to contract very quickly when moisture through soil evaporates due to temperature(climate) variation. This expansion and contraction of soil with temperature variation causes major damage to the structure as a result of which the structure life decreases and major financial loss occur in repairing the damage to save the structure from destructive damage which can be caused from any kind of threats like earthquake, floods and other acts of God.

Our project is mainly intended to evaluate the swelling behavior of expansive soils of Bannu Campus and their interdependence using “Oedometer swell test”. In order to achieve the objectives of our research work, researches accomplished on expansive soil , different Geological and Geotechnical reports on Bannu soil have been reviewed. Samples were collected from different parts of Bannu Campus considering some technical points i.e (samples were collected frpm those areas) where there were horizontal cracks near wall joints, near doors and windows joints, considering the proper drainage work provided for the rain water and also if there is some water channel or drain (nallah) is flowing near the structures. The laboratory testing section was divided in two sections, In first section Atterburg limits i.e liquid limit and plastic limit were found which shows preliminary identification of expansive soil. In second section “oedometer swell test” is performed on undisturbed soil samples which shows actual swelling behavior of soil.

INTRODUCTION

GENERAL Expansive soil, which is recognized by its considerable volume change upon exposure to moisture variation, has caused a number of problems in most structures constructed Across the globe . Recent researches in assessing the failures caused on structures built on expansive soils showed that more than 60% of the structures are damaged due to causes associated with expansive soils.

The problems are either due to misunderstanding of the behavior of the soil or lack of information on the engineering properties of the soil. Even though, expansive soils are found in different parts of the world, the engineering properties of the soil are not similar. The behavior of expansive soils varies from place to place depending upon the type of parent material, climate and topography. Researches on the engineering properties of expansive soils of Bannu have been done. Some of the researches undertaken are ‘Characterization of soils in flood plains of Bannu Basin’(G.Saeed Khan) , Physiochemical Properties Soils of Bannu District , (Wasiullah , A.U.Bhatti).in all the above researches, even though most of the

Engineering properties of expansive soils of Ethiopia are covered, consolidation

Characteristics have not been assessed.

Figure 1a: cracks caused due to expansion in soil

OBJECTIVE:

Swelling and consolidation of expansive soils are phenomena, which take place due to moisture change and effective stress change in the soil mass, respectively. In condition where both moisture change and effective stress changes take place at the same time, there will be a tendency for swelling and consolidation; the volume change being dependent on the magnitude of the effective stress change. The main objective of this thesis is to investigate the swelling behavior of Bannu campus soil which could be the reason behind the cracks produced in different parts of the campus . In addition to find the consolidation and swelling parameters of Bannu campus soil.

METHODOLOGY:

In order to achieve the objective of this thesis , researches accomplished on expansive soil has been reviewed . A small scale survey has also been done in order to select the appropriate positions for sample collection , both disturbed and undisturbed samples have been collected from the sites . In first session Atterburgs limits i.e. liquid limit and plastic limit have been found. In second session consolidation and swelling tests are carried out on disturbed samples.

LITERATURE REVIEW:

EXPANSIVE SOIL IN GENERAL

Expansive soil is a term generally applied to any soil or rock material that has a

potential for shrinking or swelling under changing moisture conditions. [2] Subsequent swelling and shrinkage of this soils due to change in moisture cause damages to civil engineering structures, particularly light buildings and pavements.

The origin of expansive soils is related to a complex combination of conditions

and processes that result in the formation of clay minerals having a particular chemical makeup which, when in contact with water, will expand. The conditions and processes which determine the clay mineralogy include composition of the parent material and degree of physical and chemical weathering to which the materials are subjected [1].

There are two fundamental molecular structures as the basic units of the lattice structure of clay soils. These are the silica tetrahedron and the alumina octahedron. The silica tetrahedron consists of a silicon atom surrounded tetrahedrally by oxygen ions. The alumina octahedron consists of an aluminum atom surrounded octahedrally by six oxygen ions. When each oxygen atom is shared by two tetrahedral, a plate- shaped layer is formed. Similarly, when each aluminum atom is shared by two octahedron, a sheet is formed.\

The silica sheets and the aluminum sheets combine to form the basic structural units of the clay particles. Various clay minerals differ in the stacking configuration. The major component of expansive soils, montmorillonite is a three layer clay mineral having a single octahedral sheet sandwiched between two tetrahedral sheets to give a 2 to 1 lattice structure.

2.2 ENGINEERING PROPERTIES OF EXPANSIVE SOIL OF BANNU

2.2.1 Little work has been done on characterization of Bannu soil

Water and Power Development Authority(WAPDA) in 1966 in their reconnaissance report of khurram basin has showed the limited permeability of soil due to finness of their structure
Project planning division of SCARP(1977) has described the structure of the soil of area as sub angular blocky
Khan and Jhamshed(1988) have discussed level of organic matter , Nitrogen and phosphorous content of soil of Bannu is low.
Nawaz (1976) has concluded that the soil of area were calcareous with alkaline reaction.

PROFILE OF NORMAL SOIL OF BANNU :

The following profile was described and sampled near village Rasul khan near Ismail khel road which off takes from Bannu_D I Khan road (Soil survey of Pakistan,1974)

The analytical data of the profile is given in the table

Table 1: Normal profile of Bannu

Horizan/Depth

Ap (0-10 cm)

B1 (10-20 cm)

Description

Color: brown/dark brown when moist and pale brown when dry; Texture: silty , clayey loam; Structure: massive ;consistency: very sticky and plastic firm when moist and hard when dry; Porosity: common interstitial pores; calcareousness; moderate; PH: alkaline; smooth boundary with lower horizon

Color: grayish brown to dark grayish brown when moist and pale brown when dry; Texture: silty clayey loam; Structure: weak, coarse sub angular blocky; consistency : very sticky and very plastic, firm when moist and hard when dry; porosity: few fine and common very fine tabular pores; calcareousness: moderate; PH: alkaline; smooth boundary with lower horizon

2.2.3 PHYSICAL PROPERITIES OF BANNU DISTRICT.

Table 2: physical properties of Bannu

Property	Mean	Maximum	Minimum
( 0 – 15 cm ) Depth
Sand	19.99	8.40	84.40
Silt	36.82	5.60	64.40
Clay	43.19	10.50	64.80
Saturation capacity	55.83	23.98	76.81
Bulk Density	1.31	1.01	1.68

Property	Mean	Minimum	Maximum
( 0 – 45 cm ) Depth
Sand	18.54	8.00	84.80
Silt	36.04	2.40	53.20
Clay	45.62	10.80	64.80
Saturation capacity	58.06	24.37	76.81

Table 2.2.3(b)

MECHANICS OF SWELLING:

GENERAL:

Swelling of expansive soils will take place under change in the environment of the

soil. Environmental change can consist of pressure release due to excavation, desiccation caused by temperature increase, and volume increase because of the introduction of moisture. By far the most important element and of most concern to the practicing engineer is the effect of water on expansive soil. There must be a potential gradient, which can cause water migration and a continuous passage through which water transfer can take place [1]. The potential gradient in expansive soils can be due to seasonal moisture fluctuation or thermal gradient, which can cause vapor and liquid moisture transfer. It is well recognized that the heaving of expansive soils may take place without the presence of free water. Vapor transfer plays an important role in providing the means for the volume increase of expansive soils.

FACTORS INFLUENCING SWELLING:

The mechanism of swelling in expansive soils is complex and is influenced by a number of factors. [2] Expansion is the result of change in the soil water system that disturbs the internal force equilibrium. The factors influencing the shrink-swell potential of a soil can be considered in three different groups. These are the soil characteristics that influence the basic nature of the internal force field, the environmental factors that influence the changes that may occur in the internal force system, and the state of stress.[2]

SOIL CHARACTERISTICS:

Soil characteristics may be considered either as micro scale or macro scale factors.

Micro scale factors include the mineralogical and chemical properties of the soil. Macro scale factors include the engineering properties of the soil which in turn dictated by the micro scale factors.

i) Micro scale factors (clay mineralogy and soil water chemistry):- clay minerals of

different types typically exhibit different swelling potentials because of variation in the electrical field associated with each mineral. The swelling capacity of an entire soil mass depends on the amount and type of clay mineral in the soil, the arrangement and specific surface area of the clay particles, and the chemistry of the soil water surrounding those particles. Soil water chemistry is important in relation to potential swell magnitude. Salt cat ions such as sodium, calcium, magnesium, and potassium are dissolved in the soil water and are adsorbed on the clay surfaces as exchangeable cat ions to balance the negative electrical surface charges. Hydration of these cat ions and adsorptive forces exerted by the clay crystals themselves can cause the accumulation of a large amount of water between the clay particles. In dry soils, salt cat ions are held close to the clay crystal surfaces by strong electrostatic forces. As water becomes available, cat ion hydration energies are sufficiently large to overcome interparticle attraction forces. Thus initially desiccated and densely packed particles are forced apart as adsorbed cat ions hydrate and become

enlarged on the addition of water.

ii) Macro scale factors (plasticity and density):- Macro scale soil properties reflect the

micro scale nature of the soil. Because they are more conveniently measured in

engineering work than micro scale factors, macro scale characteristics are primary indicators of swelling behavior. Commonly determined properties such as soil plasticity and density can provide a great deal of insight regarding the expansive potential the of the 10 soils. Soil consistency, as quantified by the Atterberg limits, is the most widely used indicator of expansive potential. Most expansive soils can exist in a plastic condition over a wide range of moisture contents. This behavior results from the capacity of expansive clay mineral to contain large amount of water between particles and yet retain a coherent structure through the interparticle electrical forces. Soil plasticity, a useful indicator of swell potential, is influenced by the same microscale factors that control the swell

potential. [2]

ENVIRNMENTAL CONDITIONS:

The potential for a soil to absorb or expel water will depend on the water content

relative to the water deficiency of the soil. Initial moisture content influences the shrinkswell potential relative to possible limits, or ranges, in moisture content. Moisture content alone is not a good indicator or predictor of shrink-swell potential. Instead, the moisture content relative to limiting moisture contents such as the plastic limit and shrinkage limit must be known. Water content changes below the shrinkage limit produce little or no change in volume. There are indications that as a soil imbibes water, little volume change occurs at water content change above the plastic limit.

The availability of water to an expansive soil profile is influenced by many environmental and manmade factors. Generally, the upper few meters of the profile are subjected to the widest ranges of potential moisture variation. Also, overburden stress is low and the soil is not restrained against movement at shallow depths. This upper stratum (active zone) of the profile therefore exhibits the major part of the shrinking and swelling. Moisture variation in the active zone of a natural soil profile is affected by climatic 11 cycles. Other obvious and direct causes of moisture variation result from altered drainage conditions or manmade sources of water, such as irrigation or leaky plumbing.

CONSOLIDATION

THEORIES OF CONSOLIDATION:

Any structure built on the ground causes increase of pressures on the underlying soil

layers. The soil layers are unable to spread laterally as the surrounding soil strata confines them. Hence there must be adjustment to the new pressure by vertical deformation. The compression of the soil mass leads to the decrease in the volume of the mass, which result in the settlement of the structure, built on the mass. The vertical compression of the soil mass under increased pressures is thus made up of the following components:

i. Deformation of the soil grains

ii. Compression of water and air with in the voids

iii. An escape of water and air from the voids

It is quite reasonable and rational to assume that the solid matter and the pore water

relatively are incompressible under the loads encountered. The change in volume of the soil mass under imposed stresses must be only due to the escape of water and air. Generally, the volume change in a soil deposit can be divided in to three stages:[12]

INITIAL CONSOLIDATION:

When a load is applied to a partially saturated soil, a decrease in volume occurs due to expulsion of and compression of air in the voids. A small decrease in volume also occurs due to compression of solid particles. The reduction in volume of the soil just after the application of the load is known as initial consolidation or initial compression. For saturated soils, the initial consolidation is mainly due to compression of solid particles.

PRIMARY CONSOLIDATION:

After initial consolidation, further reduction in volume occurs due to expulsion of water from voids. When a saturated soil is subjected to a pressure, initially all the applied pressure is taken up by water as excess pore water, as water is almost incompressible as compared with solid particles. A hydraulic gradient develops and the water starts flowing out and a decrease in volume occurs. The decrease depends up on the permeability of thesoil and is, therefore, time dependent. The reduction in volume is called primary consolidation. In fine grained soils, the primary consolidation occurs over a long time. On the other hand, in coarse grained soils, the primary consolidation occurs rather quickly due to high permeability. As water escapes from the soil, the applied pressure is gradually transferred from the water in the voids to the solid particles.

2.4.4 SECONDARY CONSOLIDATION:

The reduction in volume continues at a very slow rate even after the excess pore water pressure developed by the applied pressure is fully dissipated and the primary

consolidation is complete. This additional reduction in the volume is the called secondary consolidation. The causes for secondary consolidation are not fully established. It is attributed to the plastic readjustment of the solid particles and the adsorbed water to the new stress system. In most inorganic soil, it is generally small

Laboratory Testing section

General

Laboratory determination of the consolidation characteristics of clay soils is usually

carried out on saturated soil using an Odometer. Atterburgs limits are i.e liquid limit and plastic limit which are very helpful in determining the expansive behavior of soil. The swelling characteristics of expansive soils can also be determined conveniently using an Odometer. In Odometer test, only one dimensional consolidation and swelling characteristics of the soil are determined. The main purpose of the consolidation test is to obtain information on the compression properties of a saturated soil for use in determining the magnitude and rate of settlement of structures

In general, the consolidation test of a soil should furnish the following

information:

i. the relationship between time and percent consolidation

ii. the relationship between the increasing or decreasing load on the soil and

the change in the void ratio of the soil data on permeability of the soil as a function of that particular load

Types of samples

There are two types of samples that we usuallay get for our experiments and to perform test on that samples

Undisturbed samples

Undisturbed samples of expansive soil are recovered from Bole and Gergi area with

sampling tubes of diameter 89mm.The sample for odometer test is extracted from the tube sampler using the odometer ring which has a diameter of 7cm and a height of 2cm. The sample is then prepared for the test by trimming the ends.

Fig 3.2.1 (a) Undisturbed Sample.

3.2.2 Disturbed samples

Disturbed samples of expansive soil are also taken from Bole and Ayat area. The

disturbed samples are air dried, sieved and soaked for 24 hours. The wet soil sample is then compacted to the required density which is in the range of the density of the natural deposit and the test sample is extracted using the ring of the odometer. The samples are also prepared to have different initial moisture content.

Fig3.2.DisturbedSample

Test results:

We carried out preliminary tests for oedometer swell test and we tested four samples for that purpose and finds its atterbergs limits i.e liquid limit and plastic limit

For a soil to be expansive it must fulfill some of the criteria devised by the different researchers based on practical field data.

A soil to be expansive in nature must have plastic limit greater than 20 and liquid limit greater than 35.

So all of the four samples that we used for our preliminary test fulfill the criteria of limiting value of liquid limit and plastic limit.

So we proceed our project in pursuance of finding the expansive behaviour of UET Bannu campus soil. The results that we obtained from tests that we performed on the samples are as follows.

Liquid limit = 32.5%

Liquid limit=31.8%

Liquid limit = 38.6%

Liquid limit = 41.82%

Now the plastic limits of the above mentioned samples

Test results(oedometer swell test) :

_{A
consolidation-swell test is carried out on}_{three undisturbed}

_{samples of
expansive soils of UET Bannu campus. The test results obtained from
consolidation swell test undisturbed samples of expansive soil of
different density and initial moisture content are presented in the
form of void ratio- log pressure,and dial reading- square root of
time plots}. _{The test
results of the undisturbed samples are shown in the graph, typical of
which are shown in Figure 3.3. These results are used to determine
the relationship between consolidation and swelling characteristics
of expansive soil of UET Bannu campus. In this regard a number of
pairs of soil samples of the same dry density and different moisture
content are considered.}

_{The results are
shown as a plot between Dial guage reading of consolidation swell
test device to the square root of time taken by the readings taken at
each interval of time.}

Discussion on test results

The soil sample allowed to saturation in an oedometer undergoes swelling with a

magnitude dependent on the change in moisture content and initial dry density. Early

loading of the sample up to a pressure equal to the swelling pressure was to maintain the initial void ratio before saturation.

Further loading beyond the swelling pressure has caused the soil to undergo consolidation from which the consolidation characteristic has been determined. During unloading there is a permanent deformation for each load decrement so that after the removal of all the applied loads the void ratio was less than the initial one. This indicates that the swelling due to the moisture change at the beginning of the test will not be observed after the sequential loading and unloading.

The insitu overburden pressure for the soil samples was in the range of 25-35kPa that is much less than the swelling pressure. For any load less than the swelling pressure consolidation will not take place. For consolidation to take place, the soil should be subjected to a pressure greater than the swelling pressure. The stress history of expansive soil should be determined not only based on the relative magnitude of the present overburden pressure and the maximum pressure to which the soil was subjected in its history but also on the swelling pressure.

An expansive soil is said to be overconsolidated when the maximum pressure to which the soil was subjected in the past in greater than both the present overburden pressure and the swelling pressure otherwise, it is normally consolidated.

The consolidation swell test on undisturbed samples of expansive soil has shown that the e-logP curve for pressure greater than the swelling pressure is linear. It can be inferred from the trend of the curve that the insitu soil is normally consolidated.

In expansive soils, larger moisture change implies higher degree of disturbance in the soil structure. But the influence of the disturbance on the consolidation characteristics of expansive soil is not similar to non-expansive clay soils. In non-expansive clay soils, a soil subjected to more disturbances show a flatter e-log P curve. It can be easily observed that the samples subjected to more swelling show a larger value of compression index.

Accordingly, tests on remolded samples of different dry density and the same moisture content have also indicated that the consolidation behavior of expansive soil is different from that of non expansive clay soils. In non expansive clay soils, a soil with larger dry density will have compression index smaller than the same soil of lesser dry density, whereas, in expansive soils, a different consolidation characteristics is observed.

The observations made from the laboratory tests leads one to identify an additional factor, which governs the consolidation behavior of expansive soils. As can be seen from the test results, those factors that affect the swelling characteristics of expansive soils have also affected the consolidation characteristics of the expansive soil. Therefore, the additional factor anticipated to have an effect on consolidation characteristics of expansive soils is the swelling.It is known that consolidation is the property of the soil mass that is highly dependent on permeability which inturn depends on the structural arrangement of soil particles. On the other hand, swelling is the property of the soil particle, which depends on the mineralogy of the soil particle. In effect, both phenomena bring about volume change in the soil mass.

Swelling is a result of disturbance in internal stress equilibrium of the soil particle

by change in moisture content. To attain new internal stress equilibrium, the soil particles start to swell and a new particle arrangement will take place in the soil mass. Therefore, Swelling brings about a soil of different particle arrangement and different consolidation characteristics. The assumption in the theory of consolidation that says that volume change in the soil particle is negligible will not be reasonable in case of expansive soil

It is also observed that the potential swelling of the expansive soil ranges from 3.7% to 10.3%.

1 Chen, F.H, 1988, ‘Foundationon expansive soils’, Elsevier, Amsterdam

2- John, D.N.,and J.M.,Debora, ,1992, ‘Expansive Soils-Problems and practice in

foundationand pavement engineering’, John Wiley & Sons. Inc., New York

3- Habib, S.A., T., Kato, and D., Karube, 1995, A research paper on ‘suction

controlled one dimensional swelling and consolidation behavior of expansive soils’

4- Teklu, D., 2003, A thesis on ‘Examining the swelling pressure of Addis Ababa

expansive soils’

5- Legesse, M., 2004, A thesis on ‘Investigation on index properties of expansive soils

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