Soil structure / Soil conditions
The topo-lithosequence of Kerala along with variation in rainfall, temperature and alternate wet and dry conditions particularly from the western coast to high ranges in the east and swift flowing rivers lead to the development of different types of natural vegetation and soil. The soils of Kerala can be broadly grouped into coastal alluvium, mixed alluvium, acid saline, Kari, laterite, red, hill, black cotton and forest soils. In terms of geographical and soil conditions, Ernakulam district can be broadly classified into three physiographical units such as 1) Coastal plains, 2) Mid lands and 3) High lands. North Paravur Taluk falls under the category of coastal plains where the general elevation of coast is less than 8.0m.amsl. The major types of soil which is found in most of the regions of North Paravur Taluk are 1) silty clay, 2) silty sand and 3) laterite.
1. Single Grained Soil Structure
Single grained structures are present in cohesion less soils like gravel and sand. The grains of cohesion less soils have less surface force and more gravitational force. So, when we pour some amount of sand or gravel on the ground, the grains will settle using gravitational force rather than surface force. After attaining the final position, each grain is in contact with the surrounding grains and formed structure is called as single grained structure.
The single grained structure soils are deposited under gravitational forces but here the position of particles after settling also matters. Let us assume that all the particles are spherical in nature then maximum void ratio (e= 0.90) is occurred when they positioned in cubical array manner which is as shown in figure and is called as loosest state of soil. Similarly, Minimum void ratio (e=0.35) will occur if they positioned in diagonal packing manner as shown in figure which is called densest state.
In general, the particles are not spherical in nature, but we can say that the void ratio of single grained particles is in between 0.35 to 0.90.
To use soil for the civil engineering purpose, the void ratio of the soil should be minimum or in other words dense soil is preferable. Loose soil is very unstable and cannot withstand against vibration and shocks.
2. Honeycomb Soil Structure
Honeycomb structure found in soil contains particle of size 0.02mm to 0.002mm which are generally fine sands or silts. When this type of soil is allowed to settle on the ground, the particles will attract each other and joins one with another and forms a bridge of particles. A large void is also formed between those bridges which makes the soil very loose in nature.
The attraction of particles is due to cohesion between them, but this cohesion is just because of their size however, these soils are not plastic in nature. In fine sands, when water is added to dry fine sand bulking of sand occurs which is nothing but a structure of honeycomb.
Honeycomb structured soil is limited for static load condition. They cannot resist vibrations and shocks under building and may cause large deformations to the structure.
3. Flocculated Soil Structure
Flocculated structure is present in clay particles which contains larger surface area. These are charged particles which have positive charge on the edges and negative charge on the face of the particle. When there is net attractive force between the particles, then positive charged particles attracted towards negatively charged faces which results the formation of flocculated structure.
Clay present in the marine area is the best example for flocculated structure. Salt present in the marine water acts as electrolyte and reduces the repulsive force between the particles which leads to the formation of flocculent structure. This type of soils has high shear strength.
Because of edge to face orientation void ratio is high in this type soil and water content also optimum but they are light in weight. The compressibility is very low for this type of soils.
4. Dispersed Soil Structure
Dispersed structure also occurs in clay particles when the clay is remoulded. Remoulding reduces the sheer strength of soil which reduces the net attractive forces between the particles. Hence, due to repulsion between them, the edge to face orientation turns into face to face orientation. Finally, dispersed structure of clay will form.
This type of soil is highly compressible and less permeable. The loss of strength during remoulding is slowly achieved by the soil with the time. The process of regaining its strength after remoulding is called thixotropic.
5. Coarse Grained Skeleton Soil Structure
Coarse grained skeleton is a structure of soil which is present in composite soils containing both fine and coarse-grained particles. But, it is formed when there is large amount of coarse grained particles than the fine-grained particles. The coarse-grained particle forms a skeleton like structure and voids between them are filled by fine grained or clayey particles. If it is undisturbed, it will give good results against heavy loads. If disturbed, the strength extensively reduced.
6. Clay Matrix Soil Structure
Clay matrix structure, which also occurs in composite soils, but the amount of clay particles or fine-grained particle, is more compared to coarse grained particles. Coarse particles in this soil are separated with each other as they are less in quantity. This type of soil is very stable in nature and it has same properties of an ordinary clay deposit.
Map of North Paravur Taluk indicating soil conditions in various Panchayats
Vadakkekara is one of the plain regions. Soil in this area is alluvium. During high tides and low tides, the water from sea will enter in to the shallow streams in rural areas. Vadakkekara Panchayat is surrounded by tributaries and related streams by Periyar. The lack of caution in protecting them has a profound impact. Chittattukara is a coastal plain and have coastal alluvial soil.
Depending on the geographical area, the area under Kottuvalli Panchayat can be divided in to four categories 1) High plains, 2) Slope area, 3) Coastal plain. 4) Swamps. Kaitharam and Kottuvally portions consist of sandy soil and mud. The eastern part, Thathapilli is muddy soil and northern part is clayey soil. Valluvally koonammavu is composed of sandy soil. Kottuvally gram Panchayat is also known as agro climate zone and costal sandy zone.
Based on the geographical features Ezhikkara Gram Panchayat can be divided in to 3 categories such as 1) Coastal plain, 2) Lower pokkali fields, 3) Swamp and water logging area. The area consists of coastal alluvial soil. The unprecedented flood has caused so many alluvial soil to deposit within the Panchayat boundaries. Chendamangalam is a village with lots of rivers and streams. The Kottayil Kovilakam is the highest region in the Panchayat and the soil of this area is laterite. Rest of the areas have Red soil.
The Karumalloor Panchayat belongs to the midland region. It can be classified into four regions according to the geographical area. 1) High planes area, 2) Slope area 3) Coastal plain 4) Swamps. Manjali, Muttankunnu, Karukkunnu, Valyappanpadi, Thudalukunnu these are the high planes area in the Panchayat. Among these Muttankunnu is the highest plane. Silt soil, which is suitable for farming is seen in law lying areas of Muttankunnu. The north-east side of Aluva-Paravur road, V H Colony, Settlement are high slope area with Laterite soil. Thudalikkunnu is a high plane area with slopes and Laterite soil. On the South part, it is Clay like soil. The north part and middle area is laterite soil. On the north-west area it is silt soil. On the west of Millupadi is high plane area with slope. The Attippuzhakkavu area is Farmlands. Some area has mixture of laterite and clay, mostly in coastal area. Veliyathunad, Paruvakkad, Aduvathuruth are suitable for agriculture, marshy lands are also seen here. Karumalloor, Kallarakkal areas are plains, but Thattampadi-Angadikadavu road have marshy areas. These are normally farmland with silt soil. On the northern side of Aluva-Parvoor road is entirely of muddy farms. Thattampadi- Thanthoni, Chalil, Panatt are of flat plain area. Mundeppadam, the northern side of Aluva-Paravur road is farmlands. North-south side have silt soil whereas on the east side there is a mixture of silt soil and marshy lands, and the west lands is of sandy area. Chettikkadu, Puthukkadu are of flat plain areas. Thaithathara is a coastal area. South side of this area is marshy lands. Manakkappadythoppu is little high plane area. Anachal area is of swamps. Most of the area is of sand mixed soil. Manakkappady- Muriyakkal are of flat plane areas. On the side of Manakkappady-Mandala road have marshy land. North-west-east have sandy mixed soil and south part have clay soil. Manjali –Kunnumpuram is also a high plane area. Most of the areas are with slopes. On the east have laterite soil and centre had a mix of sandy soil and northern area has rocky sand, south-west area have sandy mixed soil.
The geographical area of Puthenvelikara can be classified in to three namely, 1) Hilly areas 2) Plain areas, 3) Coastal areas. Manacherikunnu, Kappelakunnu, Paramanasharikunnu, Enjarakunnu, Thuruthoor, Panachakunnu, Vattekattkunnu, Kodikuthiyakunnu are the major hills in the village. Red colour laterite soil is found in these hilly areas, sandy black soil and alluvial soil are found in plain areas and coastal areas. The soil here is so fertile
Alangad and varapuzha is surrounded with rivers and have alluvial soils
Characteristics of each type of soil
The type, depth and size of a footing depends on the bearing capacity of the supporting material (soil or rock), which again depends on the type of soil and many other factors such as soil structure and composition, ground water etc. Common types of soil, their bearing capacity properties and the type(s) of foundation suitable for each soil type are given below;Rock – Rock usually offers a high bearing capacity, which depends on the type of rock, state of weathering and any fissures that exist within the rock. In certain areas, rock may exist at shallow depth. Reinforced pad footings for columns and narrow strip footings for walls would be quite adequate on rock. On a slope rock surface, the foundation may have to be doweled in to the rock. Besides rock, there are five other types of
soils having particles of different sizes, which may naturally exist in different compositions.
Gravel and Gravelly Soils – Gravel, sometimes mixed with small proportions of sand/ or clay etc., exist as gravelly soils. As they have a low compressibility and a high bearing capacity, pad footings can be used for
columns and strip footings for the walls.
Sand and Sandy Soils – Sandy soils also have a high bearing capacity when densely compacted, but, relatively low bearing capacity when loose. Dense sandy soils have low compressibility but foundations placed on loosely compacted sandy soil can undergo significant settlement quickly as the load is applied. Pad footings for columns and strip footings for walls can be used. Settlements in sandy soils will be completed within a short time, mostly during construction.
Silts and Silty Soils – Silty soils have a moderate bearing capacity when dry, compacted and confined. But, their structure can easily break down when exposed to water. Silty soils are compressible when loose. Pad footings or strip footings of appropriate size can be used in silt soils as column and wall foundations respectively.
Clays and Clayey Soils – Clayey soils are made up of fine particles and are described as cohesive soils for their sticky and binding properties. They generally have a lower bearing capacity than the sandy soils. Foundation load can cause significant but gradual and prolonged settlement in some clay, which may occur over weeks and months or even years. Some clay has a very high affinity for water and expands with the absorption of water.
Organic Soils – Organic matter may exist alone as in peats that are highly compressible or mixed with clays, silts or sands. Foundations should be avoided on organic soils without appropriate ground improvement.
