RETAINING WALL AND TYPES

 What is Retaining Wall ?

Retaining wall are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to (typically a steep, near-vertical or vertical slope). 

They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. The most important consideration in proper design and installation of a retaining wall is to recognize and counteract the tendency of the retained material to move downslope due to gravity. It is important to have proper drainage behind the wall in order to limit the pressure to the wall design value. Drainage material will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. 

Types Of Retaining Wall

1. Gravity Retaining Wall
2. Reinforced Retaining Wall
3. Concrete Cantilever Wall
4. Buttressed Retaining Wall
5. Reinforced Soil Retaining Wall
6. Green Retaining Wall
7. Mechanical Stabilization Wall
8. Anchored Wall

1. Gravity Retaining Wall

Gravity wall depend on their mass to resist pressure from behind and may have better setback to improve stability by leaning back towards the retained soil. For short landscaping walls, they are often made from mortarless stone and segment concrete units. Dry stacked gravity walls is somewhat flexible and do not require a rigid footing.

2. Reinforced Retaining Wall

Reinforced concrete and reinforced masonry walls on spread foundation are gravity structures in which the stability against overturning is provided by the weight of the wall and reinforcement bars in the walls.

3. Concrete Cantilever Wall

A concrete cantilever retaining wall is one that consists of a wall that is connected to the foundation. A cantilevered wall holds back a significant amount of soil, so it must be well engineered. They are the most common type used as retaining walls. Cantilever wall rest on a slab foundation. This slab foundation is also loaded by back-fill and thus the weight of the back-fill and surcharge also stabilizes the wall against overturning and sliding.

4. Buttressed Retaining Wall

Buttressed retaining wall are cantilever wall straightened with counter forts monolithic with the back of the wall slab and base slab. The counter-forts act as tension stiffeners and connect the wall slab and the base to reduce the bending and shearing stresses. To reduce the bending moments in vertical walls of the great height, counterforts and used, spaced at distances from each other equal to or slightly larger than one-half of the height counter forts are used for high walls with heights greater than 8 to 12 m.

5. Reinforced Soil Retaining Wall

Reinforced soil can also be used as retaining walls if they are built as an integral part of the design and to act as an alternative to the use of reinforced concrete or other solutions on the grounds of economy or as a result of the ground conditions.

6. Green Retaining Wall

Green retaining walls can be used to retain more gentle slopes. A Geo cellular structure such as a series of honeycomb cells can be embedded into the surface of the slope to stabilize it, and the individual cells can then be planed.

7. Mechanical Stabilization Wall

Mechanically stabilized earth walls are walls that can tolerate some differential movement. The wall face is infilled with granular soil whilst retaining the backfill soil.

8. Anchored Wall

An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additionals strength using cables or other stays anchored in the rock or soil behind. It usually driven into the material with boring, anchors and then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete which expands to form a bulb in the soil.

 

WHAT IS CLAYEY SOIL - TYPES AND ADVANTAGES

What Is Clayey Soil

Clay is a fine-grained natural soil material containing clay minerals. Clay particles are the finest of all the soil particle measuring less than 0.002 mm in size. It consists of microscopic particles derived from the chemical decomposition of rocks. Clays develop plasticity when wet, due to a molecular film off water surrounding the clay particles, but become hard, brittle and non-plastic upon drying or firing. These soils are made of over 25 percent of clay. Most of the clay minerals are white or light in colour but natural clay show a variety of colours such as reddish or brownish because of the presence of small amount of iron oxide.

Formation Of Clayey Soil

Clays and clay minerals occur under a fairly limited range of geologic conditions. The environments of formation include soil horizons, continental and marine sediments, geothermal fields, volcanic deposits, and weathering rock formations. Most clay minerals are commonly form when rocks are in contact with air, water or steam and are also form locally from hydrothermal activity. The clay minerals are formed by the acidic weathering of feldspar rich rock such as granite in hot climates tends to produce kaolin.

Types Of Clayey Soil According To Its Percentage

1. Silt Soil     0 - 10 % Clay
2. Clay Soil 10 - 25 % Clay
3. Clay Soil 25 - 40 % Clay 
4. Clay Soil 40% Clay

Advantages Of Clayey Soil

• It is very fertile.
• It retains water and nutrients.

DIFFERENT TYPES OF SOIL

Types Of Soil

The term soil refers to the upper layer of earth crust in which plants grow. It consists of weathered rock, organic matter, air space and water. In civil engineering soil can be defined as a naturally occurring loose/uncemented /weakly cemented/unconsolidated mineral particle, organic or inorganic in character, lying over the bed rock which is formed by weathering of rock.

Types Of Soil

1. Sand
2. Silt
3. Clay
4. Loam

1. Sand

This is the first type of soil. It consists of particle of weathered rock such as silicon dioxide. Sandy soil has the largest particles where each particle can be seen from naked eyes. The large particle size increases soil aeration, improves drainage in tight soils. 

The particle size of course sand ranges from 2 - 4.75 mm, medium sand ranges from 0.425 - 2 mm. The biggest particle size of the sand gives wet or dry sandy soil a grainy texture when you rub between your finger.

2. Silt Soil

Silt is granular material of size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water and soil in a body of water such as river. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt is easily transported by moving currents and it is mainly found near the river, lake and other water bodies. The silt soil is more fertile compared to the other types of soil.

3. Clay Soil

Clay is a type of fine-grained natural soil material containing clay minerals. Clay particles are the finest of all the soil particle measuring less than 0.002 mm in size. It consists of microscopic particles derived from the chemical decomposition of rocks.The particle in this soil are tightly packed together with each other with very little or no airspace. Clay is a fine grained cohesive soil but become hard, brittle and non-plastic upon drying of firing. Most pure clay minerals are white or light coloured but natural clay show a variety of colours from impurities such as a reddish or brownish colour from small amounts of iron oxide.

4. Loam Soil

Loam is a soil composed of sand, silt and clay. These soil are fertile, easy to work  and provide good drainage. Loam is considered ideal for gardening and agricultural uses because it retains nutrients and retain water while still allowing excess water to drain away. Apart from this, it also has higher calcium and pH levels because of its inorganic origins.

STANDARD PENETRATION TEST (SPT) - PROCEDURE & ADVANTAGES

Standard Penetration Test

The standard penetration test is an in-situ test that is carried out to provide information on geotechnical engineering properties of soil. The test is carried out in borehole.The test will measure the resistance of the soil strata to the penetration undergone.The main purpose of the test is to provide an indication of the relative density of granular deposits such as sand and gravels. It can also be used to determine the confined compressive strength of cohesive soil. This test is widely used because it is a simple and inexpensive. 

Apparatus For Standard Penetration Test

1. Standard Split Spoon Sampler
2. Drop Hammer of weight 63.5 kg
3. Drilling Rig
4. Driving Head
5. Guiding Rod

Procedure For Standard Penetration Test

The test is conducted in bore hole by means of a standard split spoon sampler. Once the drilling is done to the desired depth, the drilling tool is removed and the sampler is placed inside the bore hole. The standard split spoon sampler 50.8 mm outer diameter and 35 mm inner diameter, is driven into the undisturbed soil at bottom of the bore hole by blows from a slide hammer with a weight of 63.5 kg falling through a height of 750 mm at the rate of 30 blows per minute, the sample is driven into the soil.

The number of blows of hammer needed for the tube to penetrate each 150 mm is counted. Further it is driven by 150 mm and the blows are counted. Similarly, the sample is once again further driven by 150 mm and the number of blows recorded. The number of blows recorded for the first 150 mm is not taken into consideration. The number of blows recorded for last two 150 mm intervals are added to give the standard penetration number (N). 

In case if the number of blows for 150 mm drive exceeds 50, it is taken as refusal and the test is discontinued. The standard penetration number is corrected for extensive correction and overburden correction.

Advantages Of Standard Penetration Test

• Test is simple and inexpensive.
• Able to penetrate dense layers, gravel etc.
• Test can be applicable for variety of soil conditions.

Disadvantages Of Standard Penetration Test

• Test is time consuming.
• The sample retrieved for testing is disturbed.
• The results are not very precise and highly reliable.

Vane Shear Test

Vane shear test is used to determine the undrained shear strength of cohesive soils especially soft clay. This test is done in laboratory or in field directly on the ground. This test is carried out with equipment consisting of a rod with vanes mounted to it that is inserted into the ground and rotated. This test was introduce by L Carlson and AW Skempton in 1948. This test give accurate results for soils of low shear strength i.e, less than 0.3 kg/cm2.

Apparatus Required

1. Vane Shear Apparatus
2. Vernier Callipers
3. Soil Specimen Container

Vane Shear Apparatus

Procedure Of Vane Shear Test

1. Clean the vane shear apparatus and apply grease to the lead screw for better movement off handles.

2. Take the soil specimen in container which is generally 75 mm in height and 37.5 mm in diameter.

3. Level the soil surface on the top and mount the container on the base of vane shear test apparatus using screws provided.

4. Lower the vane gradually into the soil specimen until the top of vane is at a depth of 10 -20 mm below the top of soil specimen.

5. Note down the reading of the pointer on circular graduated scale which is initial reading.

6. Rotate the vane inside the soil specimen using torque applying handle at a rate of 0.1°/sec.

7. When the specimen fails, the strain indicator pointer will move backwards on the circular graduated scale and at the point stop the test and note down the final reading of pointer.

8. The difference between initial and final reading is nothing but the angle of torque.

9. Repeat the procedure on two more soil specimens and calculate the average shear strength value.

10. Measure the diameter and height of vane using vernier caliper.

11. Sensitivity of given sooil sample is determined by repeating the above test procedure on remoulded soil which is nothing but soil obtained after rapid stirring of vane in the above test.

Sensitivity (St) = Undisturbed shear strength/Remolded shear strength.

Advantages Of Vane Shear Test

• This test is flexible.
• Vane shear test is quick and easy.

Disadvantages Of Vane Shear Test

• It cannot be conducted on the fissured clay.
• This test is not suitable for clays which contain sand or silt laminations in it.