1. Understanding Soil Mechanics: The Foundation of Every Structure
1. SOIL:
· unconsolidated material composed of solid particles, produced by disintegration of rocks.
· Void space between the particles may contain air, water or both. Also, may contain organic matter.
2. SOIL MECHANICS:
· Coined by Dr. Karl Terzaghi in 1925, who is also known as Father of Soil Mechanics.
· According to him, it is the application of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other consolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rock, regardless of whether or not they contain an admixture of organic constituents.
· In general, it is a branch of mechanics which deals with the action of forces on soil and with the flow of water in soil.
3. SOIL ENGINEERING:
· Applied science dealing with the applications of principles of soil mechanics to practical problems, and has much wider scope than soil mechanics.
· It includes site investigations, design and construction of foundations, earth retaining structures and earth structures.
4. 4. GEOTECHNICAL ENGINEERING:
· Includes: soil engineering + rock mechanics + geology.
· Synonymously used as Soil Engineering.
5. 5. Origin of Soil:
· Soil passing through this process for millions of years of geological time.
· Three phases: EROSION, TRANSPORTATION and DEPOSITION, EARTH MOVEMENT.
- Erosion Phase: cycle starts with erosional phase win which there is degradation of exposed rock by weathering processes (Physical or Chemical).
o PHYSICAL WEATHERING:
- Process may be: a) erosion of rocks caused by the action of wind, water, glaciers. b) disintegration caused by alternate freezing and thawing in cracks in the rock.
- Resulting soil particles retain the same composition as that of parent rock.
- Particles are generally of “bulky” form. Their shape be termed as angular, rounded, flat and elongated.
- Gravels and sand fall into this group.
- Structural arrangement of these are described as single grain. Each particle are in direct contact with adjoining particle, without their being any bond between them.
- State of these particles can be described as dense, medium dense or loose, depending on how these are packed together.
o CHEMICAL WEATHERING:
- Process results in changes in mineral form of parent rock due to the action of water (especially if contains acids or alkalies, oxygen and CO2).
- Results in formation of group of crystalline particles of < 2 microns known as clay mineral which have “plate like” form.
If the products of rock weathering are still located at the place where they originated, they are called residual soil, which are weakly bonded and have varying void ratio.
2. Transportation and Deposition Phase: In this phase, fragmented material is transported by agents such as wind, water, ice to new locations. Therefore, given term Transported soil-with small grain sizes and large amount of pores.
o According to the transporting agency, soils are classified as:
· Alluvial deposit: deposited by river water
· Lacustrine deposit: deposited by still water like lakes
· Marine deposit: deposited by sea water
· Aeolin deposit: transported by wind
· Glacial deposit transported by ice
6. 6. SOIL WATER RELATIONSHIP
· Soil mass, in general is a three-phase system composed of solid, liquid and gaseous phase.
· Different phases present in soil mass cannot be separated. For better understanding, all three constituents are assumed to occupy separate spaces as shown in figure below.
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· In phase diagrams, the weight of air is negligible as compared to soil solid and water and is thus assumed to be zero. The diagrammatic representation of the different phases in a soil mass is called the ‘phase diagram’, or ‘block diagram’. A three-phase diagram is applicable for a partially saturated soil (0 < S < 1)
· When all the voids are filled with water, the sample becomes saturated and thus the gaseous phase is absent; whereas, in oven dry soil sample the liquid phase is absent. Thus, in saturated and oven dry soils, the three phase system reduces to two phase system.
I
· It represents the relative ease with which the soil can be deformed.
· This term is mainly used for clayey soil and is related to water content i.e. how with change in water content the consistency of soil changes.
· Atterberg classified the consistency in 4-stages. Behaviour of soil is different in different stages.
o Solid stage
o Semi solid stage
o Plastic stage
o Liquid stage
When a clayey soil is mixed with an excessive amount of water, it may flow like a semiliquid.
If the soil is gradually dried, it will behave like a plastic, semisolid, or solid material, depending on its moisture content.
The moisture content, in percent, at which the soil changes from a liquid to a plastic state is defined as the liquid limit (LL). Similarly, the moisture content, in percent, at which the soil changes from a plastic to a semisolid state and from a semisolid to a solid state are defined as the plastic limit (PL) and the shrinkage limit (SL), respectively.
These limits are referred to as:
ATTERBERG LIMITS
- The liquid limit of a soil is determined by Casagrande’s liquid device (ASTM Test Designation D-4318) / Cone penetration and is defined as the moisture content at which a groove closure of 12.7 mm occurs at 25 blows. All soils will have similar shear strength (2.7kN/m^2) which is negligible.
- The plastic limit is defined as the minimum moisture content at which the soil crumbles when rolled into a thread of 3.18 mm in diameter (ASTM Test Designation D-4318). The shear strength of plastic limit is 100 times that of liquid limit.
- The shrinkage limit is defined as the maximum moisture content at which the soil does not undergo any further change in volume with loss of moisture (ASTM Test Designation D-427).
- Plasticity Index: Range of consistency (water content) within which soil behaves as a plastic materialThe difference between the liquid limit and the plastic limit of a soil is defined as the plasticity index (PI), or
PI =LL- PL : (If PI = 0 soil is non plastic).
- Shrinkage Index:The difference between the plastic limit and the shrinkage limit of a soil is defined as the Shrinkage index (PI), or
SI =PL- SL
- Liquidity Index:The difference between the liquid limit and the plastic limit of a soil is defined as the plasticity index (PI), or
LI = (w - PL)/(LL - PL)
Laboratory Test:· 1289.2.1.1: Soil moisture content tests—Determination of the moisture content of a soil—Oven drying method (standard method)
· 1289.3.1.1: Soil classification tests—Determination of the liquid limit of a soil—Four point Casagrande method
· 1289.3.2.1: Soil classification tests—Determination of the plastic limit of a soil—Standard method.
· 1289.3.6.1: Soil classification tests- Determination of the particle size distribution of a soil. Standard method of analysis by sieving.
· 1289.5.1.1: Soil compaction and density tests—Determination of the dry density/moisture content relation of a soil using standard compactive effort.
· 1289.5.2.1: Soil compaction and density tests—Determination of the dry density/moisture content relation of a soil using modified compactive effort.
· 1289.5.3.2: Soil compaction and density tests—Determination of the field dry density of a soil— Sand replacement method using a sand pouring can, with or without a volume displacer.
· 1289.6.1.1: Soil strength and consolidation tests—Determination of the California Bearing Ratio of a soil—Standard laboratory method for a remoulded specimen.
· 1289.6.4.1:Soil strength and consolidation tests—Determination of the compressive strength of a soil—Compressive strength of a saturated specimen tested in undrained triaxial compression without measurement of pore water pressure.
· 1289.6.7.1: Soil strength and consolidation tests—Determination of the permeability of a soil— Constant head method for a remoulded specimen.
· 1289.6.7.2: Soil strength and consolidation tests—Determination of the permeability of a soil-Falling head method for a remoulded specimen
"Now it's your turn! Do some research on the following questions and post your answers in the comment section below – let’s learn together!"
Q1. What are the different methods used for determining water content of soil in the laboratory? Briefly explain each one.
(Hint: Oven Drying, Torsion Balance/Moisture Meter, Sand Bath, Alcohol Method, Calcium Carbide Method)
Q2. Explain the Oven Drying Method step-by-step. Why is this considered the most reliable method?
Q3. What are the advantages and limitations of the Rapid Moisture Meter (Calcium Carbide Method) for water content determination on-site?
Q4. Which method would you recommend for organic soil and why?
Q5. List all the methods for determining specific gravity of soil solids. Which methods are suitable for fine-grained and coarse-grained soils?
(Hint: Density Bottle Method, Pycnometer Method, Gas Jar Method, Le-Chatelier Flask)
Q6. Describe the Pycnometer Method for specific gravity determination. Include the formula used and state the ideal soil type for this method.
Q7. Explain how the Density Bottle Method is performed and why temperature control is important in this process.
Q8. How is the Le-Chatelier Flask used for specific gravity? Where is it more commonly applied?
Q9. List and explain the methods for determining unit weight (bulk, dry, saturated, and submerged) of soil in the field and laboratory.
(Hint: Core Cutter Method, Sand Replacement Method, Water Displacement, Proctor Compaction Test)
Q10. Describe the Core Cutter Method step-by-step. For which types of soil is this method best suited?
Q11. How does the Sand Replacement Method work? Mention the significance of the calibration of the sand used.
Q12. Compare the Water Displacement Method with the Core Cutter Method for calculating unit weight. What type of sample and conditions does each method require?
Oven Drying: Soil dried at 105-110°C and weighed before/after drying. Most accurate but slow.
ReplyDeleteTorsion Balance/Moisture Meter: Uses a balance or meter to directly measure moisture loss. Quick but less accurate.
Sand Bath: Soil is heated in sand for uniform drying. Requires special equipment.
Alcohol Method: Alcohol replaces water before drying, useful for salty soils. Expensive and complex.
Calcium Carbide: Reacts with water to release gas, which is measured. Fast but less accurate.
Oven Drying Method (Gravimetric Method): Step-by-Step
ReplyDeletePreparation of the Sample:
Collect a representative soil sample, typically around 50-100g (depending on the test).
Remove any large stones, organic matter, or debris from the sample.
Weigh the Wet Sample:
Weigh the sample immediately after collection (before drying) using an accurate balance. Record the wet weight.
Place in the Oven:
Place the sample in an oven set at a temperature of 105-110°C. This temperature range is high enough to evaporate the water content but not to cause the soil to lose any of its chemical or physical structure.
Leave the sample in the oven for 24 hours or until it reaches a constant weight (usually checked every few hours after the first 24 hours).
Remove and Cool the Sample:
After the drying period, remove the soil sample from the oven using tongs or heat-resistant gloves.
Allow the sample to cool in a desiccator or at room temperature to avoid absorbing moisture from the air.
Weigh the Dry Sample:
Once cooled, weigh the sample again to determine its dry weight.
Calculate the Water Content:
The water content is determined using the formula:
Water Content (%)= Dry Weight x Wet Weight over Dry Weight
Accuracy: This method provides the most precise measure of soil moisture because it directly measures the loss of water by evaporation under controlled conditions.
Standardized: It’s a widely accepted and standardized method in soil science, ensuring consistency and comparability of results.
No Interference: Unlike methods like the calcium carbide or alcohol method, the oven drying technique doesn't rely on chemical reactions or potential interference from soil composition.
Reproducibility: Results can be easily repeated under the same conditions, making it the go-to method for validating moisture content in soils for most laboratory studies.
×100
Question 4
ReplyDeleteWhy Oven Drying for Organic Soils?
Organic soils (e.g., peat, muck, and other highly organic materials) contain a significant amount of organic matter that retains moisture differently compared to mineral soils. The Oven Drying Method is still considered reliable because it directly measures the water loss by evaporating moisture at a controlled temperature. However, organic soils may have:
Higher moisture content due to the nature of organic materials.
Higher water-holding capacity, which could make them more prone to decomposition when exposed to high temperatures for prolonged periods.
Challenges with Organic Soils in Oven Drying:
Volatility of Organic Matter: Some organic materials, when exposed to high temperatures (over 105°C), may decompose or lose volatile compounds, leading to inaccurate results.
Loss of Organic Matter: Overheating could cause a loss of organic matter, which could skew the weight difference used to calculate moisture content.
Recommended Adjustments:
Lower Temperature: For organic soils, you may choose to dry the sample at a slightly lower temperature, typically around 60-80°C. This can help avoid decomposition and loss of volatile organic compounds.
Shorter Drying Time: Reduce the drying time to minimize the risk of decomposing organic material while still ensuring most of the water content is removed.
Pre-treatment (Optional): In some cases, especially when dealing with very high organic content, you might use alcohol or sand bath methods before oven drying. The alcohol method can be particularly useful to replace water with alcohol, as alcohol evaporates more easily and doesn't affect the organic matter as much as high heat.
Alternative: Torsion Balance/Moisture Meter
Moisture meters or torsion balances may also be helpful for quick estimates of moisture in organic soils. They often provide non-destructive testing and don’t require high temperatures, thus preserving the organic content.
The Oven Drying Method is still reliable for organic soils, but be mindful of the temperature settings to avoid decomposing organic matter. If you need quicker results or want to avoid temperature-related issues, using a moisture meter or torsion balance might be a good alternative for non-invasive moisture estimation.
question 3
ReplyDeleteAdvantages:
Fast and convenient for on-site use.
Portable and doesn’t require heating or complex equipment.
Simple to operate with quick readings.
Limitations:
Less accurate than laboratory methods (e.g., oven drying).
Can be affected by soil composition.
Safety risks with handling calcium carbide.
Not suitable for large samples or extreme moisture levels.
Q1.
ReplyDelete1. Oven drying method: In oven drying method we take known weight of soil and place it in an oven for 24 hours or until it reaches a constant weight. We hen weigh the dried soil sample and calculate the water content as the difference between the initial weight (wet soil) and final weight (dry soil).
2. Torsion Balance / Moisture Meter Method: In this method A sample of soil is placed on a balance or meter that measures the weight loss as water evaporates. The moisture meter works based on the electrical resistance or capacitance, which changes with the soil's moisture content.
3. Sand Bath: A soil sample is placed in a container and placed in a sand bath where sand is heated upto 105°C and soil sample is dried in the sand bath. Weight difference is noted to find out moisture content.
4. Alcohol method: The soil sample is immersed in alcohol (which replaces the water). After the alcohol replaces the moisture, the sample is dried in an oven at a lower temperature (around 60°C) to remove the alcohol. Weight difference is noted to find out moisture content.
5. Calcium Carbide Method: A soil sample is placed in a sealed container with calcium carbide. The acetylene gas is generated as the calcium carbide reacts with the water in the soil. The amount of gas produced is measured and used to calculate the water content of the soil.
Q2.
Preparation:
Collect a representative soil sample.
Record the weight of the empty container or weighing dish (usually a metal pan or aluminum tray). This weight is called the tare weight.
Weighing the Wet Sample:
Place the soil sample (in the container) onto a balance and record the wet weight of the soil (soil + container).
Ensure that the sample is uniform and representative of the soil being tested.
Drying the Soil:
Place the soil sample in an oven set at a temperature of 105°C to 110°C.
Leave the sample in the oven for 24 hours or until it reaches a constant weight. The drying time may vary depending on the soil type and its moisture content.
Weighing the Dried Sample:
Once the drying period is complete, remove the container with the soil sample from the oven. Allow it to cool in a desiccator to prevent it from absorbing moisture from the air.
After cooling, weigh the container with the dried soil and record the dry weight.
It is the most reliable method because it has high level of accuracy and it does not include any chemical reactions.
Q1. Methods for determining water content of soil:
ReplyDeleteOven Drying: Dry sample at 105-110°C, calculate weight loss.
Torsion Balance/Moisture Meter: Measures moisture using weight change.
Sand Bath: Similar to oven drying, but uses heated sand.
Alcohol Method: Displaces water with alcohol, dries the sample.
Calcium Carbide: Reacts with water to measure moisture via pressure change.
Q2. Oven Drying Method:
Weigh wet sample.
Dry in oven at 105°C for 24 hrs.
Weigh dry sample.
Calculate moisture content.
Why reliable?: Most accurate, standard method, direct measurement.
Q3. Calcium Carbide Method:
Advantages: Fast, portable, simple to use.
Limitations: Less accurate, sensitive to soil composition, safety risks with calcium carbide.
Q4. Recommended Method for Organic Soil:
Oven Drying at lower temperature (60-80°C) to prevent decomposition.
Q5. Methods for specific gravity:
Density Bottle: Fine-grained soils.
Pycnometer: Fine-grained soils.
Gas Jar: Coarse-grained soils.
Le-Chatelier Flask: Fine-grained soils.
Q6. Pycnometer Method:
Fill pycnometer with soil and water.
Weigh and calculate displacement.
Formula:
𝐺
=
Weight of Soil
Weight of Displaced Water
G=
Weight of Displaced Water
Weight of Soil
Ideal Soil: Fine-grained soils.
Q7. Density Bottle Method:
Fill bottle with soil and water.
Weigh and calculate displacement.
Temperature control: Prevents density variation in water.
Q8. Le-Chatelier Flask:
Used for fine-grained soils. Measures volume displacement in water.
Q9. Methods for unit weight:
Core Cutter: Measures bulk density in the field.
Sand Replacement: Measures bulk density in the field.
Water Displacement: Measures volume and unit weight.
Proctor Compaction: Measures maximum dry density.
Q10. Core Cutter Method:
Insert cutter into soil.
Remove and trim excess soil.
Weigh wet sample.
Dry, weigh dry sample.
Best for: Clay, loam soils.
Q11. Sand Replacement Method:
Dig a hole, fill with calibrated sand.
Measure displaced sand.
Calibration: Ensures accurate sand volume for correct unit weight.
Q12. Water Displacement vs Core Cutter:
Water Displacement: Best for small, irregular samples.
Core Cutter: Best for cohesive, cylindrical samples in the field.
Q1.
ReplyDeleteOven Drying: The soil is weighed, dried in an oven at 105–110°C, and then weighed again to determine the weight loss and calculate the water content.
Torsion Balance/Moisture Meter: This device quickly measures the change in weight or electrical resistance to determine moisture content. It's convenient, but less accurate than oven drying.
Sand Bath: Soil is heated in a container submerged in hot sand to remove moisture. It is suitable for delicate soils. This method is slower than oven drying, but it prevents overheating.
Alcohol Method: Soil is soaked in alcohol to replace the water content, then dried and weighed. This method is ideal for clay or organic-rich soils but is more complex and requires special equipment.
Calcium Carbide Method: The soil reacts with calcium carbide to produce acetylene gas, and the pressure of the gas correlates with the soil's water content. This method is fast, but it requires specialized equipment and safety measures.
Q2.
Weigh the wet soil sample.
Measure its initial weight, including the moisture content.
Dry the soil in an oven.
Place the sample in a drying oven set to 105–110°C. Leave it to dry for 24 hours or until it reaches a constant weight.
Weigh the dried soil.
After drying, remove the sample and let it cool in a desiccator. Weigh the sample again to determine its dry weight.
Calculate the water content:
Subtract the dry weight from the wet weight, then divide by the dry weight. Multiply by 100 to express the water content as a percentage.
This method is reliable because it is accurate, standardized, and produces consistent results for various soil types.
Q3.
Advantages of the calcium carbide method:
It provides quick results (in minutes).
It is portable and ideal for field use.
Minimal sample preparation is required.
It is effective for high moisture soils.
It provides instant moisture measurement based on gas pressure.
Limitations:
Requires specialized equipment and safety precautions.
It is less accurate for low moisture content.
It is not suitable for all soil types, such as those with high organic content.
It needs regular calibration and is sensitive to temperature and pressure changes.
Q4
The Alcohol Method is recommended for organic soils because it prevents the decomposition of organic matter, which can occur with high temperatures in the oven drying method. Using alcohol instead of water and not heating preserves the soil's structure and avoids inaccuracies caused by thermal degradation. This makes the Alcohol Method ideal for accurately determining the moisture content in organic-rich soils, providing more reliable results than other methods, such as oven drying.
Q5
Methods like the Density Bottle, Pycnometer, Gas Jar, and Le-Chatelier Flask are commonly used to determine the specific gravity of soil solids. The density bottle, pycnometer, and Le-Chatelier flask are best for fine-grained soils, while the gas jar and Le-Chatelier flask are more suitable for coarse-grained soils.
Q6
The Pycnometer Method for determining specific gravity involves weighing a soil sample inside a pycnometer, then filling it with water and measuring the displacement.
Q7
The Density Bottle Method is used to measure the volume of water that is displaced by soil. To do this, you fill a calibrated bottle with a known weight of soil, and then add water to see how much is left in the bottle.
Q8
Fill Le-Chatelier flask with liquid and soil, measure displacement. Used mainly for coarse soils.
Q9
Core cutter and sand replacement are field methods. Water displacement and Proctor test are lab methods.
Q10
Drive core into soil, trim and weigh it. Best for soft, cohesive soils.
Q11
Dig hole, weigh removed soil, fill hole with sand. Sand must be calibrated to get accurate volume.
Q12
Core cutter is for soft soils in field; water displacement is for irregular lab samples. Each suits different conditions.
1. Methods for water content of soil
ReplyDeleteOven Drying: Soil is dried in oven at 105–110°C until weight is constant.
Torsion Balance/Moisture Meter: Quick method using an instrument that shows % moisture by drying soil with a lamp.
Sand Bath: Soil is dried on a sand bath heated over flame.
Alcohol Method: Soil is mixed with methylated spirit, burnt, and water evaporates.
Calcium Carbide Method (Rapid Moisture Meter): Soil mixed with calcium carbide; acetylene gas forms; pressure indicates water content.
2. Oven Drying Method (step-by-step)
Take wet soil sample and weigh.
Place in oven at 105–110°C for 24 hrs.
Take out, cool in desiccator, weigh dry soil.
Water content = (Weight of water ÷ Weight of dry soil) × 100%.
It is most reliable because all water is removed without losing solids.
3. Calcium Carbide Method (Rapid Moisture Meter)
Advantages: Quick, portable, can be used on-site.
Limitations: Not very accurate for organic soils or soils with gypsum; needs care in handling chemicals.
4. Method for organic soil
Oven drying not recommended (organic matter burns).
Recommended: Calcium Carbide Method or Alcohol Method, since they avoid burning organic matter.
5. . Methods for Specific Gravity
Density Bottle Method
Pycnometer Method
Gas Jar Method
Le-Chatelier Flask
Fine-grained soils → Density bottle, Pycnometer.
Coarse-grained soils → Le-Chatelier Flask, Gas jar.
6. The Pycnometer Method for determining specific gravity involves weighing a soil sample inside a pycnometer, then filling it with water and measuring the displacement.
7. Density Bottle Method
Take dry soil in small bottle, add water, weigh.
Compare with water only.
Temperature control important because water expands/contracts, affecting accuracy.
8. Le-Chatelier Flask
Used for coarse soils (sand, gravel).
Soil is put in flask with kerosene or water, volume change gives solids volume.
Commonly used in cement and coarse-grained soil tests.
9. Methods for Unit Weight
Core Cutter Method – small cylinder pressed into soil.
Sand Replacement Method – sand fills the hole left by excavation.
Water Displacement – irregular soil clod dipped in water, displacement gives volume.
Proctor Compaction Test – lab compaction to get unit weights.
10. Core Cutter Method
Drive a steel cylinder into soil.
Trim and remove cylinder with soil inside.
Weigh it, calculate volume (from cylinder).
Unit weight = weight/volume.
Best for soft to medium cohesive soils.
11. Sand Replacement Method
A hole is dug in ground, soil removed is weighed.
Hole is filled with calibrated sand of known density.
Volume = weight of sand used.
Unit weight = weight of soil / volume.
Calibration ensures sand density is correct for accuracy.
12. . Water Displacement vs Core Cutter
Water Displacement: For irregular clods of soil, volume found by water displaced. Works when soil cannot be cut neatly.
Core Cutter: For cohesive soils in field, needs undisturbed sample.
Water displacement works in lab; core cutter works in field.