Category: soil

Dropping Weight Method -To Determine Soil Bearing Capacity

Dropping Weight Method to Determine Soil Bearing Capacity

We can determine the bearing capacity of the soil by using the following two methods.

1. Dropping Weight Method
2. Method of Slowly Applying Load

In this article, we are discussing the dropping weight method.

Dropping Weight Method

The dropping weight method is used to determine the approximate value of the soil-bearing capacity.

⇨At first, assume a weight W that is dropped from a certain height on the bottom surface of the foundation trench.

⇨Due to falling a weight, an impression made on the soil of the bottom surface of the foundation trench, which is noted.

Now, the bearing capacity of the soil can be calculated by establishing the following equation: Let,

• R = Ultimate resistance offered by the soil.
• h = The height from where weight is dropped and up to the bottom surface of the foundation trench.
• D = Depth of the impression made on the soil of the foundation trench.
• A = Cross-sectional area.

We know that the potential energy of the falling weight is equal to the energy lost in penetration.

So we can write, Wh = Rd Or, R =Wh/d

Now, safe bearing capacity of the soil per unit area equal to = R/Af = Wh/dAf

Where f is the factor of safety.

As it is an approximate method, it is used only for the construction of the minor structure.

How To Verify The Given Bearing Capacity of Any Soil

How To Verify The Given Bearing Capacity of Any Soil

We can use the “slowly applying load” method to verify the given bearing capacity of the soil.

⇒ Let’s assume the given safe bearing capacity of the soil is 16 t/m².

⇒ To verify the bearing capacity of this soil, add an initial test load on the platform. The initial test load should be twice the safe bearing capacity, i.e. 32 t/m², and write down the settlement.

⇒ When settlement stops, the load on the platform is increased by 50 per cent of the initial load, i.e. 16 t/m² and settlement is again noted.

⇒ Now, for both cases of loading, if the settlement does not exceed 17 mm and 25 mm respectively, So, we can say that the given safe bearing capacity of the soil is correct.

Purpose & Basic Principal of Soil Stabilization

Purposes of Soil Exploration

Dropping Weight Method -To Determine Soil Bearing Capacity

The Utility of Consolidation Data

The Utility of Consolidation Data

The consolidation data is used for the following purpose:

1. Safe engineering design of structures.
2. Forecasting of the probable settlement of a structure.
3. Solving of various problems connected with the settlement of structures due to consolidation.

9 Assumption in Terzaghi Theory of Consolidation

Terzaghi Theory of Consolidation

The theoretical concept of consolidation process was developed by Terzaghi. In order to compute the amount of consolidation( or settlement) after any given time t, Terzaghi derived the following basic equation is a free surface. δu/δt = cv (δu/δz) Where, cv = Coefficient of consolidation.

The above differential equation of consolidation describes the distribution of the hydrostatic excess pressure(u) with time(t) along with depth(z).

The Assumption in the Terzaghi Theory of Consolidation

In the development of the Terzaghi theory of consolidation, the following 9 assumptions are made:

1. The soil is fully saturated.

2. The soil is homogeneous.

3. The time lag consolidation is entirely due to the low permeability of the soil.

4. Soil particles and water are incompressible.

5. Darcy’s law is strictly valid.

6. Coefficient of permeability is constant during consolidation.

7. The soil is restrained against lateral deformation.

8. Excess pore water drains out only in the vertical direction.

9. The boundary is a free surface.

Assumption of Bernoulli’s Theorem

What is Coefficient of Consolidation, Compressibility & Volume change With Their Equation

Consolidation Settlement Analysis – Soil

Consolidation Settlement Analysis

In one dimensional consolidation, the change in the height(ΔH) (i.e settlement) per unit of original height(H) equals to the change in volume(ΔV) per unit of the original volume(V).
ΔH/H = ΔV/V ………….(1).

 Consolidation Settlement (Settlement Analysis)

Now, at an initial condition, void ratio(e0) = Vv/Vs Or Vv= e0.Vs

Where Vv and Vs are the volume of voids and volume of soil solids respectively.
But, V = Vv + Vs
Or, V = e0.Vs+ Vs
Or, V = Vs(1+e0)

At compressed condition, void ratio(e) = v/Vs
Where V՛v is the volume of voids at compressed condition.
Now, V՛ = Vs(1+e)

Where V՛ and e are the volume of soil sample and void ratio respectively at the compressed condition.

Therefore, change in volume(ΔV) = V – V՛
Or, ΔV = Vs(1+e0) – Vs(1+e)
Or, ΔV = Vs(e0-e)
Or, ΔV = Vs.Δe Where [Δe (e0-e)]
Now, ΔV/V = Vs.Δe/V
Or, ΔV/V = Vs.Δe/Vs(1+e0)
Or, ΔV/V = Δe/(1+e0) ……………(2).

From compression curve of e versus Log p we get
Compression Index(Cc) = Δe/ Log10 (p/p0)
Or, Δe = CLog10 (p/p0) ………………(3).

Where p0 is the initial pressure and p is the pressure at the compressed condition.Now, from equation (1), (2), and (3) we get
ΔH/H = ΔV/V
Or, ΔH = (ΔV/V).H
Or, ΔH = {Δe/(1+e0)}.H
Or, ΔH = {CLog10 (p/p0)/(1+e0)}.H

The Equation of The Consolidation Settlement

• ΔH = {CLog10 (p/p0)/(1+e0)}.H

What is the Coefficient of Consolidation, Compressibility and Volume change With Their Equation

Coefficient of Consolidation

The term coefficient of consolidation($C_{v}$) is used to indicate the combined effects of the permeability and compressibility of soil on the rate of volume change.

It is the ratio of the coefficient of permeability(K) to the product of the coefficient of volume compressibility($m_{v}$) and unit weight of water( $\gamma _{w}$).

Equation of The Coefficient of Consolidation

•  $C_{v} = \frac{K}{m_{v}.\gamma _{w}}$

The unit of coefficient of consolidation( $C_{v}$ ) is cm²/Sec.

Coefficient of Compressibility or Coefficient of Compression(av)

Coefficient of Compressibility is defined as the decrease in void ratio per unit increase in pressure.

Equation of The Coefficient of Compressibility

•   $a_{v}$ = -Δe/Δp (negative sign indicate the decrease in void ratio with an increase in pressure).

It is a measure of compression of the soil mass. The unit of Coefficient of Compressibility($a_{v}$) is cm²/gm.

Coefficient of Volume Change or Coefficient of Volume Compressibility

The term coefficient of volume change or the coefficient of volume compressibility($m_{v}$) is defined as the change in volume of a soil mass per unit of initial volume due to an increase in pressure.

Equation of The Coefficient of Volume Change

• Mathematically, $m_{v} = \frac{a_{v}}{(1+e_{0})}$)

where $e_{0}$ is the initial void ratio.

What is Degree Of Compaction, Optimum Moisture Content & Compacting Effort -Soil

What is Porosity, Void Ratio, Water or Moisture Content, And Degree of Saturation of Soil

Define Specific Gravity,  Bulk Specific Gravity, Air Content and Percentage Air Voids-Soil

Quantity Of Soil Sample Required For Different Test

The following tests and required sample weight are:

i) Soil identification, natural moisture content test, mechanical analysis and index properties chemical test

⇛ For Cohesive Soils – Required sample weight is 1 kg

⇛ For Sand and gravels – Required sample weight 3 kg

ii) Compaction tests

⇛ For Cohesive Soils and Sands – 12.5 kg

⇛ For Gravelly Soils – 25 kg

iii) Comprehensive examination materials including soil stabilization

⇛ For Cohesive Soils and Sands – 25 to 50 kg

⇛ For Gravelly Soils – 50 to 100 kg

Types of Soil Sample

Different Grading Zone Value of Fine Aggregates(sand) As Per IS: 383

3 Types of Soil Samples – Undisturbed, Disturbed and Non representative Soil Sample

3 Types of Soil Samples

The following are the 3 types of samples which can be obtained with different sampling devices:

1. Undisturbed Soil Samples

The soil sample, in which the particle size distribution as well as the soil structure and the properties of the in situ stratum, remain preserved, is termed as undisturbed soil samples. Such soil samples are required for shear strength and consolidation tests.

This soil samples can be collected by stopping the boring process at a certain level and then inserting the appropriate sampler below the bottom of the bore.

2. Representative or Disturbed Soil Sample

The soil sample which contains the same particle size distribution as in the in situ stratum, but the natural structure of sample gets partly or entirely disturbed and modified, is called a representative or disturbed soil sample.

Such disturbed soil samples can be used for

1. Identification of soil types
2. Determining Atterberg limits, specific gravity, organic and carbonate content.
3. Compaction tests etc.

3. Non-representative Soil Sample

These are mixtures of soil from different soil strata. These samples are obtained by auger boring or sedimentation of wash boring. Such samples may help in determining the depth at which major changes in soil profile occur.

Unified Soil Classification

Textural Classification of Soil

Particle Size Classification System Of Soil

8 Causes of Settlement of Foundation

Settlement of Foundation

The settlement of the foundation can be defined as the reduction in the thickness of soil mass or as the vertical downward movement of the foundation base of a structure into the soil mass.

8 Cause of Settlement of Foundation

Settlement may be due to one or more following factors
1. Due to external static loads from structures.
2. Due to the self-weight of the soil such as recently placed fills.
3. Due to the lowering of the groundwater table.
4. Due to the flowing or slipping of soil particles.
5. Due to the excessive loading on the adjacent foundation.
6. Due to the excessive surrounding soils.
7. Due to the impact effect of vibration.
8. Due to the failure of base concrete etc.

Types of Foundation Settlement

Field Measurement of Shear Strength of Cohesive Soil By Vane Shear Test

Vane Shear Test On Soil

Vane shear test is a simple and quick test, used either laboratory or in the field.

Aim of Vane shear test

To determine the undrained shear strength of cohesive soil.

Testing Equipment For vane shear Test

⇰ A vane shear testing equipment consists of four thin plates, called vanes, welded orthogonally to a steel rod.

⇰ A torque measuring arrangement, such as a calibrated torsion spring is attached to the rod. The steel rod is rotated by worm gear and worm wheel arrangement.

The procedure of Vane Shear Test

⇰ A pit is first of all made into the ground up to a depth at which the shear strength is to be tested.

⇰ The vane tester is then pushed or driven carefully into the soil below the bottom of the pit, to a depth somewhat greater than the length of the vane.

⇰ The torque rod is now rotated (usually 1° per minute). The rotation is continued till the soil falls, which will be indicated by the sudden decrease of torque without any back movement of torque wheel.

⇰ The maximum torque (T) given till failure is measured and recorded. The shear strength (𝛕f) is calculated by using the following equation;

𝛕f = 2T / πd2[ H + d/3]

Where, H = Height of the vane And, d = Diameter of the vane.