Analysis Tool On Soil Liquefaction Potential Spreadsheet
Tag: Soil
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Principales And Practice Of Ground Improvement Free PDF
Principales And Practice Of Ground Improvement Free PDF
By Jie Han
Ground improvement is popular in many countries to solve difficult geotechnical problems, especially when construction necessarily occurs in problematic soils and under difficult geotechnical conditions.
Many recent developments in equipment, materials, and design methods have made ground improvement technologies more effective, efficient, and economic. However, the state of practice for most ground improvement technologies is that the practice is ahead of theory. Some contractors have developed their proprietary technologies, design methods, and construction techniques for their competitive advantages.
Most of the existing books on ground improvement are focused on the concept, application, and case study. However, few books have been devoted to the principles and design methods of ground improvement. This book covers both theoretical and practical aspects in the design and construction of a variety of ground improvement technologies commonly used in practice.
What is Frost Action In Soils?
What is Frost Action In Soils?
When the ambient temperature falls below freezing for several days, it is quite likely that the water in soil pores will freeze. Since the volume of water increases by about 10 percent when it freezes, the first problem is the increase in volume of the soil.
The second problem is that the freezing can cause ice crystals and lenses that are several centimeters thick to form in the soil. These two problems can result in heaving of the subgrade (frost heave), which may result in significant structural damage to the pavement.
In addition, the ice lenses melt during the spring (spring thaw), resulting in a considerable increase in the water content of the soil. This increase in water significantly reduces the strength of the soil, causing structural damage of the highway pavement known as “spring break-up.”
In general, three conditions must exist for severe frost action to occur:
1. Ambient temperature must be lower than freezing for several days.
2. The shallow water table that provides capillary water to the frost line must be
available.
3. The soil must be susceptible to frost action.
The first condition is a natural phenomenon and cannot be controlled by humans. Frost action therefore will be more common in cold areas than in warm areas if all other conditions are the same.
The second condition requires that the groundwater table be within the height of the capillary rise, so that water will be continuously fed to the growing ice lenses.
The third condition requires that the soil material be of such quality that relatively high capillary pressures can be developed, but at the same time that the flow of water through its pores is restricted.
Granular soils are therefore not susceptible to frost action because they have a relatively high coefficient of permeability. Clay soils also are not highly susceptible to frost action because they have very low permeability, so not enough water can flow during a freezing period to allow the formulation of ice lenses.
Sandy or silty clays or cracked clay soils near the surface, however, may be susceptible to frost action. Silty soils are most susceptible to frost action. It has been determined that 0.02 mm is the critical grain size for frost susceptibility.
For example, gravels with 5 percent of 0.02 mm particles are in general susceptible to frost action, whereas well-graded soils with only 3 percent by weight of their material finer than 0.02 mm are susceptible, and fairly uniform soils must contain at least 10 percent of 0.02 mm particles to be frostsusceptible.
Soils with less than 1 percent of their material finer than the critical size are rarely affected by frost action.
Current measures taken to prevent frost action, include removing frost-susceptible soils to the depth of the frost line and replacing them with gravel material, lowering the water table by installing adequate drainage facilities, using impervious membranes or chemical additives, and restricting truck traffic on some roads during the spring thaw.
Permeability of Soil: Definition, Darcy’s Law and Tests
Permeability of Soil: Definition, Darcy’s Law and Tests
Definition of Permeability:
It is defined as the property of a porous material which permits the passage or seepage of water (or other fluids) through its interconnecting voids.
A material having continuous voids is called permeable. Gravels are highly permeable while stiff clay is the least permeable, and hence such a clay may be termed impermeable for all practical purpose.
The study of seepage of water through soil is important for the following engineering problems:
1. Determination of rate of settlement of a saturated compressible soil layer.
2. Calculation of seepage through the body of earth dams and stability of slopes for highways.
3. Calculation of uplift pressure under hydraulic structure and their safety against piping.
4. Groundwater flow towards well and drainage of soil.
Darcy’s Law (1856) of Permeability:
For laminar flow conditions in a saturated soil, the rate of the discharge per unit time is proportional to the hydraulic gradient.
q = kiA
v = q/A = Ki … (7.1)
Where q = discharge per unit time
A = total cross-sectional area of soil mass, perpendicular to the direction of flow
i = hydraulic gradient
k = Darcy’s coefficient of permeability
v = velocity of flow or average discharge velocity
If a soil sample of length L and cross-sectional area A, is subjected to differential head of water h1 – h2, the hydraulic gradient i will be equal to [(h1 – h2)/L] and we have q = k. [(h1 – h2)/L].A.
When hydraulic gradient is unity, k is equal to V. Thus, the coefficient of permeability, or simply permeability is defined as the average velocity of flow that will occur through the total cross-sectional area of soil under unit hydraulic gradient. Dimensions are same as of velocity, cm/sec.
The coefficient of permeability depends on the particle size and various other factors. Some typical values of coefficient of permeability of different soils are given in Table 7.1.
Discharge Velocity and Seepage Velocity:
The total cross-sectional area of the soil mass is composed of sectional area of solids and voids, and since flow cannot occur through the sectional areas of solids, the velocity of flow is merely an imaginary or superficial velocity.
The true and actual velocity with which water percolates through a soil is called the velocity of percolation or seepage velocity. It is the rate of discharge of percolating water per unit of net sectional area of voids perpendicular to the direction of flow.
In accordance with the Darcy’s Law, the velocity of flow through soil mass is directly proportion to the hydraulic gradient for laminar flow condition only. It is expected that the flow to be always laminar in case of fine-grained soil deposits because of low permeability and hence low velocity of flow.
However, in case of sands and gravels flow will be laminar upto a certain value of velocity for each deposit and investigations have been carried out to find a limit for application of Darcy’s law.
According to researchers, flow through sands will be laminar and Darcy’s law is valid so long as Reynolds number expressed in the form is less than or equal to unity as shown below –
Where v = velocity of flow in cm/sec
Da = size of particles (average) in cm.
It is found that the limiting value of Reynolds number taken as 1 is very approximate as its actual value can have wide variation depending partly on the characteristic size of particles used in the equation.
Factors affecting permeability are:
1. Grain size
2. Properties of pore fluid
3. Void ratio of the soil
4. Structural arrangement of the soil particle
5. Entrapped air and foreign matter
6. Adsorbed water in clayey soil
4. Effect of degree of saturation and other foreign matter
k will decrease if air is entrapped in the voids thus reducing its degree of saturation. Percolating water in the field may have some gas content, it may appear more realistic to use the actual field water for testing in the laboratory.
Organic foreign matter also has tendency to move towards critical flow channels and choke them up, thus decreasing permeability.
5. Effect of adsorbed water – The adsorbed water surrounding the fine soil particle is not free to move, and reduces the effective pore space available for the passage of water.
Capillarity-Permeability Test:
The set-up for the test essentially consists of a transparent tube about 40 mm in diameter and 0.35 m to 0.5 m long in which dry soil sample is placed at desired density and water is allowed to flow from one end under a constant head, and the other end is exposed to atmosphere through air vent.
At any time interval t, after the commencement of the test, Let the capillary water travel through a distance x, from point P to Q. At point P, there is a pressure deficiency (i.e., a negative head) equal to hc of water.
If the coefficient of permeability is designated as ku at a partial saturation S, the above expression can be rewritten as –
In order to find the two unknowns k and hc in the above equation, the first set of observations are taken under a head h1. As the capillary saturation progresses the values of x are recorded at different time intervals t.
The values of x2 are plotted against corresponding time intervals t to obtain a straight line whose slope, say m, gives the value of [(x22 – x21)/(t2 – t1)] . The second set of observation are taken under an increased head h2 and values of x2 plotted against corresponding values of t to obtain another straight line, whose slope m2 will give the value [(x22 – x21)/(t2 – t1)].
By substitution in Eqn. 7.1, we obtain the following two equations, which are solved simultaneously to get k and hc.
The porosity n required in the above equation is computed from the known dry weight of soil, its volume and specific gravity of soil particles.
Permeability of Stratified Soil Deposits:
In general, natural soil deposits are stratified. Each layer may be homogeneous and isotropic. When we consider flow through the entire deposit the average permeability of deposit will vary with the direction of flow relative to the bedding plane. The average permeability for flow in horizontal and vertical directions can be readily computed.
Average Permeability Parallel to Bedding Plane:
Figure 7.9 shows several layers of soil with horizontal stratification. Let Z1, Z2, ….Zn be the thickness of layers with permeabilities k1, k2, … kn.
For flow parallel to bedding plane the hydraulic gradient i will be same for all layers. The total discharge through the deposit will be the sum of discharges through individual layers.
Average Permeability Perpendicular to Bedding Plane:
For flow in the vertical direction for the soil layers shown in Fig. 7.10.
In this case the velocity of flow, v will be same for all layers the total head loss will be sum of head losses in individual layers.
h = h1 + h2 + h3 + … + hn (i)
If kz denotes average permeability perpendicular to bedding plane, applying Darcy’s law, we have –
How to Test Compaction of Soil?
How to Test Compaction of Soil?
Compaction test is conducted in the laboratory to determine the relation between the dry density and the water content of the given soil compacted with standard compaction energy and to determine the OMC corresponding to the MDD.
The OMC obtained from the laboratory compaction test will help in deciding the amount of water to be used for compaction in the field. The MDD obtained from the laboratory compaction test helps in knowing the dry density achievable in the field compaction and also as a check for quality control.
Based on the compacted energy used in compacting the soil in the laboratory test, the laboratory compaction tests are of two types:
1. Standard Proctor test.
2. Modified Proctor test.
1. Standard Proctor Test:
In this test, the soil is compacted in three layers, each layer subjected to blows of a rammer with falling weight of 5.5 pounds (2.6 kgf) falling through a height of 12 in. in a cylindrical mold of internal diameter of 4 in. and effective height of 4.6 in.
The compaction parameters in a standard Proctor test are – W is the weight of rammer blow = 5.5 pounds, h is the height of fall = 12 in. = 1 ft, n is the number of blows per layer = 25, l is the number of layers = 3, and Vm is the volume of the mold = 1/30 cubic ft. The total compaction energy imparted on the soil per unit volume in this test is –
IS – 2720 (Part 7) – 1980 recommends the Indian Standard light compaction method based on standard Proctor test in metric system. The standard Proctor test or IS light compaction can be used as a criteria for the compaction of subgrades on highways and earth dams, where light rollers are used.
2. Modified Proctor Test:
Advances in construction technology resulted in the development of heavier rollers, which impart higher compaction energy during field compaction. To provide a laboratory control criterion for the higher compaction energy, the modified Proctor test was developed and standardized by the American Association of State Highway (and Transportation) Officials (AASHO or AASHTO) and by US Army Corps of Engineers for airfield construction.
In modified Proctor test, the soil is compacted in the same mold as in standard Proctor test, which has internal diameter of 4 in. and affective height of 4.6 in. giving a total internal volume of 57.805 cubic in. or 1/30 cubic ft. The rammer is bigger with a hammer of weight 10 pounds (4.5 kgf) falling through a height of 18 in. The soil is compacted in five layers, each layer being given 25 blows.
The compaction parameters in modified Proctor test are as follows – W is the weight of hammer blow = 10 pounds, h is the height of fall = 18 in. = 1.5 ft, n is the number of blows per layer = 25,l is the number of layers = 5, and Vm is the volume of the mold = 1/30 cubic ft.
The compaction energy imparted to the soil, per unit volume, in the modified Proctor test is:
Thus, the compaction energy in the modified Proctor test is (56250/12375) 4.55 times that in standard Proctor test.
If the soil fraction retained on 20 mm sieve is more than 5%, a bigger mold of 5.9 in. (15 cm) internal diameter and 5 in. (12.73 cm) internal height giving a total volume of 137.04 cubic in. or 1/12.611 cubic ft (2250 cm3) is used. When the bigger mold is used, the soil is compacted with 56 blows for each layer.
IS – 2720 (Part VII) – 1980 and Part VIII-1983 have recommended procedures corresponding to these two tests as follows:
1. IS light compaction test.
2. IS heavy compaction test.
1. IS Light Compaction Test:
The objective of the IS light compaction test is to determine the relation between the water content and the dry density of compacted soil and to determine the MDD and OMC from this test. The compaction energy used to compact the soil corresponds to that of standard Proctor test.
The compaction parameters in IS light compaction test are as follows – W is the weight of hammer blow = 2.6 kgf, h is the height of fall = 31 cm, n is the number of blows per layer = 25, and I is the number of layers = 3, and Vm is the volume of the mold = 1000 cubic centimeter (cc). The total compaction energy imparted on the soil in this test is –
E = Whnl = 2.6 × 31 × 25 × 3 = 6045 kgf cm
The total compaction energy imparted on the soil per unit volume in this test is –
E = 6045/1000 = 6.045 kgf cm/cm3
2. IS Heavy Compaction Test:
The IS heavy compaction test is similar to IS light compaction text except for the following differences:
1. A heavy rammer of 4.9 kgf falling weight that falls through a height of 45 cm is used for compacting the soil in IS heavy compaction.
2. The soil is compacted in five layers of equal thickness in the compaction mold.
3. The initial water content to be used in the first trial is 3%-5% for sandy and gravelly soils and 12%-16% below plastic limit for cohesive soils.
To increase the accuracy of the test results, it is desirable to reduce the increment of water in the region of OMC.