Phase Relationships of Soil

Basic soil properties and parameters can be subdivided into physical, index, and engineering categories. One of the youngest disciplines of civil engineering, soil mechanics, involves the study of soil, its behaviour and its application as an engineering material. In soil, there are three kinds of standard composition: solid, liquid and gas.

Terzaghi (1948) once said, “Soil Mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they contain an admixture of organic constituent.”

The Three Phases of Soil

soil is generally a three-phase material which contains solids (soil particles), water (in liquid state) and air (in gas state). Within the soil, it contains solid particles and voids, and whereby the voids contain water and/or air. The phase diagram that contains all three states is called partially saturated (unsaturated soil), while the diagram with solids and water is called fully saturated (wet soil condition) and lastly, dry soil with soil solids with lots of voids.

The soil's physical properties generally include mass density, particle sizes, specific gravity, and water content. Within these physical properties, these are the essential terms used in soil mechanics (as below):

• The water content of a soil sample represents the weight of free water contained in the sample expressed as a percentage of its dry weight.
• Degree of saturation of the soil sample is the ratio, often expressed as a percentage, of the volume of free water, contained in the sample to its total volume of voids and has an important influence on soil behaviour.
• Void ratio to the volume occupied by the soil particles defines the void ratio, in other words the volume of voids in a mixture divided by the volume of solids.
• Porosity, which is a measure of the relative amount of openings and voids (air or gases), is the ratio of void volume to the total volume of soil, and it represents the storage capacity of the geologic material.
• Specific gravity of a substance is a comparison of its density to that of water.

The Phases Relationship in Soil

For defining purposes of the soil physical and index properties, it is more convenient to represent the soil components (in physical forms) by projecting it into a block or phase diagram (as figure above). In soil mechanics, the unit weight of soil varies in which depending on the amount of water contained in the soil. This is often known as the relationship of weights (W) and volumes (V) in soil.

Notes and Legends:

1. Specific gravity, G_s = 2.67 +/- 0.05 for inorganic soils.
2. Unit weight of water = 62.4 lb/ft³ or 1001 kg/m³ for freshwater and 64.0 lb/ft³ or 1026.7 kg/m³ for seawater.

• W_total, W_t = Total weight of soil mass
• W_solids, W_s = Dry weight of soil mass
• W_water, W_w = Weight of water in soil mass
• V_total, V_t = Total volume of soil
• V_solids, V_s = Volume of solids in soil mass
• V_voids, V_s = Volume of voids in soil mass
• V_water, V_w = Volume of water in soil mass

The Phases Relationship in Terms of Formulation

The phase relations in soil materials, between weight and volume can be further derived and then produced above formulas.

Further Formulation of The Phases Relationship in Soil

The series of soil weight-volume relationships is formulated in three categories: moisture, dry and saturated. Hmm, life could be easier after this…for soil mechanics…

In conclusion, these are the basic fundamentals in terms of the relationship between the soil and engineering materials acquired through soil mechanics subject.

Particle Distribution for Soil Test

For classification of soil for engineering purposes, we oath to know the distribution of the grain sizes in any given soil mass especially the one obtained from the construction site or burrow pits. Particle size distribution test, also known as sieve analysis test is a method used to determine the grain (granular) size distribution of soil samples.

The sieves are normally made of woven wires with square openings and steel body frames. It has different numbers which respect to the opening sizes. BS Sieve Aperture and ASTM Sieve Aperture sizes are mostly the same especially from 4.75 mm to 63 μm, and slightly different from 75 mm to 6.3 mm.

The Objective and Scope of Test

The sieve analysis (grain size analysis) is widely used in the classification of soils. The data obtained from grain size distribution curves is used in the design of filters for earth dams and to determine the suitability of soil for road/highway construction, embankment fill of dam, airport runway/taxiway, etc. The information that we obtained from sieving test could be used to predict soil water movement although permeability tests are more generally used.The objective of this test is to determines the relative proportions of different granular sizes as they are passing through certain sieve sizes. Thus, the percentage of sand, gravel, silt and clay can be obtained from the sieve analysis test.

The Technical Standards
The sieve analysis of soil test is accordance to ASTM D-422 (American Society for Testing and Materials) or BS 1377: Part 2 1990 (British Standards) as both are the most widely used technical standards in construction. The dry sieving of soil is the simplest and cheapest method among others.

The Required Apparatus

As per figure above:

• Stack of Sieve Aperture sizes (including the cover and pan)
• Electronic Balance (decimal reading to 0.01 g)
• Rubber pestle, mortar (for crushing the soil if lumped), and brush
• Mechanical sieve vibrator (shaker)
• Oven Dry (thermostatically controlled temperature)

The Test Procedure / Method
Here is how we going to do it:

1. Take out the dried soil samples from the oven dry and weighs about 500 g (normal amount used for any soil samples the greatest particle size of which is 4.75 mm).
2. The dried soil particles should be first crush (in lumped) using the rubber pestle and mortar.
3. Determine the mass of sample accurately and label as W_total (in g).
4. Then prepare a stack of sieve aperture sizes with larger opening sizes of sieve at the top (having lower number) and down to the last one with smaller opening sizes (having higher number). Not forgetting the sieve pan underneath and cover on top.
5. Weigh all sieves and the pan separately if necessary (mostly neglected).
6. Pour the soil slowly into the stack of sieves from the top and place the cover, put the stack onto the sieve shaker (vibrator), tighten the clamps, adjust the time within 5 to 10 minutes and turn it on…shake it baby…
7. When times out, take it out and measure the mass of each sieve aperture + retained soil inside, from the top sieve until the pan. This procedure should be done carefully…one by one…
8. Record down the weight in the result sheet and ready for calculation and plotting of graph.

The Calculations
Both calculation methods taken into account with an example of test sheet result:-

Calculation following BS 1377: Part 2 1990:

Calculation following ASTM D-422:

The Results Documentation
Draw graph of log sieve size vs % finer. The graph is known as grading curve. Corresponding to 10%, 30% and 60% finer, obtain diameters from graph these are D10, D30, D60, using these obtain Cc and Cu which further represent how well the soil is graded i.e whether the soil is well-graded, gap-graded or poorly graded.

The Graphs
Graph for BS 1377: Part 2 1990:

Graph for ASTM D-422:

US Standard Sieve Sizes

Referring to the graph;
Uniformity Coefficient, C_u = D₆₀ / D₁₀ = 0.9 / 0.16 = 5.625

Coefficient of Gradation, C_c = (D₃₀)² / (D₆₀ x D₁₀) = (0.37)² / (0.9 x 0.16) = 0.95

Things to Remember

A few thing remember during the sieve analysis testing:

• Make sure the sieve aperture in dry condition and properly cleaned from any particles by poke them out using brush before commencing the test.
• Make sure to double check the stack of sieve aperture sizes arrangement in order before shaking begins.
• Make sure the balance have an adequate battery for a long run (if lots of soil sample to be test).
• The sieve shaker should be in good condition as well for a long run.
• The oven-dry and the balance calibration certificate still valid (haven’t due yet) for an accurate results.
• Do not shake the soil sample with the shaker for too long as the finer particles could easily lost. For more accurate results especially doing some research or independent lab test, manual approach is recommended.

The sieve analysis of soil test above is applicable not only to soil samples but can be tested upon aggregate, cement, and sand samples. The procedure would be the same as well as the calculation method and the graph plotting. The particle distribution test would eventually allows the grading of soil particles. Happy testing…

Foundation In GeoTechnical Perspective

E

very structure that sits on the Earth's surface requires a foundation. The functionality of the foundation is to transfer or transmit the building structural loads (dead and live loads) equally and safely on the ground. A larger and heavier concrete, steel, or masonry building would need its foundations, which when deeper inside the ground until it reaches the hard ground or bedrock. This is an essential method for carrying the building's massive loading because the variety of soil, rock, and water conditions that soon meet below the surface of the ground is unpredictable. The highly acclaimed knowledge in understanding the ground condition and foundation design is a highly specialised field of geotechnical engineering.

Foundation Systems

Distribute vertical loads so the settlement of a building is either negligible or uniform under all parts of the building, The foundation system for any structure is the critical link in the transmission of its loading down to the ground (on the surface or beneath the surface). With the load bearing directly on the soil, the foundation system must:

• Relatively high stresses in the superstructure have to be safely transferred to the much softer and weaker soil,
• Anchor the building’s superstructure to prevent uplifting due to wind and earthquake forces,
• The best solution would be to place the supports of the structure on solid rock, but this is seldom possible,
• In most cases, solid rocks or bedrock lies deep inside the ground, with softer and weaker soil layers above it.

The most critical factor in determining the foundation system of a building is the classification and bearing capacity of the soil.

Loading and Settlements of Foundations:

• Types of loads: Dead, live, inclination thrusts and uplift, water table, wind and earthquake forces.
• Types of settlements: Uniform and differential (Differential settlement must be minimized, depends on the soil conditions at site and distribution of loads on columns which supporting the structure).
• Requirements of safety: Structure-foundation system safe against settlements that would lead to collapse (Foundation settlement should not damage the structure and must be technically and economically feasible).

Types Of Foundation

1) Shallow Foundation System

Shallow Foundation, Spread Footing in construction...

Shallow foundations system normally located just below the lowest part of the structure, which means that it is placed relatively close to the surface of the ground. The loads were transferred from the building to the soil by providing a large enough area of the foundation to reduce pressure below the ones allowed by the strength of the soil. This will prevent an excessive settlement and bearing failure of the structure.

Types: Spread Foundation and Mat / Raft Foundation

2) Deep Foundation System

Deep Foundation, Caisson Pile in construction...

In the case of deep foundations, the means of support is usually a drilled shaft, a group of piles or a pier. Significant buildings in areas underlain with thick, cohesive soil deposits carry the loads vertically to more competent strata or bedrock primarily to control settlement or gradually transmit the load to the soil by friction and at a greater depth below the structure.

Types: Pile, Pile walls, Diaphragm wall, and Caissons pile

Advantages of Deep foundation:

• Cost (at affordable price)
• Construction Procedure (simple to follow)
• Material (mostly using concrete)
• Labor (doesn’t need any expertise)

Designing A Foundation:

• Information on the Working Loads – get from Structural Engineers or Architects
• Information on Sub-surface conditions – get from Site Investigation report
• Established Design Criteria
• Foundations must be designed to satisfy 3 general criteria:

It must be located properly so as not to be adversely affected by outside influence,

It must be safe from bearing capacity failure,

It must be safe from excessive settlement.

Factors Affecting Foundation Choice

Primary Factors Affecting Foundation Choice:

• Sub-surface soil
• Ground water table conditions
• Building structural requirements

Secondary Factors Affecting Foundation Choice:

• Construction access, methods and site conditions
• Environmental factors
• Building Codes and Regulations
• Impact on surrounding structures
• Construction schedule
• Construction risks

Depth and Location of Foundations

The depth and the location of foundations are dependent on as follows:

Significant Soil Volume Changes:

• Some soils shrink and swell significantly upon drying and wetting respectively,
• The specific depth & volume change relationship for a particular soil is dependent on the type of soil and level of groundwater,
• Volume change is usually insignificant below a depth of 1.5 to 3.0 m and does not occur below the Ground Water Table (GWT),
• In general, the soil beneath the centre of a structure is more protected from sun and precipitation hence moisture content changes and resulting soil movement are relatively more minor.

Adjacent Structures and Property Lines:

• Existing structures may be damaged by the construction of new foundations nearby,
• After new foundations have been constructed, the load that they place on the soil may cause settlement of existing structures,
• Damage to existing structures by new construction may result in liability problems. Thus new structures should be located and designed very carefully,
• In general, the deeper of the new foundation and the closer to the older structure, the greater will be the potential for damage and movement,
• A general rule is that a straight line drawn downward and outward at a 45^o angle from the end of the bottom of any higher footing should not intersect any existing footing,
• As a footing is wider than the building it supports, part of the footing may extend across the property line and may encroach on adjacent land.

Groundwater:

• Presence of groundwater near a footing is undesirable because:
• Footing construction below GWT is difficult and expensive,
• Groundwater around a footing can reduce the strength of soils,
• It may also cause hydrostatic uplift,
• Frost action may increase,
• Waterproofing problems.

Underground Defects:

• This includes faults, caves and mines,
• Human-made discontinuities such as sewer lines, underground cables and utilities must be considered,
• Structures should never be built on or near tectonic faults (plate movements) that may slip,
• A survey upon the underground utility lines should be made before any excavation in order to avoid damages to utilities during excavation.

Type of Soils and Characteristics:

• Soils (particulate earth material): Boulder (too large to be lifted by hands), cobble (a particle that can be lifted by a single hand), gravel aggregates (course-grained particle larger than 6.4mm), sand (frictional, size varies from 6.4 to 0.06mm), silts (frictional, low surface area to volume ratio, size varies from 0.06 mm to 0.002mm) and clays (cohesive – fine-grained – high surface area to volume ratio, a size smaller than 0.002 mm)
• Rocks: Broken into regular and irregular sizes by joints
• Peat: Soils not suitable for foundations of any structure

Problems due to Settlement can Arise when:

• Soil property changes at different points under the same structure,
• When construction of the building proceeds fast (mostly in modern times cases),
• When an additional heavy load (e.g. a tower in old times – Pisa) is added to stabilise it,
• Groundwater is pumped out; Notorious instances examples like Venice and Mexico City tragedy.

The Conclusions

The foundation type is selected in consultation with the geotechnical engineer. The factors to be considered are the soil strength, the soil type, the variability of the soil over the area and with increasing depth, and the susceptibility of the soil and the building to deflection. I consider the foundation the most critical part of any structure that requires very careful structuring before the next stage can be done…

New Force Driving Earth's Tectonic Plates

Reconstruction of the Indo-Atlantic Ocean at 63 million years, during the time of the superfast motion of India which Scripps scientists attribute to the force of the Reunion plume head. The arrows show the relative convergence rate of Africa (black arrows) and India (dark blue) relative to Eurasia before, during and after (from left to right) the period of maximum plume head force. The jagged red and brown lines northeast of India show two possible positions of the trench (the subduction zone) between India and Eurasia depending on whether the India-Eurasia collision occurred at 52 million years or 43 million years. (Credit: Scripps Institution of Oceanography, UC San Diego)

Science Daily  — Bringing fresh insight into long-standing debates about how powerful geological forces shape the planet, from earthquake ruptures to mountain formations, scientists at Scripps Institution of Oceanography at UC San Diego have identified a new mechanism driving Earth's massive tectonic plates.

Scientists who study tectonic motions have known for decades that the ongoing "pull" and "push" movements of the plates are responsible for sculpting continental features around the planet. Volcanoes, for example, are generally located at areas where plates are moving apart or coming together. Scripps scientists Steve Cande and Dave Stegman have now discovered a new force that drives plate tectonics: Plumes of hot magma pushing up from Earth's deep interior. Their research is published in the July 7 issue of the journalNature.

Using analytical methods to track plate motions through Earth's history, Cande and Stegman's research provides evidence that such mantle plume "hot spots," which can last for tens of millions of years and are active today at locations such as Hawaii, Iceland and the Galapagos, may work as an additional tectonic driver, along with push-pull forces.

Their new results describe a clear connection between the arrival of a powerful mantle plume head around 70 million years ago and the rapid motion of the Indian plate that was pushed as a consequence of overlying the plume's location. The arrival of the plume also created immense formations of volcanic rock now called the "Deccan flood basalts" in western India, which erupted just prior to the mass extinction of dinosaurs. The Indian continent has since drifted north and collided with Asia, but the original location of the plume's arrival has remained volcanically active to this day, most recently having formed Réunion island near Madagascar.

The team also recognized that this "plume-push" force acted on other tectonic plates, and pushed on Africa as well but in the opposite direction.

"Prior to the plume's arrival, the African plate was slowly drifting but then stops altogether, at the same time the Indian speeds up," explains Stegman, an assistant professor of geophysics in Scripps' Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics. "It became clear the motion of the Indian and African plates were synchronized and the Réunion hotspot was the common link."

After the force of the plume had waned, the African plate's motion gradually returned to its previous speed while India slowed down.

"There is a dramatic slow down in the northwards motion of the Indian plate around 50 million years ago that has long been attributed to the initial collision of India with the Eurasian plate," said Cande, a professor of marine geophysics in the Geosciences Research Division at Scripps. "An implication of our study is that the slow down might just reflect the waning of the mantle plume-the actual collision might have occurred a little later."

Funding for the research was provided by the National Science Foundation