SOIL

Soil and its Origin• Soil is one of the most important material that influenced mankind in his struggle for survival from the earliest times up to the present. It is the material on which we grow our food, build our homes, etc.

• In this studies, the soil referred to is the material used to build with or on, that acts in combination with other forces of nature to make structures and landforms, that which is classified under the field of Geotechnics.

• Soil maybe defined as the accumulation of unconsolidated sediments and deposits of solid particles as a result from the disintegration of rock.

• Rock defined as a natural aggregate of minerals that are connected by strong bonding of attractive forces considered as consolidated materials.

Types of Rock

1. Igneous rock is the result from the cooling and hardening of molten rock called magma, which has originated from deep within the earth. The rapid cooling of molten magma called lava which was blown up by volcanoes solidify into small crystals and possessed a fine interlocking texture.

2. Sedimentary rocks are formed from accumulated deposits of soil particles or remains of certain organisms that have hardened by pressure or cemented by minerals.

Examples:

Limestone Dolostone

Shale Conglomerate

3. Metamorphic rocks are rock resulting from metamorphism due changes in temperature, pressure and plastic flow, changing the original rock structure and mineral composition of the rock.

Marble SlateQuartzite

• There are numerous types of rocks to enumerate, but, the construction industry and Engineers concerned are not generally affluent with the name of rocks but with their properties, especially in-place properties such as hardness, strength, durability, soundness and all other properties needed to build on or with.

Categories of Soil1.Residual soil (Sedimentary soil) is

formed from weathering (mechanical or chemical) of rock or accumulation of organic materials, and remain at the location of their origin.

2. Transported soils are those materials transported from their place of origin.

Soil Types• The major categories of soils are gravel,

and, silt and clay. Gravel and Sand are universally considered as coarse grained soil because of their individual particles that are large enough to distinguish without magnification. On the other hand, Silt and Clays are considered as fine grained soil because of their tiny particles.• Clay soil is plastic. It is sticky and when

dried possesses very strong resistance to crushing.• Silt soil possesses little or no plasticity

and when dried has very little or no strength at all.

• Dilatancy is a phenomenon wherein a small amount of moist silt sample is shaken on the palm of the hand, water will appear on the surface of the sample, but disappear when shaking stops.

• Silty sand is a material that is mostly sand but includes silt.• Silty clay is a material that is mostly

clay but includes silt.

Clay Silt

Characteristics of Soil1. The grain size.

2. Grain shape

3. Surface texture and electrical surface charges resulting from the chemical composition and molecular structures.

General Characteristic Classification of Soil Particles

1. Gravel

Consist of rock fragments more or less rounded by water action or abrasion.

Gravel Typesa. Quartz – the hardest of the common

rock forming mineral.b. Rounded pebbles and boulderc. Slightly worn gravel (include other

materials or rocks such as granite, schist, basalt or limestone.)

2. Coarse Sand

Is usually rounded like gravel with which it is found and generally contains the same particles.

3. Fine Sand

Has particles that are more angular than the coarse and fine sand particles.

4. Silt Grain

Is usually similar to fine sands and has the same mineral composition. They are found as rock flour in glacial moraines. Silt contains:

a. Pumiceb. Loessc. Materials foreign to the

associated sand.

5. Clay

Is plate like, scale like, or rod like in shape as a result from chemical weathering.

6. Colloidal Clay

Are finer clay particles that remain suspended in water and do not settle under the force of gravity.

As a rule of thumb: “Soils rich for crop growing are usually poor for road building”.

Classification for SoilsPedology is the science of soils. It is

the basis for the pedological classification, of which the principle behind is that like soils are developed on like slope when like materials are weathered in like fashion.

Under the pedological classification, soils produced in like fashion from the same parent rock is assumed to possess similar engineering properties that also requires similar engineering treatment.

Three Textural Classification of Soil1. Sand 2. Silt 3. Clay

Primary Classification Tools1. Mechanical Analysis 2. Liquid and Plastic Limit Test

Soil CompositionEngineers are interested to know the

properties of soil deposits, especially the strength and compressibility since they are necessary in determining the bearing capacity and stability of the foundations. Strength and compressibility of the soil are directly related to:

1. Soil density weight per unit volume

2. Water content of the soil3. Void Ratio5. Degree of saturation

FIGURE: Relationship between volume and weight/mass of a soil bulk

The Relationship between Mass Volume and Weight Volume

From the above figure, the total weight of the soil volume is: weight of the solid (Ws) plus weight of the water (Ww) is equal to the total weight (WT) thus:

WT = Ws + Ww

Similarly, for the measurement of the mass:

Total Mass = Mass of Solid + Mass of WaterMT = Ms + Mw

Therefore, the total volume of the soil sample or bulk includes the solid or liquid or air or gas. The volume by water and air may collectively be called the Volume of Voids, thus

VT = Vs + (Vw + Va)

=Volume of Solid + Volume of voidsVT = Vs + Vv

The relationship between the weight and volume for any material will be

W = V x Gs x UW

While the Mass and Volume will be: M = V x Gs x DW

Where:W – Weight of the material (solid, liquid or gas)V – Volume occupied by the materialg – Specific gravity of the materialUW – Unit weight of the water at temperature stated

UW = 62.4 lb/ft3 = 980 dynes/cm3 = 9.81 KN/m3

DW – Density of water (1 g/cm3 or 1000 kg/m3 or 1.95 slugs/ft3)

For soil mechanics:Ws = Vs x Gs x UW

Ms = Vs x Gs x DW

Ww = Vw x Gw x UW = Vw x UW since Gw = 1

Mw = Vw x Gw x DW

• The specific gravity of most commonly occurring rock or soil materials is between 2.30 and 3.10.• The specific gravity of soil solids lies within

the range of 2.60 to 2.75.

The unit weight of a soil U could be expressed as

Wet Unit Weight (Uwet) = WT/VT

Dry Unit Weight (Udry) = Ws/VT

For soil density:Wet Density (Dwet) = MT/VT

Dry Density (Ddry) = Ms/VT

Water content (w) is the ratio of the weight of water in a soil volume to the weight of soil solids, or the mass of water in a soil to the mass of solids, thus:

w = x 100% or x 100%

Where:w - Water content in percentageWw – Weight of waterWs – Weight of dry soilMw – Mass of waterMs – Mass of a Solid

The relationship of water content and the weight of dry soil to the total weight of a soil volume should be

WT = Ws + Ww

Then, WT = Ws (1 +) or Ws = WT/(1 + )

The void and porosity in relation with the solid and void follows:

e = =

The void ratio is expressed in decimal number, and porosity (p) is conventionally expressed as percentage, thus:

p = = Vv/VT

And, the relationship between void ratio and porosity is:

p = or e =

The Degree of Saturation (S) indicate the portion of the void spaces in a soil material that is filled with water. The degree of saturation is expressed in percentage.

S = x 100% or S =

Full saturation or 100% saturation means that all voids are filled.

Example No. 1A cubic foot of soil sample weighing 130 lbs

is taken from a test pit. The entire sample is thoroughly dried and finally weighed 120 lbs. Determine the water content, wet unit weight and the dry unit weight.

Example No. 2Compute for the wet density, dry unit

weight, void ratio, water content and degree of saturation for a sample of moist soil which has a mass of 20 kg and occupies a total volume of 0.008 m3 and 0.006 m3 volume of solid. The sample is dried in an oven and weighs 18 kg. The specific gravity of the soil is 2.60.

Submerged Soil (Effective or Buoyant Unit Weight) is the weight of a saturated soil, surrounded by water, per unit volume.

Usub soil = Usat soil - UW or Usub soil = x UW

Relative Density (Dr) is an index that quantifies the degree of packing between the loosest and densest possible state of coarse-grained soils as determine by experiments.

Dr = (emax - e) / (emax – emin)

Example No. 3A sample of undistributed fine sand

having a volume of 0.09 ft3, dry unit weight of 10 lbs and specific gravity (Gs) of 2.60 was tested at the maximum density with the void at 0.40 and at the minimum density with the void ratio at 0.90. Determine the relative density of the undistributed sample.Dr (%) Description

0 – 15 Very Loose 15 – 35 Loose 35 – 65 Medium Loose 65 – 85 Dense 85 - 100 Very Dense

TABLE: Description based on Relative Density