DIRECT DETERMINATION OF k OF SOILS IN PLACE BY PUMPING TEST

The most reliable information concerning the permeability of a deposit of coarse grained material below the water table can usually be obtained by conducting pumping tests in the field.

Although such tests have their most extensive application in connection with dam foundations, they may also prove advisable on large bridge or building foundation jobs where the water table must be lowered.

The arrangement consists of a test well and a series of observation wells. The test well is sunk through the permeable stratum up to the impermeable layer.

A well sunk into a water bearing stratum, termed an aquifer, and tapping free flowing ground water having a free ground water table under atmospheric pressure, is termed a gravity or unconfined well. A well sunk into an aquifer where the ground water flow is confined between two impermeable soil layers, and is under pressure greater than atmospheric, is termed as artesian or confined well.

Observation wells are drilled at various distances from the test or pumping well along two straight lines, one oriented approximately in the direction of ground water flow and the other at right angles to it.

A minimum of two observation wells and their distances from the test well are needed. These wells are to be provided on one side of the test well in the direction of the ground water flow.

The test consists of pumping out water continuously at a uniform rate from the test well until the water levels in the test and observation wells remain stationary. When this condition is achieved the water pumped out of the well is equal to the inflow into the well from the surrounding strata.

The water levels in the observation wells and the rate of water pumped out of the well would provide the necessary additional data for the determination of k.

As the water from the test well is pumped out, a steady state will be attained when the water pumped out will be equal to the inflow into the well. At this stage the depth of water in the well will remain constant.

The draw down resulting due to pumping is called the cone of depression. The maximum draw down DQ is in the test well. It decreases with the increase in the distance from the test well.

The depression dies out gradually and forms theoretically, a circle around the test well called the circle of influence. The radius of this circle is called the radius of influence of the depression cone.

GENERAL TYPES OF SOIL BASIC INFORMATION AND TUTORIALS

It has been discussed earlier that soil is formed by the process of physical and chemical weathering. The individual size of the constituent parts of even the weathered rock might range from the smallest state (colloidal) to the largest possible (boulders). This implies that all the weathered constituents of a parent rock cannot be termed soil.

According to their grain size, soil particles are classified as cobbles, gravel, sand, silt and clay. Grains having diameters in the range of 4.75 to 76.2 mm are called gravel. If the grains are visible to the naked eye, but are less than about 4.75 mm in size the soil is described as sand.

The lower limit of visibility of grains for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075 to 0.002 mm are termed as silt and those that are finer than 0.002 mm as clay. This classification is purely based on size which does not indicate the properties of fine grained materials.

Residual and Transported Soils

On the basis of origin of their constituents, soils can be divided into two large groups:

1. Residual soils, and

2. Transported soils.

Residual soils are those that remain at the place of their formation as a result of the weathering of parent rocks. The depth of residual soils depends primarily on climatic conditions and the time of exposure. In some areas, this depth might be considerable. In temperate zones residual soils are commonly stiff and stable.

An important characteristic of residual soil is that the sizes of grains are indefinite. For example, when a residual sample is sieved, the amount passing any given sieve size depends greatly on the time and energy expended in shaking, because of the partially disintegrated condition.

Transported soils are soils that are found at locations far removed from their place of formation. The transporting agencies of such soils are glaciers, wind and water. The soils are named according to the mode of transportation. Alluvial soils are those that have been transported by running water. The soils that have been deposited in quiet lakes, are lacustrine soils.

Marine soils are those deposited in sea water. The soils transported and deposited by wind are aeolian soils. Those deposited primarily through the action of gravitational force, as in land slides, are colluvial soils.

Glacial soils are those deposited by glaciers. Many of these transported soils are loose and soft to a depth of several hundred feet. Therefore, difficulties with foundations and other types of construction are generally associated with transported soils.

Organic and Inorganic Soils

Soils in general are further classified as organic or inorganic. Soils of organic origin are chiefly formed either by growth and subsequent decay of plants such as peat, or by the accumulation of fragments of the inorganic skeletons or shells of organisms. Hence a soil of organic origin can be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter.

Names of Some Soils that are Generally Used in Practice

Bentonite is a clay formed by the decomposition of volcanic ash with a high content of montmorillonite. It exhibits the properties of clay to an extreme degree. Varved Clays consist of thin alternating layers of silt and fat clays of glacial origin. They possess the undesirable properties of both silt and clay.

The constituents of varved clays were transported into fresh water lakes by the melted ice at the close of the ice age. Kaolin, China Clay are very pure forms of white clay used in the ceramic industry. Boulder Clay is a mixture of an unstratified sedimented deposit of glacial clay, containing unsorted rock fragments of all sizes ranging from boulders, cobbles, and gravel to finely pulverized clay material.

Calcareous Soil is a soil containing calcium carbonate. Such soil effervesces when tested with weak hydrochloric acid. Marl consists of a mixture of calcareous sands, clays, or loam. Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when wet. Boulder clays or glacial till is also sometimes named as hardpan. Caliche is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited from ground water.

Peat is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures. Loam is a mixture of sand, silt and clay.

Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown. Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.

NAMES OF SOME SOILS THAT ARE GENERALLY USED IN CIVIL ENGINEERING PRACTICE

Bentonite is a clay formed by the decomposition of volcanic ash with a high content of montmorillonite. It exhibits the properties of clay to an extreme degree.

Varved Clays consist of thin alternating layers of silt and fat clays of glacial origin. They possess the undesirable properties of both silt and clay. The constituents of varved clays were transported into fresh water lakes by the melted ice at the close of the ice age.

Kaolin, China Clay are very pure forms of white clay used in the ceramic industry.

Boulder Clay is a mixture of an unstratified sedimented deposit of glacial clay, containing unsorted rock fragments of all sizes ranging from boulders, cobbles, and gravel to finely pulverized clay material.

Calcareous Soil is a soil containing calcium carbonate. Such soil effervesces when tested with weak hydrochloric acid.

Marl consists of a mixture of calcareous sands, clays, or loam.

Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when wet.

Boulder clays or glacial till is also sometimes named as hardpan.

Caliche is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited from ground water.

Peat is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures.

Loam is a mixture of sand, silt and clay.

Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown.

Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.

FORMATION OF SOILS BASIC INFORMATION AND TUTORIALS

HOW THE SOILS ARE FORMED?

Soil is defined as a natural aggregate of mineral grains, with or without organic constituents, that can be separated by gentle mechanical means such as agitation in water. By contrast rock is considered to be a natural aggregate of mineral grains connected by strong and permanent cohesive forces.

The process of weathering of the rock decreases the cohesive forces binding the mineral grains and leads to the disintegration of bigger masses to smaller and smaller particles. Soils are formed by the process of weathering of the parent rock.

The weathering of the rocks might be by mechanical disintegration, and/or chemical decomposition.

Mechanical Weathering

Mechanical weathering of rocks to smaller particles is due to the action of such agents as the expansive forces of freezing water in fissures, due to sudden changes of temperature or due to the abrasion of rock by moving water or glaciers.

Temperature changes of sufficient amplitude and frequency bring about changes in the volume of the rocks in the superficial layers of the earth's crust in terms of expansion and contraction. Such a volume change sets up tensile and shear stresses in the rock ultimately leading to the fracture of even large rocks.

This type of rock weathering takes place in a very significant manner in arid climates where free, extreme atmospheric radiation brings about considerable variation in temperature at sunrise and sunset.

Erosion by wind and rain is a very important factor and a continuing event. Cracking forces by growing plants and roots in voids and crevasses of rock can force fragments apart.

Chemical Weathering

Chemical weathering (decomposition) can transform hard rock minerals into soft, easily erodable matter.

The principal types of decomposition are hydmtion, oxidation, carbonation, desilication and leaching. Oxygen and carbon dioxide which are always present in the air readily combine with the elements of rock in the presence of water.

COMPOSITION AND STRUCTURE OF CLAY MINERALS

COMPOSITION OF CLAY MINERALS

The word 'clay' is generally understood to refer to a material composed of a mass of small mineral particles which, in association with certain quantities of water, exhibits the property of plasticity.

According to the clay mineral concept, clay materials are essentially composed of extremely small crystalline particles of one or more members of a small group of minerals that are commonly known as clay minerals.

These minerals are essentially hydrous aluminum silicates, with magnesium or iron replacing wholly or in part for the aluminum, in some minerals. Many clay materials may contain organic material and water-soluble salts.

Organic materials occur either as discrete particles of wood, leaf matter, spores, etc., or they may be present as organic molecules adsorbed on the surface of the clay mineral particles. The water-soluble salts that are present in clay materials must have been entrapped in the clay at the time of accumulation or may have developed subsequently as a consequence of ground water movement and weathering or alteration processes.

Clays can be divided into three general groups on the basis of their crystalline arrangement and it is observed that roughly similar engineering properties are connected with all the clay minerals belonging to the same group.

STRUCTURE OF CLAY MINERALS

Clay minerals are essentially crystalline in nature though some clay minerals do contain material which is non-crystalline (for example allophane). Two fundamental building blocks are involved in the formation of clay mineral structures.

They are:

1. Tetrahedral unit.

2. Octahedral unit.

The tetrahedral unit consists of four oxygen atoms (or hydroxyls, if needed to balance the structure) placed at the apices of a tetrahedron enclosing a silicon atom which combines together to form a shell like structure with all the tips pointing in the same direction. The oxygen at the bases of all the units lie in a common plane.

EXCAVATION CALCULATION BASIC INFORMATION AND TUTORIALS

Excavation is measured by the cubic yard for the quantity takeoff (27 cf # 1 cy). Before excavation, when the soil is in an undisturbed condition, it weighs about 100 pounds per cf; rock weighs about 150 pounds per cf.

The site plan is the key drawing for determining earthwork requirements and is typically scaled in feet and decimals of a foot. There is usually no reason to change to units of feet and inches; however, at times they must be changed to decimals. Remember that when estimating quantities, the computations need not be worked out to an exact answer.

Swell and Compaction.

Material in its natural state is referred to as bank materials and is measured in bank cubic yards (bcy). When bank materials are excavated, the earth and rocks are disturbed and begin to swell.

This expansion causes the soil to assume a larger volume; this expansion represents the amount of swell and is generally expressed as a percentage gained above the original volume.

Uncompacted excavated materials are referred to as loose materials and are measured in loose cubic yards (lcy).When loose materials are placed and compacted (as fill) on a project, it will be compressed into a smaller volume than when it was loose, and with the exception of solid rock it will occupy less volume than in its bank condition.

This reduction in volume is referred to as shrinkage. Shrinkage is expressed as a percentage of the undisturbed original or bank volume.

Materials that have been placed and compacted are referred to as compacted materials and are measured in compacted cubic yards (ccy). Bank, loose, and compacted cubic yards are used to designate which volume we are talking about.

Figure 9.1 is a table of common swell and shrinkage factors for various types of soils. When possible tests should be performed to determine the actual swell and shrinkage for the material.

FIGURE 9.1. Swell and Shrinkage Factors. (Solid rock when compacted is less dense than its bank condition.)

UNDERWATER SOIL SAMPLING BASIC INFORMATION AND TUTORIALS

Common constructions that require some kind of underwater exploration program include bridge piers, port structures, pipelines, oil well platforms, land recovery (fills to extend the shore line or for an island), and the like. It is usually necessary to collect enough data to make a strength estimate.

Soil shear strength determines how much pile embedment is required or whether a fill will require special construction procedures. Estimates of settlement are also often required—both how much and how long it will take. This is very critical for land recovery operations, since the client will want to know when enough settlement has occurred so that construction of surface facilities can begin.

The in situ testing and recovery procedures for underwater samples, either in a freshwater or a saltwater environment, are not much different from those for dry land for water depths up to about 45 m. The principal differences are that the testing or drilling equipment is mounted on a barge that is towed to the test location and securely anchored and that casing is used, at least to the water bed and possibly 1 or 2 meters into the bed.

The casing strength is the principal cause for limiting the depth to about 45 m. For this situation the barge is securely anchored using four to six anchors so it does not shift or twist. Sometimes divers are used to observe visually if any construction difficulty will be encountered or if there are any existing underwater obstructions.

A barge-mounted drilling rig (drilling over the side) is a common method for drilling in rivers, in lake beds, and in the shallower water along the continental shelf for bridges, port structures, or land recovery. Penetration, vane, and pressuremeter tests described in the following sections can be made in the borings.

In deeper water (up to 1,000+ m) wave action requires alternative exploration equipment, such as a small ship converted to a drilling platform by installing a center well of 460 to 610 mm diameter from the deck through the hull and adding a drill rig. This configuration is sometimes called a drill ship.

Submarine-type vessels (sometimes called submersibles) are also used. In very deep water a platform might be constructed, off of which the exploration crew might work. Any of these equipment options will allow recovery of samples of reasonable quality.

Where wave action occurs, it is necessary to use casing with flexible joints, and a casing diameter large enough to allow passage of the sampling (or test device) tube. In deeper water the drill pipe may act as the casing (again using flexible joints). In this case the lower end of the pipe holds the auger bit, which produces an over-sized hole.

At the desired level a sampler is lowered through the drill pipe to the base of the hole and either driven or pushed into the soil below the bit.

There are also projectile-type devices that are lowered to the ocean floor from the drill ship to recover soil samples. Servomechanisms commanded from the surface may be used to force a sample tube into the soil using the weight of either the surface vessel or some kind of reaction device placed on the seafloor.

A projectile device may contain a gas or explosive charge to propel a sample tube into the soil, again using the weight of the total device as a reaction. Most of these types of devices are patented and/or proprietary. Deepwater divers are sometimes used to recover samples or to inspect the reaction device.

In situ tests are currently considered preferable to sample recovery, particularly for strength testing. It is difficult to recover good-quality samples from underwater because of the change in pore pressure when the sample is brought above water. As a minimum, air bubbles tend to come out of the pore water and occupy a greater volume, causing the sample to expand or even explode.

If the sample is still in the sample tube, the expansion may cause the sample to extend out of the tube end(s). Depending on the equipment, the sample recovery tube (about 50- to 75-mm ID and 610 to 100O+ mm in length) may be pushed or driven.

A pushed sample is generally of better quality than one obtained by driving the tube into the soil. Shorter tube lengths generally produce better-quality samples, since side friction is significant with all tube samples; if the sample is too long, it may become compressed from side friction between the sample and the inside walls of the sampler.

At a given site a few samples should be recovered for visual inspection and possibly some index tests (w#, W>L, Ip). A driven-tube recovered sample will often have excessive disturbance for strength testing, but the blow count to drive the tube gives some indication of soil strength, somewhat like the SPT test described in the next section.

A number of underwater exploration methods are described in ASTM (1971) and appear among the references cited by Focht and Kraft (1977), which the interested reader may wish to consult. Using the in situ vane test for underwater exploration is described in ASTM (1988). Olsen et al. (1986) described an elaborate marine sampling and testing program undertaken in 1979-1980.

MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS

Natural Cementation and Aging

All soils undergo a natural cementation at the particle contact points. The process of aging seems to increase the cementing effect by a variable amount. This effect was recognized very early in cohesive soils but is now deemed of considerable importance in cohesionless deposits as well. 

The effect of cementation and aging in sand is not nearly so pronounced as for clay but still the effect as a statistical accumulation from a very large number of grain contacts can be of significance for designing a foundation.

Overconsolidation

A soil is said to be normally consolidated (nc) if the current overburden pressure (column of soil overlying the plane of consideration) is the largest to which the mass has ever been subjected. It has been found by experience that prior stresses on a soil element produce an imprint or stress history that is retained by the soil structure until a new stress state exceeds the maximum previous one. The soil is said to be overconsolidated (or preconsolidated) if the stress history involves a stress state larger than the present overburden pressure.

Mode of Deposit Formation

Soil deposits that have been transported, particularly via water, tend to be made up of small grain sizes and initially to be somewhat loose with large void ratios. They tend to be fairly uniform in composition but may be stratified with alternating very fine material and thin sand seams, the sand being transported and deposited during high-water periods when stream velocity can support larger grain sizes. These deposits tend to stabilize and may become very compact (dense) over geological periods from subsequent overburden pressure as well as cementing and aging processes.

Quality of the Clay

The term clay is commonly used to describe any cohesive soil deposit with sufficient clay minerals present that drying produces shrinkage with the formation of cracks or fissures such that block slippage can occur. Where drying has produced shrinkage cracks in the deposit we have a fissured clay. 

This material can be troublesome for field sampling because the material may be very hard, and fissures make sample recovery difficult. In laboratory strength tests the fissures can define failure planes and produce fictitiously low strength predictions (alternatively, testing intact pieces produces too high a prediction) compared to in situ tests where size effects may either bridge or confine the discontinuity.

Soil Water

Soil water may be a geological phenomenon; however, it can also be as recent as the latest rainfall or broken water pipe. An increase in water content tends to decrease the shear strength of cohesive soils. An increase in the pore pressure in any soil will reduce the shear strength. 

A sufficient increase can reduce the shear strength to zero—for cohesionless soils the end result is a viscous fluid. A saturated sand in a loose state can, from a sudden shock, also become a viscous fluid. This phenomenon is termed liquefaction and is of considerable importance when considering major structures (such as power plants) in earthquake-prone areas.

THE UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) BASIC INFORMATION TUTORIALS

The Unified Soil Classification System is based on the airfield classification system developed by A. Casagrande during World War II. With some modification it was jointly adopted by several U.S. government agencies in 1952. Additional refinements were made and it is currently standardized as ASTM D 2487-93. It is used in the U.S. and much of the world for geotechnical work other than roads and highways.

In the unified system soils are designated by a two-letter symbol: the first identifies the primary component of the soil, and the second describes its grain size or plasticity characteristics. For example, a poorly graded sand is designated SP and a low plasticity clay is CL. Five first-letter symbols are used:

G for gravel

S for sand

M for silt

C for clay

O for organic soil

Clean sands and gravels (having less than 5% passing the No. 200 sieve) are given a second letter P if poorly graded or W if well graded. Sands and gravels with more than 12% by weight passing the No. 200 sieve are given a second letter M if the fines are silty or C if fines are clayey.

Sands and gravels having between 5 and 12% are given dual classifications such as SP-SM. Silts, clays, and organic soils are given the second letter H or L to designate high or low plasticity. The specific rules for classification are summarized as follows and described in detail in ASTM D 2487.

Organic soils are distinguished by a dark-brown to black color, an organic odor, and visible fibrous matter. For soils that are not notably organic the first step in classification is to consider the percentage passing the No. 200 sieve.

If less than 50% of the soil passes the No. 200 sieve, the soil is coarse grained, and the first letter will be G or S; if more than 50% passes the No. 200 sieve, the soil is fine grained and the first letter will be M or C.

For coarse-grained soils, the proportions of sand and gravel in the coarse fraction (not the total sample) determine the first letter of the classification symbol. The coarse fraction is that portion of the total sample retained on a No. 200 sieve.

If more than half of the coarse fraction is gravel (retained on the No. 4 sieve), the soil is gravel and the  first letter symbol is G. If more than half of the coarse fraction is sand, the soil is sand and the first letter symbol is S.

SOIL GRAIN SIZE CHARACTERISTICS BASIC INFORMATION AND TUTORIALS

Large-grained materials such as cobbles and boulders are sometimes considered to be soil. The differentiation of cobbles and boulders depends somewhat on local practice, but boulders are generally taken to be particles larger than 200 to 300 mm or 9 to 12 in.

The Unified Soil Classification System suggests that boulders be defined as particles that will not pass a 12-in. (300 mm) opening. Cobbles are smaller than boulders and range down to particles that are retained on a 3-inch (75 mm) sieve.

Gravels and sands are classified as coarse-grained soils; silts and clays are fine-grained soils. For engineering purposes, gravel is defined as soil that passes a 3-inch (75 mm) sieve and is retained by a No. 4 sieve (4.75 mm or 0.187 in.) or No. 10 sieve (2.00 mm or 0.078 in.), depending on the classification system.

Sand is defined as soil particles smaller than gravel but retained on a No. 200 sieve (0.075 mm or about 0.003 in.). Soils passing the No. 200 sieve may be silt or clay. Although grain-size criteria were used in some older classification systems to differentiate silt from clay, the two systems described herein make this differentiation based on plasticity rather than grain size.

The grain-size characteristics of soils that are predominantly coarse grained are evaluated by a sieve analysis. A nest of sieves is prepared by stacking sieves one above the other with the largest opening at the top followed by sieves of successively smaller openings and a catch pan at the bottom.

Opening sizes of commonly used sieves are shown in Table 15.1.

A sample of dry soil is poured onto the top sieve, the nest is covered, and it is then shaken by hand or mechanical shaker until each particle has dropped to a sieve with openings too small to pass, and the particle is retained.

The cumulative weight of all material larger than each sieve size is determined and divided by the total sample weight to obtain the percent retained for that sieve size, and this value is subtracted from 100% to obtain the percent passing that sieve size.

Results are displayed by plotting the percent passing (on a linear scale) against the sieve opening size (on a log scale) and connecting the plotted points with a smooth curve referred to as a grain-size distribution curve.

GROUND FILLING AND COMPACTION BASIC AND TUTORIALS

Scarcity of good building land will often necessitate building on areas of fill. A variety of materials can be found in filled sites, ranging from quarry and mining waste to household and industrial refuse. Sites filled with refuse can give rise to problems of internal combustion, methane gas and other toxic chemicals; therefore building on these should be avoided whenever possible.

If the fill is fairly shallow then the most sensible option is to use piled foundations. The augured pile described earlier is often not suitable in fill if large stones and rubble are likely to be encountered and an alternative method is to use a driven pile.

One option is to use a driven pile made up of individual hollow pre-cast concrete sections, typically 300 400mm diameter. Using a special crane the pile is driven down into the ground adding extra sections as necessary. It has reached its correct depth when repeated hammer blows only produce minimal downward movement of the pile; this is known as a ‘set’ and is specified by an engineer.

As the fill naturally consolidates over the years there may be a downward force on the piles due to the friction of the ground against the pile sides. This ‘down-drag’ is rarely even and the resulting differential movement can cause cracking of the building.

In practice it is difficult to sleeve the whole length of a pile and several manufacturers prefer to coat the pile sections with a bituminous compound during manufacture. Under slow rates of strain the bituminous compound acts as a viscous fluid and reduces the down-drag (or up-lift) on the pile.

Deeper fill is best dealt with by the use of rafts and, as explained earlier, the raft spreads the load from the walls over the whole ground floor area. Some movement is to be expected and it is therefore essential to make sure that the services which enter or leave the property have flexible connectors immediately adjacent to the external wall.

Rafts, when designed for poor-quality ground, or ground where subsidence is expected, can be very expensive and have to be designed by structural engineers. However, they are a fast form of construction with minimal excavation and are sometimes also used on soft clays as an alternative to the reinforced wide strip foundation.

The pictures show a simple raft foundation formed from 150mm reinforced concrete slab and a more complicated raft foundation with downstand beams.

Occasionally it is possible to provide some form of ground treatment and use traditional strip foundations. On very large housing sites this can be cheaper than the use of rafts or piling. There are a variety of methods which attempt to increase the stability and bearing capacity of the ground. One method, called vibro compaction, involves the use of a crane-mounted poker which is driven into the ground.

A spinning eccentric weight inside the poker causes it to vibrate and this helps to compact the surrounding ground. The poker is then slowly extracted from the ground at the same time as sand is pumped through the poker to fill the void.

The operation is then repeated at 2 or 3m intervals to form a regular grid across the site. Vibro-replacement is another ground treatment; a treatment more suitable for cohesive soils. A special poker or piling rig forms a grid of stone columns in the ground, at the same time compressing the surrounding soil and increasing its density.

The stone piles act as weak columns transferring the building loads to a firmer strata. An alternative form of ground treatment is called dynamic compaction. This sounds a very grand title but in fact just involves dropping a weight of several tonnes on to the ground from a crane. It is not suitable if there are existing buildings in the immediate vicinity.

SUBSOIL DRAINAGE BASIC AND CIVIL ENGINEERINGTUTORIALS

SUBSOIL DRAINAGE BASIC INFORMATION
What Are Subsoil Drainage System?


Subsoil Drainage ~ Building Regulation C2 requires that subsoil drainage shall be provided if it is needed to avoid:-

a) the passage of ground moisture into the interior of the building or
b) damage to the fabric of the building.

Subsoil drainage can also be used to improve the stability of the ground, lower the humidity of the site and enhance its horticultural properties. Subsoil drains consist of porous or perforated pipes laid dry jointed in a rubble filled trench.

Porous pipes allow the subsoil water to pass through the body of the pipe whereas perforated pipes which have a series of holes in the lower half allow the subsoil water to rise into the pipe.

This form of ground water control is only economic up to a depth of 1„500, if the water table needs to be lowered to a greater depth other methods of ground water control should be considered.

The water collected by a subsoil drainage system has to be conveyed to a suitable outfall such as a river, lake or surface water drain or sewer.

In all cases permission to discharge the subsoil water will be required from the authority or owner and in the case of streams, rivers and lakes, bank protection at the outfall may be required to prevent erosion.

Subsoil Drainage Systems ~ the lay out of subsoil drains will depend on whether it is necessary to drain the whole site or if it is only the substructure of the building which needs to be protected.

The latter is carried out by installing a cut off drain around the substructure to intercept the flow of water and divert it away from the site of the building. Junctions in a subsoil drainage system can be made using standard fittings or by placing the end of the branch drain onto the crown of the main drain.

NB. connections to surface water sewer can be made at inspection chamber or direct to the sewer using a saddle connector † it may be necessary to have a catchpit to trap any silt.

SOIL CLASSIFICATION METHODS IN FOUNDATION DESIGN BASICS AND TUTORIALS

SOIL CLASSIFICATION METHODS IN FOUNDATION DESIGN BASIC INFORMATION
What Are The Methods Of Classifying Soils In Foundation Design?

It is necessary for the foundation engineer to classify the site soils for use as a foundation for several reasons:

1. To be able to use the database of others in predicting foundation performance.
2. To build one's own local database of successes (or any failures).
3. To maintain a permanent record that can be understood by others should problems later develop and outside parties be required to investigate the original design.
4. To be able to contribute to the general body of knowledge in common terminology via journal papers or conference presentations. After all, if one is to partake in the contributions of others, one should be making contributions to the general knowledge base and not be just a "taker."


The Unified Soil Classification System (USCS) of Table 2-1 is much used in foundation work. A version of this system has been standardized by ASTM as D 2487 (in Volume 04.08: Soil and Rock; Dimension Stone; Geosynthetics). The standardized version is similar to the original USCS as given by Casagrande (1948) but with specified percentages of sand or gravel passing specific sieves being used to give the "visual description" of the soil.

The original Casagrande USCS only classified the soil using the symbols shown in Table 2-1 (GP, GW, SM, SP, CL, CH, etc.), based on the indicated percentages passing the No. 4 and No. 200 sieves and the plasticity data. The author has always suggested a visual description supplement such as the following:

It is evident in this table that terms "trace" and "with" are somewhat subjective. The soil color, such as "blue clay," "gray clay," etc., is particularly useful in soil classification.

In many areas the color—particularly of cohesive soils—is an indication of the presence of the same soil stratum as found elsewhere. For example the "soft blue clay" on the soil profile of Fig. 2-4 for Chicago has about the same properties at any site in the Chicago area.

In foundation work the terms loose, medium, and dense, , and consistency descriptions such as soft, stiff, very stiff, etc., are also commonly used in foundation soil classification. Clearly, all of these descriptive terms are of great use to the local geotechnical engineer but are somewhat subjective.

That is, there could easily be some debate over what is a "medium" versus a "dense" sand, for example. The D 2487 standard removed some of the subjectiveness of the classification and requires the following terminology:

< 15% is sand or gravel use name (organic clay, silt, etc.)
15% < x < 30% is sand or gravel describe as clay or silt with sand, or clay or silt with gravel
> 30% is sand or gravel describe as sandy clay, silty clay, or gravelly clay, gravelly silt

The gravel or sand classification is based on the percentage retained on the No. 4 (gravel) sieve or passing the No. 4 and retained on the No. 200 (sand) sieves. This explanation is only partial, as the new standard is too lengthy to be presented in detail.

Although not stated in D 2487, the standard is devised for using a computer program3 to classify the soil. Further, not all geotechnical engineers directly use the ASTM standard, particularly if their practice has a history of success using the original USC system.

FIVE (5) MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS

MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS
What Are The 5 Major Factors That Affect The Engineering Properties of Soils?


Most factors that affect the engineering properties of soils involve geological processes acting over long time periods. Among the most important are the following.

1. Natural Cementation and Aging

All soils undergo a natural cementation at the particle contact points. The process of aging seems to increase the cementing effect by a variable amount. This effect was recognized very early in cohesive soils but is now deemed of considerable importance in cohesionless deposits as well.

The effect of cementation and aging in sand is not nearly so pronounced as for clay but still the effect as a statistical accumulation from a very large number of grain contacts can be of significance for designing a foundation. Care must be taken to ascertain the quantitative effects properly since sample disturbance and the small relative quantity of grains in a laboratory sample versus site amounts may provide difficulties in making a value measurement that is more than just an estimate.

Field observations have well validated the concept of the cementation and aging process. Loess deposits, in particular, illustrate the beneficial effects of the cementation process where vertical banks are readily excavated.

2. Overconsolidation

A soil is said to be normally consolidated (nc) if the current overburden pressure (column of soil overlying the plane of consideration) is the largest to which the mass has ever been subjected. It has been found by experience that prior stresses on a soil element produce an imprint or stress history that is retained by the soil structure until a new stress state exceeds the maximum previous one.

The soil is said to be overconsolidated (or preconsolidated) if the stress history involves a stress state larger than the present overburden pressure.

Overconsolidated cohesive soils have received considerable attention. Only more recently has it been recognized that overconsolidation may be of some importance in cohesionless soils. A part of the problem, of course, is that it is relatively easy to ascertain overconsolidation in cohesive soils but very difficult in cohesionless deposits.

The behavior of overconsolidated soils under new loads is different from that of normally consolidated soils, so it is important— particularly for cohesive soils—to be able to recognize the occurrence.

3. Mode of Deposit Formation

Soil deposits that have been transported, particularly via water, tend to be made up of small grain sizes and initially to be somewhat loose with large void ratios.

They tend to be fairly uniform in composition but may be stratified with alternating very fine material and thin sand seams, the sand being transported and deposited during high-water periods when stream velocity can support larger grain sizes.

These deposits tend to stabilize and may become very compact (dense) over geological periods from subsequent overburden pressure as well as cementing and aging processes.

Soil deposits developed'where the transporting agent is a glacier tend to be more varied in composition. These deposits may contain large sand or clay lenses. It is not unusual for glacial deposits to contain considerable amounts of gravel and even suspended boulders.

Glacial deposits may have specific names as found in geology textbooks such as moraines, eskers, etc.; however, for foundation work our principal interest is in the uniformity and quality of the deposit. Dense, uniform deposits are usually not troublesome. Deposits with an erratic composition may be satisfactory for use, but soil properties may be very difficult to obtain.

Boulders and lenses of widely varying characteristics may cause construction difficulties. The principal consideration for residual soil deposits is the amount of rainfall that has occurred. Large amounts of surface water tend to leach materials from the upper zones to greater depths. A resulting stratum of fine particles at some depth can affect the strength and settlement characteristics of the site.

4. Quality of the Clay

The term clay is commonly used to describe any cohesive soil deposit with sufficient clay minerals present that drying produces shrinkage with the formation of cracks or fissures such that block slippage can occur.

Where drying has produced shrinkage cracks in the deposit we have a fissured clay. This material can be troublesome for field sampling because the material may be very hard, and fissures make sample recovery difficult. In laboratory strength tests the fissures can define failure planes and produce fictitiously low strength predictions (alternatively, testing intact pieces produces too high a prediction) compared to in situ tests where size effects may either bridge or confine the discontinuity.

A great potential for strength reduction exists during construction where opening an excavation reduces the overburden pressure so that expansion takes place along any fissures. Subsequent rainwater or even local humidity can enter the fissure so that interior as well as surface softening occurs.

A clay without fissures is an intact clay and is usually normally consolidated or at least has not been over consolidated from shrinkage stresses. Although these clays may expand from excavation of overburden, the subsequent access to free water is not so potentially disastrous as for fissured clay because the water effect is more nearly confined to the surface.

5. Soil Water

Soil water may be a geological phenomenon; however, it can also be as recent as the latest rainfall or broken water pipe. An increase in water content tends to decrease the shear strength of cohesive soils. An increase in the pore pressure in any soil will reduce the shear strength.

A sufficient increase can reduce the shear strength to zero—for cohesionless soils the end result is a viscous fluid. A saturated sand in a loose state can, from a sudden shock, also become a viscous fluid. This phenomenon is termed liquefaction and is of considerable importance when considering major structures (such as power plants) in earthquake-prone areas.

When soil water just dampens sand, the surface tension produced will allow shallow excavations with vertical sides. If the water evaporates, the sides will collapse; however, construction vibrations can initiate a cave-in prior to complete drying.

The sides of a vertical excavation in a cohesive soil may collapse from a combination of rainfall softening the clay together with excess water entering surface tension cracks to create hydrostatic water pressure. In any case, the shear strength of a cohesive soil can be markedly influenced by water.

Even without laboratory equipment, one has probably seen how cohesive soil strength can range from a fluid to a brick-like material as a mudhole alongside a road fills during a rain and subsequently dries. Ground cracks in the hole bottom after drying are shrinkage (or tension) cracks.

SOIL LOADS AND HYDROSTATIC PRESSURE BASIC AND TUTORIALS

SOIL LOADS AND HYDROSTATIC PRESSURE BASIC
What Are Soil Loads And Hydrostatic Pressure?


The notation H is used for lateral soil loads, loads due to hydrostatic pressure, and the pressure of bulk materials. Soil lateral loads and hydrostatic pressure are introduced in ASCE 7 Sec. 3.2 and IBC Sec. 1610.

Soil lateral loading most commonly occurs at retaining walls. It is relatively unusual for wood to be directly loaded by retained soils. One notable exception to this is permanent wood foundations, used in some regions of the United States.

While soil lateral loading will most often come from a geotechnical investigation report, ASCE 7 Table 3-1 provides design lateral soil pressures (in psf, per foot of soil depth) for a range of soil classifications.

Where retaining walls are provided, it is possible to develop hydrostatic pressure in addition.

Hydrostatic pressure is most often avoided by providing drains behind retaining walls. In cases where it is not possible to provide drains, design for combined soil and hydrostatic lateral pressures is required.

In conditions where hydrostatic lateral pressures can develop, it is possible to also have upward hydrostatic pressures on adjacent floor slabs. These upward pressures would also use the notation H in load combinations.

The notation H is also defined in ASCE 7 and the IBC to include pressure of bulk materials. Although no discussion of this use is provided, it is thought to include pressure due to storage of grain, aggregates, or other bulk solids that can exert lateral pressures.

TESTING APPARATUS FOR SITE SOIL LABORATORY BASICS AND TUTORIALS

TESTING APPARATUS FOR SITE SOIL LABORATORY BASIC INFORMATION
What Are The Testing Apparatus For Site Soil Laboratories?

The usual apparatus suitable for a small soils laboratory on site, to be run by the resident engineer’s staff after proper instruction from a geotechnical engineer, is set out below.

For moisture content determinations
1. Beam balance weighing by 0.01 g divisions.
2. Drying oven, thermostatically controlled. (Not absolutely essential. For rough measurement of moisture content the sample can be dried on a flat tray over a stove.)
3. Six drying trays.

For grading analyses of soils
4. Aset of BS sieves (woven wire) with lid and pan for each different diameter: (a) 300 mm dia – 38, 25, 19, 13, 10 mm. (These can also be used for testing concrete aggregate gradings.) (b) 200 mm dia – 7, 5 and 3 mm, and Nos. 7, 14, 25, 52, 72, 100 and 200.
5. Balance weighing up to 25 kg.
6. Balance capable of weighing up to 7 kg by 1 g divisions.

For in situ density test (sand replacement method)
BS 1377 Part 9:1990 gives four tests of which Test 2.2 is the most useful because it can be used on fine, medium and coarse grained soils. A metal tray with a 200 mm diameter hole cut in it is placed on the formation and material is excavated via the hole.

The volume of the excavation is measured by pouring uniformly graded sand into it whose bulk density has been measured.

Apparatus required (additional to 1, 2, 3 and 6 above):
7. Small tools for excavating hole.
8. A rigid metal tray 500 mm square or larger with a 200 mm diameter hole cut in it.
9. Dried clean sand all passing No. 25 sieve but retained on No. 52 or 100 sieve and suitable airtight containers for storing it. (About 20 kg of this sand will be required initially.)
10. A pouring cylinder (as BS 1377 Part 9 Fig. 4).
11. Acalibrating container 200 mm diameter by 250 mm (as BS 1377 Part 9 Fig. 5).
12. Air-tight containers for the excavated soil.

The method can be applied to larger test holes in soils containing some gravel; the sand being poured in layers from a can with a top spout. A length of hose is attached to the spout with a conical tin shield wired to the lower end, so the sand has only a short standard free fall. Tests to fill measured containers can show the accuracy in ascertaining the bulk density of the sand as poured.

GRAIN SIZE CHARACTERISTICS OF SOILS BASIC AND TUTORIALS

SOIL GRAIN SIZE CHARACTERISTICS BASIC INFORMATION

What Are The Grain Size Characteristics of Soils?

Large-grained materials such as cobbles and boulders are sometimes considered to be soil. The differentiation of cobbles and boulders depends somewhat on local practice, but boulders are generally taken to be particles larger than 200 to 300 mm or 9 to 12 in. 

The Unified Soil Classification System suggests that boulders be defined as particles that will not pass a 12-in. (300 mm) opening. Cobbles are smaller than boulders and range down to particles that are retained on a 3-inch (75 mm) sieve. 

Gravels and sands are classified as coarse-grained soils; silts and clays are fine-grained soils. For engineering purposes, gravel is defined as soil that passes a 3-inch (75 mm) sieve and is retained by a No. 4 sieve (4.75 mm or 0.187 in.) or No. 10 sieve (2.00 mm or 0.078 in.), depending on the classification system. 

Sand is defined as soil particles smaller than gravel but retained on a No. 200 sieve (0.075 mm or about 0.003 in.). Soils passing the No. 200 sieve may be silt or clay. Although grain-size criteria were used in some older classification systems to differentiate silt from clay, the two systems described herein make this differentiation based on plasticity rather than grain size.

The grain-size characteristics of soils that are predominantly coarse grained are evaluated by a sieve analysis. A nest of sieves is prepared by stacking sieves one above the other with the largest opening at the top followed by sieves of successively smaller openings and a catch pan at the bottom. 

Opening sizes of commonly used sieves are shown in Table 15.1. A sample of dry soil is poured onto the top sieve, the nest is covered, and it is then shaken by hand or mechanical shaker until each particle has dropped to a sieve with openings too small to pass, and the particle is retained.

The cumulative weight of all material larger than each sieve size is determined and divided by the total sample weight to obtain the percent retained for that sieve size, and this value is subtracted from 100% to obtain the percent passing that sieve size. 

Results are displayed by plotting the percent passing (on a linear scale) against the sieve opening

size (on a log scale) and connecting the plotted points with a smooth curve referred to as a grain-size

distribution curve.

A sample of some grain-size distribution curves is presented in Fig. 15.1. The notation Dxx refers to the size D , in mm, for which xx percent of the sample by weight passes a sieve with an opening equal to D . The D10 size, sometimes called the effective grain size, is the grain diameter for which 10% of the sample (by weight) is finer. 

It is determined from the grain-size distribution curve at the point where the curve crosses a horizontal line through the 10% passing value on the y axis. Other D sizes are found in a similar manner. The D 50 size, called the median grain size, is the grain diameter for which half the sample (by weight) is smaller and half is larger.

Two parameters are used to describe the general shape of the grain-size distribution curve. The coefficient of uniformity, Cu , is:

Cu = D60/D10

The coefficient of curvature, Cc, is:

Cc = (D30)^2/ D60 D10

SOIL CLASSIFICATION AND PROPERTIES BASICS AND TUTORIALS

BASIC PROPERTIES AND CLASSIFICATION OF SOILS
Soil Mechanics Tutorials

1. REFERENCE.

Soil designations in this manual conform to the Unified Soil Classification per ASTM D2487, Classification of Soil for Engineering Purposes.

2. UTILIZATION.

Classify soils in accordance with the Unified System and include appropriate group symbol in soil descriptions. A soil is placed in one of 15 categories or as a borderline material combining two of these categories. Laboratory tests may be required for positive identification.

a. Sands and Gravels.

Sands are divided from gravels on the No. 4 sieve size, and gravels from cobbles on the 3-inch size. The division between fine and medium sands is at the No. 40 sieve, and between medium and coarse sand at the No. 10 sieve.

b. Silts and Clays.

Fine-grained soils are classified according to plasticity characteristics determined in Atterberg limit tests.

c. Organic Soils.

Materials containing vegetable matter are characterized by relatively low specific gravity, high water content, high ignition loss, and high gas content. Decrease in liquid limit after oven-drying to a value less than three-quarters of the original liquid limit is a definite indication of an organic soil.

The Unified Soil Classification categorizes organic soils based on the plotted position on the A-line chart. However, this does not describe organic soils completely. For the characteristics of the Unified Soil Classification System pertinent to roads and airfields, see NAVFAC DM-5.4.

3. TYPICAL PROPERTIES. Some typical properties of soils should be based on laboratory and/or field testing, and engineering evaluation.

SOIL IDENTIFICATION (SOIL MECHANICS) REQUIREMENTS BASICS AND TUTORIALS

SOIL MECHANICS SOIL IDENTIFICATION

Soil Mechanics Tutorials

REQUIREMENTS.

A complete engineering soil identification includes: (a) a classification of constituents, (b) the description of appearance and structural characteristics, and (c) the determination of compactness orconsistency in situ.

a. Field Identification. Identify constituent materials visually according to their grain size, and/or type of plasticity characteristics per ASTM Standard D2488, Description of Soils (Visual-Manual Procedure). (1) Coarse-Grained Soils. Coarse-grained soils are those soils where more than half of particles finer than 3-inch size can be distinguished by the naked eye.

The smallest particle that is large enough to be visible corresponds approximately to the size of the opening of No. 200 sieve used for laboratory identification. Complete identification includes grain size, color, and/or estimate of compactness.

(a) Color. Use color that best describes the sample. If there are two colors describe both colors. If there are more than two distinct colors, use multi-colored notation.

(b) Grain Size. Identify components and fractions in accordance Coarse-Grained Soils.

(c) Grading. Identify both well graded and poorly graded sizes as, under Supplementary Criteria for Visual Identification.

(d) Assigned Group Symbol. Use Table 3 for estimate of group symbols based on the Unified Classification System.

(e) Compactness. Estimate compactness in situ by measuring resistance to penetration of a selected penetrometer or sampling. If the standard penetration test is performed, determine the number of blows of a 140 pound hammer falling 30 inches required to drive a 2-inch OD, 1-3/8 inch ID split barrel sampler 1 foot.

The number of blows thus obtained is known as the standard penetration resistance, N. The split barrel is usually driven 18 inches. The penetration resistance is based on the last 12 inches.

1) Description Terms.

2) Compactness Based on Static Cone Penetration Resistance, q+c,. Reference 2, Cone Resistance as Measure of Sand Strength, by Mitchell and Lunne, provides guidance for estimating relative density with respect to the cone resistance.

 If q+c, and N values are measured during the field exploration, a q+c,-N correlation could be made, and is used to describe compactness. If N is not measured, but q+c, is measured, then use 7.1-N = q+c,/4 for sand and fine to medium gravel and N = q+c,/5 for sand.

(f) Describe, if possible, appearance and structure such as angularity, cementation, coatings, and hardness of particles.

(g) Examples of Sample Description: Medium dense, gray coarse to fine SAND, trace silt, trace fine gravel (SW). Dry, dense, light brown coarse to fine SAND, some silt (SM).

SOIL DEPOSITS GEOLOGIC ORIGIN AND MODE OF OCCURRENCE BASICS AND TUTORIALS

GEOLOGIC OCCURRENCE OF SOIL DEPOSITS
Soil Mechanics Tutorials

a. Principal Soil Deposits

Soil deposits grouped in terms of origin (e.g., residual, colluvial, etc.) and mode of occurrence (e.g., fluvial, lacustrine, etc.).

b. Importance.

 A geologic description assists in correlating experiences between several sites, and in a general sense, indicates the pattern of strata to be expected prior to making a field investigation (test borings, etc.).

Soils with similar origin and mode of occurrence are expected to have comparable if not similar engineering properties.

For quantitative foundation analysis, a geological description is inadequate and more specific classification is required.

A study of references on local geology should precede a major subsurface exploration program.

c. Soil Horizon.

Soil horizons are present in all sedimentary soils and transported soils subject to weathering. The A horizon contains the maximum amount of organic matter; the underlying B horizon contains clays, sesquioxides, and small amounts of organic matter.

The C horizon is partly weathered parent soil or rock and the D horizon is unaltered parent soil and rock.