Engineering and Construction

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Generally, soil groups listed toward the top of Figure 2.2 are more desirable for supporting building foundations than those listed further down. The higherlisted soils tend to have better  soil engineering properties, that is, they tend  to have greater loadbearing capacity,  to be more stable, and to react less to
changes in moisture content. Rock is  generally the best material on which  to found a building. When rock is too deep to be reached economically, the designer must  choose from the strata  of different soils that lie closer to the  surface and design a foundation to per- form satisfactorily in the selected soil.

Figure 2.5 gives some conservative values of  loadbearing capacity for  various types of soil. These values give only an approximate idea of the  relative strengths of different soils;  the strength of any particular soil is
also dependent on factors such as  the presence or absence of water, the  depth at which the soil lies beneath  the surface, and, to some extent, the manner in which the foundation acts  upon it. In practice, the designer may  also choose to reduce the pressure  of the foundations on the soil to well  below these values in order to reduce  the potential for building settlement.

Figure 2.5 Presumptive surface bearing values of various soil types, from the 2006 IBC. Classes 3, 4, and 5 refer to the soil group symbols in Figure 2.2.

The stability of a soil is its ability to retain its structural properties under  the varying conditions that may occur  during the lifetime of the building. In  general, rock, gravels, and sands tend  to be the most stable soils, clays the least  stable, and silts somewhere in between.

Many clays change size under chang-ing subsurface moisture conditions,  swelling considerably as they absorb  water and shrinking as they dry. In the  presence of highly expansive clay soils,  a foundation may need to be designed  with underlying void spaces into which  the clay can expand to prevent structural damage to the foundation itself.

When wet clay is put under pressure,  water can be slowly squeezed out of  it, with a corresponding gradual reduction in volume. In this circumstance, long-term settlement of a foundation bearing on such soil is a risk that must  be considered. Taken together, these properties make many clays the least predictable soils for supporting buildings. (In Figure 2.2, the fine-grained  soil groups indicated as having a liquid  limit greater than 50 are generally the  ones most affected by water content,  exhibiting higher plasticity (moldability) and greater expansion when wet  and lower strength when dry.

In regions of significant  earth- quake risk, stability of soils during seismic events is also a concern. Sands and  silts with high water content are particularly susceptible to liquefaction, that
is, a temporary change from solid to  liquid state during cyclic shaking. Soil  liquefaction can lead to loss of support  for a building foundation or excessive  pressure on foundation walls.

The drainage characteristics of a  soil are important in predicting how  water will fl ow on and under building  sites and around building substructures. Where a coarse-grained soil is composed of particles mostly of the
same size, it has the greatest possible volume of void space between particles, and water will pass through it most readily. Where coarse-grained soils are composed of particles with a diverse range of sizes, the volume of void space between particles is reduced, and such soils drain water less eficiently. Coarse-grained soils consisting of particles of all sizes are termed well graded or poorly sorted, those with a smaller range of particle sizes are termed  poorly gradedor well sorted, and those with particles mostly of one size are termed uniformly graded (Figure 2.6).

Figure 2.6 Two gravel samples, illustrating differences in range of particle sizes or grading. The left-hand sample, with a diverse range of particle sizes, comes from a well-graded sandy gravel. On the right is a sample of a uniformly graded gravel in which there is little variation in size among particles.

Because of their smaller particle size,fine-grained soils also tend to drain water less efficiently:  Water
passes slowly through very fine sands and silts and almost not at all through many clays. A building site with clayey or silty soils near the surface drains poorly and is likely to be muddy and covered with puddles during rainy periods, whereas a gravelly site is likely to remain dry. Underground, water passes quickly through strata of gravel and sand but tends to accumulate above layers of clay and fine silt. An excel-lent way to keep a basement dry is to surround it with a layer of uniformly graded gravel or crushed stone. Water passing through the soil toward the building cannot reach the basement without first falling to the bottom of
this porous layer, from where it can be drawn off in perforated pipes before it accumulates (Figures 2.60–2.62). It does little good to place perforated drainage pipes directly in clay or silt because water cannot fl  ow  through the impervious soil toward the pipes.

Figure 2.60 Two methods of relieving water pressure around a building substructure by drainage. The gravel drain (left) is hard to do well because of the dificulty of depositing the crushed stone and backfill soil in neatly separated, alternating layers. The drainage mat (right) is easier and often more economical to install.

Figure 2.62 A diagrammatic representation of the placement of sheet membrane waterproofing around a basement. A mud slab of low- strength concrete was poured to serve as a base for placement of the horizontal membrane. Notice that the vertical and horizontal membranes join to wrap the basement completely in a waterproof enclosure.

Rarely is the soil beneath a building site composed of a single type. Beneath most buildings, soils of various
types are arranged in superimposed layers (strata) that were formed by various past geologic processes. Frequently, soils in any one layer are themselves also mixtures of different soil groups, bearing descriptions such as well-graded gravel with silty clay and sand, poorly graded sand with clay, lean clay with gravel, and so on. Determining the suitability of any particular site’s soils for support of a building foundation, then, depends on the be-haviors of the various soils types and how they interact with each other and  with the building foundation.

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a. Vibrations of foundation-soil systems can adequately be represented by simple mass-spring-dashpot systems.  The model for this simple system consists of a concentrated mass, m, supported by a linear elastic spring with a spring constant, k, and a viscous damping unit (dashpot) having a damping constant, c.  The system is excited by an external force, e.g., Q = Qo sin (ωt), in which Qo is the amplitude of the exciting force, ω = 2πfo is the angular frequency (radians per second) with fo the exciting frequency (cycles per second), and t is time in seconds.

Figure 17-2.  Response curves for the single-degree-of-freedom system with viscous damping.

b. If the model is oriented as shown in the insert in figure 17-2(a), motions will occur in the vertical or z direction only, and the system has one degree of freedom (one coordinate direction (z) is needed to describe the motion).  The magnitude of dynamic vertical motion, Az, depends upon the magnitude of the external excitation, Q, the nature of Qo, the frequency, fo, and the system parameters m, c, and k.  These parameters are customarily combined to describe the "natural frequency" as follows:

c. Figure 17-2(a) shows the dynamic response of the system when the amplitude of the exciting force, Qo, is constant.  The abscissa of the diagram is the dimensionless ratio of exciting frequency, fo, divided by the natural frequency, fn, in equation (17-1).  The ordinate is the dynamic magnification factor, Mz, which is the ratio of Az to the static displacement, Az  = (Qo/k). Different response curves correspond to different values of D.

d. Figure 17-2(b) is the dynamic response of the system when the exciting force is generated by a rotating mass, which develops:

e. The ordinate Mz.  (fig 17-2(b)) relates the dynamic displacement, Az, to me e/m.  The peak value of the response curve is a function of the damping ratio and is given by the following expression:

For small values of D, this expression becomes 1/2D. These peak values occur at frequency ratios of f

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Introduction.
a. Vibrations caused by steady state or transient loads may cause settlement of soils, excessive motions of foundations or structures, or discomfort or distress to personnel.  Some basic design factors for dynamic loading are treated in this section.  Design of a foundation system incorporates the equipment loading, subsurface material properties, and geometrical proportions in some analytical procedure.

b. Figure 17-1 shows some limiting values of vibration criteria for machines, structures, and personnel.

Figure 17-1.  Response spectra for tibraton limits.

On this diagram, vibration characteristics are described in terms of frequency and peak amplitudes of
acceleration, velocity, or displacement.  Values of frequency constitute the abscissa of the diagram and peak velocity is the ordinate.  Values of peak displacement are read along one set of diagonal lines and labelled in displacement (inches), and peak acceleration values are read along the other set of diagonal lines and labelled in various amounts of g, the acceleration of gravity.  The shaded zones in the upper right-hand corner indicate possible structural damage to walls by steady-state vibrations.  For structural safety during blasting, limit peak velocity to 2.0 inches per second and peak acceleration to 0.10g for frequencies exceeding 3 cycles per second.  These limits may occasionally have to be lowered to avoid being excessively annoying to people.

c. For equipment vibrations, limiting criteria consist of a maximum velocity of 1.0 inch per second up to a frequency of about 30 cycles per second and a peak acceleration of 0.15g above this frequency.  However, this upper limit is for safety only, and specific criteria must be established for each installation.  Usually, operating limits of equipment are based on velocity criteria; greater than 0.5 inch per second indicates extremely rough operation and machinery should be shut down; up to 0.10 inch per second occurs for smooth, well-balanced equipment; and less than 0.01 inch per second represents very smooth operation.

d. Figure 17-1 also includes peak velocity criteria for reaction of personnel to steady-state vibrations. Peak velocities greater than 0.1 inch per second are "troublesome to persons," and peak velocities of 0.01 inch per second are just "barely noticeable to persons." It is significant that persons and machines respond to equivalent levels of vibration.

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a. Remove and replace.  Removal of poor soil and replacement with the same soil treated by compaction, with or without admixtures, or by a higher quality material offer an excellent opportunity for producing high-strength, relatively incompressible, uniform foundation conditions.  The cost of removal and replacement of thick deposits is high because of the need for excavation and materials handling, processing, and recompaction.  Occasionally, an expensive dewatering system also may be required.  Excluding highly organic soils, peats and sanitary landfills, virtually any inorganic soil can be processed and treated so as to form an acceptable structural fill material.

b. Lime treatment.   This treatment of plastic fine-grained soils can produce high-strength, durable
materials.  Lime treatment levels of 3 to 8 percent by weight of dry soil are typical.

c. Portland cement.  With treatment levels of 3 to 10 percent by dry weight, portland cement is particularly well suited for low-plasticity soils and sand soils.

d. Stabilization using fills.

(1) At sites underlain by soft, compressible soils and where filling is required or possible to establish the final ground elevation, load-bearing structural fills can be used to distribute the stresses from light structures.

Compacted sands and gravels are well suited for this application as are also fly ash, bottom ash, slag, and various lightweight aggregates, such as expended shale, clam and oyster shell, and incinerator ash. Admixture stabilizers may be incorporated in these materials to increase their strength and stiffness.

(2) Clam and oyster shells as a structural fill material over soft marsh deposits represent a new development.  The large deposits of clam and oyster or reef shells that are available in the Gulf States coastal areas can be mined and tasportd short distances economically.  Clam shells are 1/4, to 1/2 inch in diameter; whereas, oyster shells, which are coarser and more elongated, are 2 to 4 inches in size.  When dumped over soft ground, the shells interlock; if there are finmes and water present, some cementation develops owing to the high calcium carbonate (>90 percent) content.  In the loose state, the shell unit weight is about 63 pounds per square foot; after construction, it is about 95 pounds per square foot.  Shell embenkments "float" over very soft ground; whereas, conventional fills would sink out of sight.  About a 5-foot-thick layer is required to be placed in a single lift.  The only compaction used is from the top of the lift, so the upper several inches are more tightly knit and denser than the rest of the layer.

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The supporting capacity of soft, compressible ground may be increased and settlement reduced through use of compression reinforcement in the direction parallel to the applied stress or tensile reinforcement in planes normal to the direction of applied stress.  Commonly used compression reinforcement elements include mix-in-place piles and walls.  Strips and membranes are used for tensile reinforcement, with the latter sometimes used to form a moisture barrier as well.

a. Mix-in-place piles and walls.   Several procedures are available, most of them patented or proprietary, which enable construction of soil-cement or soil-lime in situ.  A special hollow rod with rotating vanes
is augered into the ground to the desired depth.

Simultaneously, the stabilizing admixture is introduced.

The result is a pile of up to 2 feet in diameter.  Cement, in amounts of 5 to 10 percent of the dry soil weight, is best for use in sandy soils.  Compressive strengths in excess of 200 kips per square foot can be obtained in hese materials.  Lime is effective in both expansive plastic clays and in saturated soft clay.  Compressive strengths of about 20 to 40 kips per square foot are to be expected in these materials.  If overlapping piles are formed, a mix-in-place wall results.

b. Vibroreplacement stone columns.   A vibroflot is used to make a cylindrical, vertical hole under its own weight by jetting to the desired depth.  Then, 1/2- to 1- cubic yard coarse granular backfill, usually gravel or crushed rock 3/4 to 1 inch is dumped in, and the vibroflot is used to compact the gravel vertically and radially into the surrounding soft soil.  The process of backfilling and compaction by vibration is continued until the densified stone column reaches the surface.

c. Strips and membranes.

(1) Low-cost, durable waterproof membranes, such as polyethylene, polypropolylene asphalt, and polyester fabric asphalt, have had application as moisture barriers.  At the same time, these materials have sufficient tensile strength that when used in envelope construction, such as surrounding a well- compacted, fine-grained soil, the composite structure has a greater resistance to applied loads than conventional construction with granular materials.  The reason is that any deformation of the enveloped soil layer
causes tension in the membrance, which in turn produces additional confinement on the soil and thus increases its resist- ance to further deformation.

(2) In the case of a granular soil where moisture infiltration is not likely to be detrimental to
strength, horizontally bedded thin, flat metal or plastic strips can act as tensile reinforcing elements.

Reinforced earth has been used mainly for earth retaining structures; however, the feasibility of using reinforced earth slabs to improve the bearing capacity of granular soil has been demonstrated.

(3) Model tests have shown that the ultimate bearing capacity can be increased by a factor of 2 to 4 for the same soil unreinforced.  For these tests, the spacing between reinforcing layers was 0.3 times the
footing width.  Aggregate strip width was 42 percent of the length of strip footing.

d. Thermal methods.   Thermal methods of foundation soil stabilization, freezing or heating, are
complex and their costs are high.

(1) Artificial ground freezing.   Frozen soil is far stronger and less pervious than unfrozen ground.
Hence, artificial ground freezing has had application for temporary underpinning and excavation stabilization.
More recent applications have been made to back- freezing soil around pile foundations in permafrost and maintenance of frozen soil under heated buildings on permafrost.  Design involves two classes of problems;
namely, the structural properties of the frozen ground to include the strength and the stress-strain-time behavior, and thermal considerations to include heat flow, transfer of water to ice, and design of the refrigeration system.

(2) Heating.  Heating fine-grained soils to moderate temperatures, e.g., 1000C+, can cause drying and accompanying strength increase if subsequent rewetting is prevented.  Heating to higher temperatures can result in significant permanent property improvements, including decreases in water sensitivity, swelling, and compressibility; and increases in strength.

Burning of liquid or gas fuels in boreholes or injection of hot air into 6- to 9-inch-diameter boreholes can produce 4- to 7-foot-diameter strengthened zones, after continuous treatment for about 10 days.  Dry or partly saturated weak clayey soils and loess are well suited for this type of treatment, which is presently regarded as experimental.

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a. Preloading.   Earth fill or other material is placed over the site to be stabilized in amounts sufficient
to produce a stress in the soft soil equal to that anticipated from the final structures.  As the time required for consolidation of the soft soil may be long (months to years), varying directly as the square of the layer thickness and inversely as the hydraulic conductivity, preloading alone is likely to be suitable only for stabilizing thin layers and with a long period of time available prior to final development of the site.

b. Surcharge fills.   If the thickness of the fill placed for pre-loading is greater than that required to induce stresses corresponding to structure-induced stresses, the excess fill is termed a surcharge fill.

Although the rate of consolidation is essentially independent of stress increase, the amount of
consolidation varies approximately in proportion to the stress increase.  It follows, therefore, that the preloading fill plus surcharge can cause a given amount of settlement in shorter time than can the preloading fill alone.  Thus, through the use of surcharge fills, the time required for preloading can be reduced significantly.

(1) The required surcharge and loading period can be determined using conventional theories of
consolidation.  Both primary consolidation and most of the secondary compression settlements can be taken
out in advance by surcharge fills.  Secondary compression settlements may be the major part of the total settlement of highly organic deposits or old sanitary landfill sites.

(2) Because the degree of consolidation and applied stress vary with depth, it is necessary to determine if excess pore pressures will remain at any depth after surcharge removal.  If so, further primary consolidation settlement under permanent loadings would occur.  To avoid this occurrence, determine the duration of the surcharge loading required for points most distant from drainage boundaries.

(3) The rate and amount of preload may be controlled by the strength of the underlying soft soil.
Use berms to maintain foundation stability and place fill in stages to permit the soil to gain strength from consolidation.  Predictions of the rates of consolidation strength and strength gain should be checked during fill placement by means of piezometers, borings, laboratory tests, and in situ strength tests.

c. Vertical drains.

(1) The required preloading time for most soft clay deposits more than about 5 to 10 feet thick will be large.  The consolidation time may be reduced by providing a shorter drainage path by installing vertical sand drains.  Sand drains are typically 10 to 15 inches in diameter and are installed at spacings of 5 to 15 feet.

A sand blanket or a collector drain system is placed over the surface to facilitate drainage.  Other types of drains available are special cardboard or combination plastic-cardboard drains.  Provisions should be made to monitor pore pressures and settlements with time to determine when the desired degree of precompression has been obtained.

(2) Both displacement and nondisplacement methods have been used for installing sand drains.  Although driven, displacement drains are less expensive than augered or "bored" nondisplacement drains; they should not be used in sensitive deposits or in stratified soils that have higher hydraulic conductivity in the horizontal than in the vertical direction.  Vertical drains are not needed in fibrous organic deposits because the hydraulic conductivity of these materials is high, but they may be required in underlying soft clays.

d. Dynamic consolidation (heavy tamping).
Densification by heavy tamping has also been reported as an effective means for improving silts and clays, with preconstruction settlements obtained about 2 to 3 times the predicted construction settlement.  The time required for treatment is less than for surcharge loading with sand drains.  The method is essentially the same as that used for cohesionless soils, except that more time is required.

Several blows are applied at each location followed by a 1- to 4-week rest period, then the process is repeated.

Several cycles may be required.  In each cycle the settlement is immediate, followed by drainage of pore water.  Drainage is facilitated by the radial fissures that form around impact points and by the use of horizontal and peripheral drains.  Because of the necessity for a time lapse between successive cycles of heavy tamping when treating silts and clays, a minimum treatment area of 18,000 to 35,000 square yards (4 to 8 acres) is necessary for economical use of the method.  This method is presently considered experimental in saturated clays.

e. Electroosmosis.   Soil stabilization by electroosmosis may be effective and economical under the following conditions: (1) a saturated silt or silty clay soil, (2) a normally consolidated soil, and (3) a low pore
water electrolyte concentration.  Gas generation and drying and fissuring at the electrodes can impair the
efficiency of the method and limit the magnitude of consolidation pressures that develop.  Treatment results in nonuniform changes in properties between electrodes because the induced consolidation depends on the voltage, and the voltage varies between anode and cathode.  Thus, reversal of electrode polarity may be desirable to achieve a more uniform stress condition.

Electroosmosis may also be used to accelerate the consolidation under a preload or surcharge fill.  The method is relatively expensive.

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Grouting is a high-cost soil stabilization method that can be used where there is sufficient confinement to permit required injection pressures.  It is usually limited to zones of relatively small volume and to special problems.  Some of the more important applications are control of groundwater during construction; void filling to prevent excessive settlement; strengthening adjacent foundation soils to protect against damage during excavation, pile driving, etc.; soil strengthening to reduce lateral support requirements; stabilization of loose sands against liquefaction; foundation underpinning; reduction of machine foundation vibrations; and filling solution voids in calcareous materials.

a. Grout types and groutability.  Grouts can be classified as particulate or chemical.  Portland cement is
the most widely used particulate grouting material. Grouts composed of cement and clary are also widely used, and lime-slurry injection is finding increasing application.  Because of the silt-size particles in these
materials, they cannot be injected into the pores of soils finer than medium to coarse sand.  For successful grouting of soils, use the following guide


Type I portland cement, Typeportland cement, and  Type III portland cement, Type III portland cement, and processed bentonite cannot be used to penetrate soils finer than 30, 40, and 60 mesh sieve sizes, respectively.

Different types of grouts may be combined to both coarse- and fine-grained soils.

b. Cement and soil-cement grouting.  See TM 5-818-6/AFM 88-32 for discussion of planning and
implementation of foundation grouting with cement and soil-cement.

c. Chemical grouting.  To penetrate the voids of finer soils, chemical grout must be used.  The most
common classes of chemical grouts in current use are silicates, resins, lignins, and acrylamides.  The viscosity of the chemical-water solution is the major factor controlling groutability.  The particle-size ranges over which each of these grout types is effective is shown in figure 16-6.

Figure 16-6.  Soil particle sizes suitable for different grout types and several concentrations and viscosities shown.

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The methods in this group are similar to those described in the preceding section except that the vibrations are supplemented by active displacement of the soil and, in the case of vibroflotation and compaction piles, by backfilling the zones from which the soil has been displaced.

a. Compaction piles.  Partly saturated or freely draining soils can be effectively densified and strengthened by this method, which involves driving displacement piles at close spacings, usually 3 to 6 feet on centers.  One effective procedure is to cap temporarily the end of a pipe pile, e.g., by a detachable plate, and drive it to the desired depth, which may be up to 60 feet.  Either an impact hammer or a vibratory driver can be used.  Sand or other backfill material is introduced in lifts with each lift compacted concurrently with withdrawal of the pipe pile.  In this way, not only is the backfill compacted, but the compacted column has also expanded laterally below the pipe tip forming a caisson pile.

b. Heavy tamping (dynamic consolidation).

(1) Repeated impacts of a very heavy weight (up to 80 kips) dropped from a height of 50 to 130 feet are applied to points spaced 15 to 30 feet apart over the area to be densified.  In the case of cohesionless soils, the impact energy causes liquefaction followed by settlement as water drains.  Radial fissures that form around the impact points, in some soils, facilitate drainage.  The method has been used successfully to treat soils both above and below the water table.

(2) The product of tamper mass and height of fall should exceed the square of the thickness of layer to be densified.  A total tamping energy of 2 to 3 blows per square yard is used.  Increased efficiency is
obtained if the impact velocity exceeds the wave velocity in the liquefying soil.  One crane and tamper can treat from 350 to 750 square yards per day.  Economical use of the method in sands requires a minimum treatment area of 7500 square yards.  Relative densities of 70 to 90 percent are obtained.  Bearing capacity increases of 200 to 400 percent are usual for sands and marls, with a corresponding increase in deformation modulus.  The cost is reported as low as one-fourth to one-third that of vibroflotation.

(3) Because of the high-amplitude, low- frequency vibrations (2-12 Hz), minimum distances should be maintained from adjacent facilities as follows:


c. Vibroflotation.

(1) A cylindrical penetrator about 15 inches in diameter and 6 feet long, called a vibroflot, is attached to an adapter section containing lead wires and hoses. The whole assembly is handled by a crane.  A rotating eccentric weight inside the vibroflot develops a horizontal centrifugal force of about 10 tons at 1800 revolutions per minute.  Total weight is about 2 tons.

(2) To sink the vibroflot to the desired treatment depth, a water jet at the tip is opened and acts in conjunction with the vibrations so that a hole can be advanced at a rate of about 3.6 feet per minute; then the bottom jet is closed, and the vibroflot is withdrawn at a rate of about 0.1 foot per minute.  Newer, heavier vibroflots operating at 100 horsepower can be withdrawn at twice this rate and have a greater effective penetration depth.  Concurrently, a cohesionless sand or gravel backfill is dumped in from the ground surface and densified.  Backfill consumption is at a rate of about 0.7 to 2 cubic yards per square yard of surface. In partly saturated sands, water jets at the top of the vibroflot can be opened to facilitate liquefaction and densification of the surrounding ground.  Liquefaction occurs to a radial distance of 1 to 2 feet from the surface of the vibroflot.

Most vibroflotation applications have been to depths less than 60 feet, although depths of 90 feet have been attained successfully.

(3) A relationship between probable relative density and vibroflot hold spacings is given in figure 16-4.  Newer vibroflots result in greater relative densities.  Figure 16-5 shows relationships between
allowable bearing pressure to limit settlements to 1 inch and vibroflot spacing.  Allowable pressures for "essentially cohesionless fills" are less than for clean sand deposits, because such fills invariably contain some fines and are harder to densify.

(4) Continuous square or triangular patterns are often used over a building site.
Alternatively, it may be desired to improve the soil only at the locations of individual spread footings.  Patterns and spacings required for an allowable pressure of 3 tons per square foot and square footings are given in table 16-3.

Figure 16-4.  Relative density as a function of vibroflot hole spacings.

Figure 16-5.  Allowable bearing pressure on cohesionless soil layers stabilized by vibroflotation.

Table 16-3.  Vibroflotation Patterns for Isolated Footings for an Allowable Bearing Pressure.

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Vibrocompaction methods (blasting, terraprobe, and vibratory rollers) can be used for rapid densification of saturated cohesionless soils (fig 16-1).  The ranges of grain-size distributions suitable for treatment by vibrocompaction, as well as vibroflotation, are shown in figure 16-2.  The effectiveness of these methods is greatly reduced if the percent finer than the No.  200 sieve exceeds about 20 percent or if more than about 5 percent is finer than 0.002 millimeter, primarily because the hydraulic conductivity of such materials is too low to prevent rapid drainage following liquefaction.

Figure 16-1.  Applicable grain-size ranges for different stabilization methods.

Figure 16-2.  Range of particle-size distributions suitable for densification by vibrocompaction.

The usefulness of these methods in partly saturated sands is limited, because the lack of an increase of pore water pressure impedes liquefaction.  Lack of complete saturation is less of a restriction to use of blasting because the high-intensity shock wave accompanying detonation displaces soil, leaving depressions that later can be backfilled.

a. Blasting.

(1) Theoretical design procedures for densification by blasting are not available and continuous on-site supervision by experienced engineers having authority to modify procedures as required is esential if this treatment method is used.  A surface heave of about 6 inches will be observed for proper charge sizes and placement depths.  Surface cratering should be avoided.

Charge masses of less than 4 to more than 60 pounds have been used.  The effective radius of influence for charges using (M = lb) 60 percent dynamite is as follows:


Charge spacings of 10 to 25 feet are typical.  The center of charges should be located at a depth of about two-thirds the thickness of the layer to be densified, and three to five successive detonations of several spaced charges each are likely to be more effective than a single large blast.  Little densification is likely to result above about a 3-foot depth, and loosened material may remain around blast points.  Firing patterns should be established to avoid the "boxing in" of pore water. Free-water escape on at least two sides is desirable.

(2) If blasting is used in partly saturated sands or loess, preflooding of the site is desirable.  In one technique, blast holes about 3 to 3/2 inches in diameter are drilled to the desired depth of treatment, then small charges connected by prima cord, or simply the prima cord alone, are strung the full depth of the hole.  Each hole is detonated in succession, and the resulting large diameter holes formed by lateral displacement are backfilled. 
A sluiced-in cohesionless backfill will densify under the action of vibrations from subsequent blasts.  Finer grained backfills can be densified by tamping.

b. Vibratingprobe (terraprobe).

(1) A 30-inch-outside-diameter, open- ended pipe pile with 3/, -inch wall thickness is suspended
from a vibratory pile driver operating at 15 Hz.  A probe length 10 to 15 feet greater than the soil depth to be stabilized is used.  Vibrations of 7%- to 1-inch amplitude are in a vertical mode.  Probes are made at spacings of 3 to 10 feet.  After sinkage to the desired depth, the
probe is held for 30 to 60 seconds before extraction.

The total time required per probe is typically 21/2 to 4 minutes. Effective treatment has been accomplished at
depths of 12 to 60 feet.  Areas in the range of 450 to 700 square yards may be treated per machine per 8-hour shift.

(2) Test sections about 30 to 60 feet on a side are desirable to evaluate the effectiveness and required probe spacing.  The grain-size range of treated soil should fall within limits shown in figure 16-2.  A square pattern is often used, with a fifth probe at the center of each square giving more effective increased densification than a reduced spacing.  Saturated soil conditions are necessary.  Underlying soft clay layers may dampen vibrations.

c. Vibratory rollers.   Where cohesionless deposits are of limited thickness, e.g., less than 6 feet, or
where cohesionless fills are being placed, vibratory rollers are likely to be the best and most economical means for achieving high density and strength.  Use with flooding where a source of water is available.  The effective depth of densification may be 6 feet or more for the heaviest vibratory rollers (fig 16-3a).  For a fill placed in successive lifts, a density-depth distribution similar to that in figure 16-3b results.  It is essential that the lift thickness, soil type, and roller type be matched. Properly matched systems can yield compacted layers at a relative density of 85 to 90 percent or more.

Figure 16-3.  Sand densification using vibratory rollers.

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General.

a. The applicability and essential features of foundation soil treatments are summarized in tables 16-1
and 16-2 and in figure 16-1.  The depth of stabilization generally must be sufficient to absorb most of the foundation pressure bulb.

b. The relative benefits of vibrocompaction, vibrodisplacement compaction, and precompression
increase as load intensity decreases and size of loaded area increases.  Soft, cohesive soils treated in place are generally suitable only for low-intensity loadings.  Soil stabilization of wet, soft soils may be accomplished by addition of lime; grout to control water flow into excavations to reduce lateral support requirements or to reduce liquefaction or settlement caused by adjacent pile driving; seepage control by electroosmosis; and temporary stabilization by freezing.  The range of soil grain sizes for which each stabilization method is most applicable is shown in figure 16-1.

Figure 16-1.  Applicable grain-size ranges for different stabilization methods.

Table 16-1.  Stabilization of Soils for Foundations of Structures

Table 16-2.  Applicability of Foundation Soil Improvement for Different Structures and Soil Types

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If a slide is being stabilized by flattening the slope or by using a buttress or retaining structure,  the shear strength at time of failure corresponding to a factor of safety of 1 should be calculated.  This strength can be used to evaluate the safety factor of the slope after stabilization.  Methods for stabilizing slopes and landslides are summarized in table 8-1.  Often one or more of these schemes may be used together.  Schemes I  through V are listed approximately in order of increasing cost.

Table 8-1.  Methods of Stabilizing Slopes and Landslides

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a. Stiff-fissured clays and shales.   The shearing resistance of most stiff-fissured clays and shales may be far less than suggested by the results of shear tests on undisturbed samples.  This result is due, in part, to prior shearing displacements that are much larger than the displacement corresponding to peak strength.  Slope failures may occur progressively, and over a long period of time the shearing resistance may be reduced to the residual value-the minimum value that is reached only at extremely large shear displacements.  Temporary slopes in these materials may be stable at angles that are steeper than would be consistent with the mobilization of only residual shear strength.  The use of local experience and empirical correlations are the most reliable design procedures for these soils.

b. Loess.  Vertical networks of interconnected channels formed by decayed plant roots result in a high
vertical permeability in loess.  Water percolating downward destroys the weakly cemented bonds between
particles,  causing rapid erosion and slope failure.

Slopes in loess are frequently more stable when cut vertically to prevent infiltration.  Benches at intervals can
be used to reduce the effective slope angle.  Horizontal surfaces on benches and at the top and bottom of the
slope must be sloped slightly and paved or planted to prevent infiltration.  Ponding at the toe of a slope must be prevented.  Local experience and practice are the best guides for spacing benches and for protecting slopes against infiltration and erosion.

c. Residual soils.  Depending on rock type and climate, residual soils may present special problems with
respect to slope stability and erosion.  Such soils may contain pronounced structural features characteristic of
the parent rock or the weathering process, and their characteristics may vary significantly over short distances.  It may be difficult to determine design shear strength parameters from laboratory tests.

Representative shear strength parameters should be determined by back-analyzing slope failures and by using empirical design procedures based on local experience.

d. Highly sensitive clays.  Some marine clays exhibit dramatic loss of strength when disturbed and can
actually flow like syrup when completely remolded.

Because of disturbance during sampling, it may be difficult to obtain representative strengths for such soils from laboratory tests.  Local experience is the best guide to the reliability of laboratory shear strength values for such clays.

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Excavation slope instability may result from failure to control seepage forces in and at the toe of the slope, too steep slopes for the shear strength of the material being excavated, and insufficient shear strength of subgrade soils.  Slope instability may occur suddenly, as the slope is being excavated, or after the slope has been standing for some time.  Slope stability analyses are useful in sands, silts, and normally consolidated and overconsolidated clays, but care must be taken to select the correct strength parameter.  Failure surfaces are shallow in cohesionless materials and have an approximately circular or sliding wedge shape in clays.

      a. Cohesionless slopes resting on firm soil or rock.  The stability of slopes consisting of cohesionless soils depends on the angle of internal friction  φ’, the slope angle, the unit weight of soil, and pore pressures.
Generally, a slope of 1 vertical (V) on 1 1/2 horizontal (H) is adequate; but if the slope is subjected to seepage or sudden drawdown, a slope of 1V on 3H.is commonly employed.  Failure normally occurs by surface raveling or shallow sliding.  Where consequences of failure may be important, required slopes can be determined using simple infinite slope analysis.  Values of  φ’ for stability analyses are determined from laboratory tests or estimated from correlations (para 3-6).  Pore pressure due to seepage reduces slope stability, but static water pressure, with the same water level inside and outside the slopes, has no effect.  Benches, paved ditches, and planting on slopes can be used to reduce runoff velocities and to retard erosion.  Saturated slopes in cohesionless materials may be susceptible to liquefaction and flow slides during earthquakes, while dry slopes are subject to settlement and raveling.  Relative densities of 75 percent or larger are required to ensure seismic stability, as discussed in later.

      b. Cohesive slopes resting on firm soil or rock.
The stability of slopes consisting of cohesive soils depends on the strength of soil, its unit weight, the slope
height, the slope angle, and pore pressures.  Failure usually occurs by sliding on a deep surface tangent to
the top of firm materials.  For relatively high slopes that drain slowly, it may be necessary to analyze the stability for three limiting conditions:

          (1) Short-term or end-of-construction condition.   Analyze this condition using total stress methods, with shear strengths determined from Q tests on undisturbed specimens.  Shear strengths from unconfirmed compression tests may be used but generally may show more scatter.  This case is often the only one analyzed for stability of excavated slopes.  The possibility of progressive failure or large creep deformations exists for safety factors less than about 1.25 to 1.50.

          (2) Long-term condition.   If the excavation is open for several years,  it may be necessary to analyze this condition using effective stress methods, with strength parameters determined from S tests or R
tests on undisturbed specimens.  Pore pressures are governed by seepage conditions and can be determined using flow nets or other types of seepage analysis.  Both internal pore pressures and external water pressures
should be included in the analyses.  This case generally does not have to be analyzed.

          (3) Sudden drawdown condition,  or other conditions where the slope is consolidated under one loading condition and is then subjected to a rapid change in loading, with insufficient time for drainage.   Analyze this condition using total stress methods,  with shear strengths measured in R and S tests.  Shear strength shall be based on the minimum of the combined R and S envelopes.  This.case is not normally encountered in excavation slope stability.

      c. Effect of soft foundation strata.  The critical failure mechanism is usually sliding on a deep surface tangent to the top of an underlying firm layer.  Short-term stability is usually more critical than long-term stability.

The strength of soft clay foundation strata should be expressed in terms of total stresses and determined using Q triaxial compression tests on undisturbed specimens or other methods described previously.

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The report should contain sufficient description of field and laboratory investigation, subsurface conditions, typical test data, basic assumptions, and analytical procedures to permit detailed review of the conclusions, recommendations, and final design. The amount and type of information to be presented shall be consistent with the scope of the investigation. For some structures, a cursory review of foundation conditions may be adequate. For major structures, the following outline shall be used as a guide:

      a. A general description of the site, indicating principal topographic features in the vicinity. A plan map that shows the surface contours, the location of the proposed structure, and the location of all borings should be included.

      b. A description of the general and local geology of the site, including the results of the geological studies.

      c. The results of field investigations, including graphic logs of all foundation and borrow borings, locations of and pertinent data from piezometers, and a general description of subsurface materials, based on the borings. The information shall be presented in accordance with Government standards. The boring logs should indicate how the borings were made, type of sampler used, split-spoon penetration resistance, and other field measurement data.

      d. Groundwater conditions, including data on seasonal variations in groundwater level and results of field pumping tests, if performed.

      e. A general description of laboratory tests performed, range of test values, and detailed test data on
representative samples. Atterberg limits should be plotted on a plasticity chart, and typical grain-size curves on a grain-size distribution plot. Laboratory test data should be summarized in tables and figures as appropriate. If laboratory tests were not performed, the basis for determining soil or rock properties should be presented, such as correlations or reference to pertinent publications.
     
      f. A generalized geologic profile used for design, showing properties of subsurface materials and design values of shear strength for each critical stratum.

The profile may be described or shown graphically.

      g.  Where alternative foundation designs are prepared, types of foundations considered, together with evaluation and cost data for each.

      h. A table or sketch showing the final size and depth of footings or mats and lengths and types of piles, if used.

      i. Basic assumptions for loadings and the computed factors of safety for bearing-capacity calculations, as outlined later.

      j. Basic assumptions, loadings, and results of settlement analyses, as outlined later; also, estimated swelling of subgrade soils. The effects of computed differential settlements, and also the ef- fects of swell, on the structure should be discussed.

      k. Basic assumptions and results of other analyses.

      l. An estimate of dewatering requirements, if necessary. The maximum anticipated pumping rate and flow per foot of drawdown should be presented.

      m. Special precautions and recommendations for construction. Possible sources for fill and backfill should also be given. Compaction requirements should be described.

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There are many grouting methods that can be used to strengthen the on-site soil (see Table 15.4). For example, in order to stabilize the ground, fluid grout can be injected into the ground to fill in joints, fractures, or underground voids (Graf, 1969; Mitchell, 1970). For the releveling of existing structures, one option is mudjacking, which has been defined as a process whereby a water and soil-cement or soil-lime cement grout is pumped beneath the slab, under pressure, to produce a lifting force that literally floats the slab to the desired position (Brown, 1992). Other site improvement grouting methods are as follows:

• Compaction grouting. A commonly used site improvement technique is compaction grouting, which consists of intruding a mass of very thick consistency grout into the soil, which both dis- places and compacts the loose soil (Brown and Warner, 1973; Warner, 1978, 1982). Compaction grouting has proved successful in increasing the density of poorly compacted fill, alluvium, and compressible or collapsible soil. The advantages of compaction grouting are less expense and dis- turbance to the structure than foundation underpinning, and it can be used to relevel the structure.

The disadvantages of compaction grouting are that it is difficult to analyze the results, it is usually ineffective near slopes or for near-surface soils because of the lack of confining pressure, and there is the danger of filling underground pipes with grout (Brown and Warner, 1973).

• Jet grouting (columnar). This process is used to create columns of grouted soil. The grouted columns are often brittle and may provide little or no resistance to lateral movements and may be broken by lateral ground movements (Seed, 1991).

• Deep mixing. Jetting or augers are used to physically mix the stabilizer and soil. There can be overlapping of treated columns in order to create a more resistant treated zone.

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INTRODUCTION
Geotechnical earthquake engineering can be defined as that subspecialty within the field of geotechnical engineering that deals with the design and construction of projects in order to resist the effects of earthquakes. Geotechnical earthquake engineering requires an understanding of basic geotechnical principles as well as an understanding of geology, seismology, and earthquake engineering. In a broad sense,  seismology can be defined as the study of earthquakes. This would include the internal behavior of the earth and the nature of seismic waves generated by the earthquake.

For geotechnical earthquake engineering of soils and foundations, the types of activities that may need to be performed by the geotechnical engineer include the following:

• Subsurface exploration for geotechnical earthquake engineering, i.e., the screening investigation and the quantitative evaluation.
• Total stress and effective stress analyses for geotechnical earthquake engineering
• Bearing capacity analyses for geotechnical earthquake engineering
• For the design earthquake, determining the peak ground acceleration and magnitude of the earthquake
• Investigating the possibility of liquefaction at the site
• Investigating the stability of slopes for the additional forces imposed during the design earthquake
• Calculating the settlement of the foundation caused by the design earthquake
• Performing calculations to assess the effect of the earthquake on retaining walls
• Considering foundation alternatives to mitigate the effects of earthquakes

In terms of the accuracy of the calculations used to determine the earthquake induced soil move-
ment, Tokimatsu and Seed (1984) conclude:

It should be recognized that, even under static loading conditions, the error associated with the estimation of settlement is on the order of ±25 to 50%. It is therefore reasonable to expect less accuracy in predicting settlements for the more complicated conditions associated with earthquake loading.... In the application of the methods, it is essential to check that the final results are reasonable in the light of available experience.

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There are many different methods to deal with collapsible soil. If there is a shallow deposit of collapsible soil, then the deposit can be removed and recompacted during the grading of the site. In some cases, the soil can be densified, such as by compaction grouting, to reduce the collapse potential of the soil. Another method to deal with collapsible soil is to flood the building footprint or force water into the collapsible soil stratum by using wells. As the wetting front moves through the ground, the collapsible soil will densify and reach an equilibrium state. Flooding or forcing water into collapsible soil should not be performed if there are adjacent buildings because of the possibility of damaging these structures. Also, after the completion of the flooding process, subsurface exploration and laboratory testing should be performed to evaluate the effectiveness of the process.

There are also foundation options that can be used for sites containing collapsible soil. A deep foundation system that derives support from strata below the collapsible soil could be constructed.

Also, post-tensioned foundations or mat slabs can be designed and installed to resist the anticipated settlement from the collapsible soil.

As previously mentioned, the triggering mechanism for the collapse of fill or natural soil is the introduction of moisture. Common reasons for the infiltration of moisture include water from irrigation, broken or leaky water lines, and an altering of surface drainage that allows rainwater to pond near the foundation. Another source of moisture infiltration is from leaking pools. For example, Fig. 7.5 shows a photograph of a severely damaged pool shell that cracked due to the collapse of an underlying uncompacted debris fill. For sites with collapsible soil, it is good practice to emphasize the importance of positive drainage (no ponding of water at the site) and an immediate repair of any leaking utilities.