Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification

Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification Soil improvement and ground modification methods chapter 4 objectives and improvements from soil densification

Objectives and Improvements

from Soil Densification

This chapter provides an overview of the objectives and improvements attained by densification of soil, including effects on fundamental soil engi-neering properties, basic geotechnical design, and special attention to lique-faction phenomenon An explanation is made to discern the differences between shallow and deep densification The fundamental differences between methodological processes used to densify different soil types is addressed along with an introduction to how different equipment can achieve these different densification processes The ending sections of this chapter describe the effects of soil densification on each of the basic soil engineering behaviors that are important to design of various geotechnical projects

4.1 OVERVIEW OF SOIL DENSIFICATION

Without much question, the most common method of soil and/or ground improvement is densification Most fundamental, desired, engineering properties of soils can be achieved and/or improved by creating a denser packing of soil grains These include soil shear strength (critical to founda-tion bearing capacity, slope stability, liquefacfounda-tion mitigafounda-tion, etc.), mini-mized compressibility and settlement, increased stiffness, resiliency and durability, reduced permeability, and so forth Depending on the approach and equipment used, densification may be applicable to a wide range of soil types and site conditions, including soft fine-grained marine sediments, liquefiable sands, heterogeneous fills, sinkholes, municipal wastes, and even low-level nuclear waste (Schexnayder and Lukas, 1992a,b) In todays prac-tice, “unusable” sites no longer exist

In general, there are two fundamentally different categories of soil den-sification with important differences in mechanisms Compaction is the pro-cess by which soil is densified by eliminating (or squeezing out) air from void space between grains Consolidation is the process by which soil is densified

by eliminating (or squeezing out) water from void space between grains

A big difference between the two is that compaction occurs almost

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Soil Improvement and Ground Modification 2015 Elsevier Inc.

immediately after application of a load or densification process Consolida-tion, on the other hand, is time dependent, and is a function of the soil per-meability or rate that water will be expelled from the soil Consolidation is also a function of length of travel path, in that the longer the travel path, the longer the time to consolidate

4.1.1 Shallow vs Deep Densification

There are many methods and technologies available for achieving increased soil density that will be described herein But first it would be useful to dif-ferentiate between “shallow compaction” and “deep densification.” Shallow compaction typically refers to processes where soil is worked at the ground surface or where material is placed and compacted in layers This may involve compacting existing near surface soils in place, but more commonly refers to engineered fill, where soil is placed in controlled lifts (i.e., layers, typ-ically 20-30 cm¼8-12 in thick) and compacted to achieve a minimum desired (usually specified) density This type of application is often called earthwork construction and results in earth structures such as prepared soil foundations, engineered slopes and embankments (including earth dams and levees), transportation projects, and stable backfill to create level ground (e.g., behind retaining walls) Deep densification usually refers to in situ processes where existing subsurface soils are densified by a variety of methods such as blasting, vibrocompaction, deep dynamic compaction, compaction grouting, and others that will be described inChapter 6 Forced consolidation techniques may be sidered a method of dewatering, but for practical purposes may also be con-sidered a form of in situ densification for fine-grained soils One of the essential differences between deep densification of primarily granular soils and densi-fication of saturated fine-grained soils by consolidation is the time required for consolidation, controlled largely by the permeability of the soils being den-sified Ground improvement methods employing preloading or forced consol-idation will be introduced here, but described and addressed in greater detail in

Section 6.1.6inChapter 6under the heading of Deep Densification and in

Chapter 9, which is dedicated to this improvement application

The approach taken in this text is to address each general category of densification separately Shallow compaction will be covered inChapter 5 Deep densification will be covered inChapter 6 In each chapter, the objec-tives, methodological approaches, application and equipment choices, design specifications, and QA/QC will be described

4.1.2 Processes and Equipment

When discussing or designing for densification applications, it is important

to understand some of the basic processes of compaction techniques One should know how the soil grains are physically rearranged during a compac-tion process and how the compaccompac-tion energy is delivered This will largely

be a function of the soil type being densified and the equipment being used There are a variety of densification processes that can be administered by means of different equipment and methods

The most efficient way to compact granular, primarily cohesionless soils is with the assistance of vibrations This is due to the fact that cohesionless soils attain all of their strength from friction between grains The introduction of vibrational loads “shakes” the particles so that the frictional resistance between grains is overcome When combined with static load and/or impact load, vibrations can help to attain high levels of compaction A notable example

of this is the use of a vibratory table for the maximum density test of cohe-sionless soils (ASTM D4253) Compaction equipment is available for both shallow and deep compaction processes that employ vibrations, principally through oscillatory motors with controllable frequencies of oscillation For shallow compaction, vibratory rollers are available that apply both static load through their own weight combined with vibrations The most effective vibration frequencies for clean sands have been found to be between approx-imately 25 and 30 Hz (Xanthakos et al., 1994) In situ densification of deep cohesionless deposits is often achieved through application of induced vibra-tions through specialized vibratory probes (vibroflot) or by other dynamic means These will be further described inChapter 6

The application of static loads has been conventionally applied through a range of heavy, steel drum, tired, or tracked vehicles for shallow or surface compaction An additional densification technique applying a static load is preloading, where large loads approximately equivalent to (or sometimes greater) than the final constructed project load are placed on a site to allow soil compression and settlement to occur prior to the actual construction, thus alleviating postconstruction distress Static compaction is applicable for most soil types, but is most effective for use with well-graded and cohesive soils

In addition to static and vibratory compaction (and combinations of the two), other loading methods that employ somewhat different or modified equipment include impact, tamping, kneading, and so forth A more detailed description of shallow compaction equipment is provided inChapter 5

4.2 ENGINEERING IMPROVEMENT OBJECTIVES

As mentioned previously, several fundamental soil engineering properties may be enhanced by densification Each of the major improvement objec-tives is addressed in the following sections

4.2.1 Bearing Capacity, Strength, and Stiffness

One of the most important engineering properties of interest for design and performance of structures built on, of, or within the earth is soil strength This generally refers to shear strength, as soils tend to fail in shear Soil shear strength is fundamental to analyses and design for engineering use of earth materials A background discussion of soil shear strength was provided in

Chapter 3 Basic soil mechanics teaches us that the capacity of a soil to sup-port bearing loads (bearing capacity), the ability of soil to stand up to lateral forces imposed by retaining walls, excavations, and so forth (lateral earth pressures), and the stability of sloping ground or sloped earth structures (slope stability), all rely on the shear strength of soil

As described inSection 3.1.2, soil shear strength is a limiting state of shear stress as a function of applied load Theoretical values of strength can be determined as a combination of the combined components of cohesive and frictional strengths under a set of limiting stress conditions Soil mechan-ics theory, further supported by laboratory tests, has shown that, for the same states of stress conditions, a soil with grains arranged in a tighter packing con-figuration (denser) will have higher frictional strength or greater frictional resistance Thus, attaining a greater degree of density will generally result

in increased shear strength, leading to greater bearing capacity, greater slope stability, ability to resist higher, lateral earth pressures, and so on

Stiffness (or stress-strain behavior) will also generally increase with increased density Stiffness is an important parameter for many engineering components where smaller tolerances on deformations are needed or desir-able Soil fabric or “structure” (arrangement of soil grains) may also account for variations in soil stiffness, especially for cohesive soils whose structure plays a critical role in response characteristics For granular soils with more rounded or “bulky” grains, the soil “structure” is essentially just a matter of grain packing.Figure 4.1depicts how a soil made up of rounded grains can

be arranged into a denser state by simply packing grains in a closer config-uration For fine-grained cohesive soil, moisture (or water content) at the time of compaction and the compaction method can be vitally important

in controlling the arrangement of soil grains This is described in some detail

inChapter 5 Research has indicated that some variation in stiffness may also occur for granular soils at low confining stresses compacted at different mois-ture levels (Carrier, 2000)

4.2.2 Compressibility and Settlement

As densification of a soil will reduce void space within the soil mass, it will also reduce the compressibility of the ground, as the soil structure will already

be in a compressed state The fundamental goal of reduced compressibility is

to reduce future settlement of the ground under the load of built structures

or of a prepared engineered site Depending on soil type, depth, structure, and stress-deformation characteristics, soil compressibility (and hence the amount of expected settlement or deformation under load) may be com-prised of elastic and/or inelastic components

The compressibility of granular soils is directly related to soil density (or degree of compaction) For the most part, the settlement of granular soil is essentially “immediate” and for practical purposes is often considered elastic Settlement of structures founded on granular soils can be estimated using rel-atively simple equations (Hookes law) employing such soil parameters as the elastic (Youngs) modulus (Es), and Poissons ratio (ms), as described in

Chapter 3 Denser sands will have higher Esandmsvalues and will therefore exhibit less settlement, as can be seen by examining Equation (3.4) Thus, densification of sands and other mostly granular soils is therefore an

“immediate” result unless saturated As will be described later inChapter 6, there are a number of deep densification techniques for granular soil with a maximum of approximately 15-20 “fines” (minus #200 material) But as will be discussed, some percentage of fines can actually improve the overall density and stiffness

Compressibility and settlement of cohesive materials, particularly satu-rated clays, is often a controlling design parameter for many structures As

Figure 4.1 Packing arrangement of rounded “bulky” soil grains: (a) loose, (b) dense.

described inChapter 3, the dominant portion of settlement in saturated clays occurs as a result of consolidation Consolidation settlement can be signif-icantly reduced by densifying these types of soils This involves techniques that will preconsolidate the ground prior to construction and application of final load Due to the typically low permeability of clay and the significant time required to consolidate these materials, especially when significant depths are encountered, densification techniques for these soil types often employ methods to expedite consolidation

4.2.3 Permeability and Seepage

With the decrease in void space when a soil is densified, it intuitively follows that the permeability of the soil will be decreased All else being held equal, this assumption is essentially true But for cohesive soils or soils containing appreciable amounts of clay, the permeability and seepage rate will also be heavily dependent on soil structure Fortunately, the structure of clayey soils can be controlled to a great degree by the compaction conditions and method (and equipment) of compaction used These attributes will be dis-cussed further inChapter 5

4.2.4 Volume Stability (Shrinking and Swelling)

Volume stability is an important parameter, as it has been noted that exces-sive shrinking, and particularly swelling, has been known to cause millions of dollars of damage each year to roadways, airfields, and foundations Repeated shrinking and swelling of expansive clayey soils in alternating wet-ting and drying cycles of certain soils has also been attributed to downward slope movement Volume stability is not easily achieved by merely densify-ing soil In fact, as will be discussed inChapter 5, soil densification may actu-ally aggravate swelling potential in some soils dependent on compaction conditions

4.2.5 Liquefaction Phenomenon and Mitigation

Several of the available soil and ground improvement applications are intended to mitigate liquefaction that may result from seismic (earthquake) events An overview of liquefaction phenomenon and ground/soil condi-tions that provide susceptibility to this type of soil failure was presented

inSection 3.1.2 As noted inChapter 3, three fundamental conditions must

be present for initiation of liquefaction:

(1) The soil must be essentially cohesionless, such that all of its shear strength results from intergranular friction and shear strength is a direct function

of effective stress

(2) The soil must be in a “loose” condition in that applied shear stress will cause a tendency for compression or contraction of the soil mass (3) The soil must be saturated and effectively undrained so that any increase

in loads will tend to generate positive water pressures, thereby decreas-ing effective stress

If any of these conditions are not met, then soil liquefaction is unlikely Some general methodological approaches to mitigate liquefaction occur-rence is presented throughout discussions of the ground modification tech-niques in this text These mitigation approaches can be fulfilled by several different (or combination of) ground improvement techniques In principle,

to mitigate liquefaction potential, one must eliminate one or more of the causative or susceptibility factors This simply means that if (1) density is increased, or (2) water saturation is eliminated, or (3) the material is made

to be “cohesive” by means of additional intergranular strength, the soil deposit would be rendered less likely to liquefy under dynamic (earthquake) loads Each of these variables can be addressed by means of ground improve-ment methods Densifying soil is one of the most accepted and well-defined means to achieve this goal while enjoying several other gains in engineering performance

RELEVANT ASTM STANDARDS

D4253-06 Standard Test Methods for Maximum Index Density and Unit Weight of Soils Using a Vibratory Table, V4.09

D7263-09 Standard Test Methods for Laboratory Determination of Density (Unit Weight) of Soil Specimens, V4.09

REFERENCES

Carrier III, W.D., 2000 Compressibility of a compacted sand J Geotech Geoenviron Eng.

126 (3), 273–275.

Schexnayder, C., Lukas, R.G., 1992a Dynamic compaction of nuclear waste Civil Engi-neering ASCE, New York, pp 64-65.

Schexnayder, C., Lukas, R.G., 1992b The use of dynamic compaction to consolidate nuclear waste Grouting, Soil Improvement and Geosynthetics ASCE, Geotechnical Special Publication No 30.

Xanthakos, P.P., Abramson, L.W., Bruce, D.A., 1994 Ground Control and Improvement John Wiley & Sons, Inc, 910 pp.