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<ul><li><p>FOUNDATION ANDCIVIL ENGINEERINGSITE DEVELOPMENTSite hydrology and land planning are two initial factors that influence land use and foundationdesign. Part 1 addresses these concerns. Site hydrology involves both subsurface and surface watercontent and movement. Land planning develops construction techniques intended to accommodatehydrologic problems and provide best use of the parcel. Coverage of the topic will be rathercursoryas a rule, foundation engineers are not involved with the early stages of development, butan awareness of the potential problems is beneficial.</p><p>P A R T 1</p><p>Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.</p><p>Any use is subject to the Terms of Use as given at the website.</p><p>Source: PRACTICAL FOUNDATION ENGINEERING HANDBOOK</p></li><li><p>Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.</p><p>Any use is subject to the Terms of Use as given at the website.</p><p>FOUNDATION AND CIVIL ENGINEERING SITE DEVELOPMENT</p></li><li><p>SECTION 1A</p><p>WATER BEHAVIOR IN SOILSROBERT WADE BROWN</p><p>1A.1 MOISTURE REGIMES 3.1 1A.5 RUN-OFF 1.81A.2 SOIL MOISTURE VERSUS 1A.6 GROUNDWATER </p><p>WATER TABLE 1.4 RECHARGE 1.91A.3 SOIL MOISTURE VERSUS 1A.7 CLAY SOIL 1.9</p><p>AERATION ZONE 1.5 1A.8 SOIL MOISTURE VERSUS 1A.3.1 Transpiration 1.5 ROOT DEVELOPMENT 1.91A.3.2 Gravity and 1A.8.1 Summary: Soil Moisture </p><p>Evaporation 1.6 Behavior 1.151A.4 PERMEABILITY VERSUS 1A.9 CONCLUSIONS 1.20</p><p>INFILTRATION 1.7 REFERENCES 1.21</p><p>Site hydrology and land planning are two initial factors that influence land use and foundation de-sign. This section addresses these concerns. Site hydrology involves both subsurface and surfacewater content and movement. Land planning develops construction techniques intended to accom-modate hydroponic problems and provide best use of a parcel of land. The coverage will be rathercursory. As a rule, foundation engineers are not initially involved with the early stages of develop-ment. An awareness of the potential problems is, however, beneficial.</p><p>1A.1 MOISTURE REGIMES</p><p>The regime of subsurface water can be divided into two general classifications: the aeration zoneand the saturation zone. The saturation zone is more commonly termed the water table or ground-water, and it is, of course, the deepest. The aeration zone includes the capillary fringe, the interme-diate belt (which may include one or more perched water zones), and, at the surface, the soil waterbelt, often referred to as the root zone (Fig. 1A.1). Simply stated, the soil water belt provides mois-ture for the vegetable and plant kingdoms; the intermediate belt contains moisture essentially indead storageheld by molecular forces; and the perched ground water, if it occurs, develops essen-tially from water accumulation either above a relatively impermeable stratum or within an unusuallypermeable lens. Perched water occurs generally after heavy rain and is relatively temporary. Thecapillary fringe contains capillary water originating from the water table. The soil belt can containcapillary water available from rains or watering; however, unless this moisture is continually re-stored, the soil will eventually desiccate through the effects of gravity, transpiration, and/or evapora-tion. When it does so, the capillary water is lost. The soil belt is also the zone that most critically in-fluences both foundation design and stability. This will be discussed in the following sections. Asstated, the more shallow zones have the greatest influence on surface structures. Unless the watertable is quite shallow, it will have little, if any, material influence on the behavior of foundations ofnormal residential structures. Furthermore, the surface of the water table, the phreatic boundary,will not normally deflect or deform except under certain conditions, such as when it is in the prox-imity of a producing well. Then the boundary will draw down or recede.</p><p>Engineers sometimes allude to a natural buildup of surface soil moisture beneath slab founda-tions due to the lack of evaporation. This phenomenon is often referred to as center doming or cen-</p><p>1.3</p><p>Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.</p><p>Any use is subject to the Terms of Use as given at the website.</p><p>Source: PRACTICAL FOUNDATION ENGINEERING HANDBOOK</p></li><li><p>ter lift (refer to Sec. 7A.3). If the source for this moisture is assumed to be the water table and if thewater table is deeper than about 10 ft (3 m),* the boundary (as well as the capillary fringe) is notlikely to dome; hence, no transfer of moisture to the shallow soils would be likely. The othersource of moisture could involve the capillary or osmotic transfer from underlying soils to the dryer,more shallow soils. When expansive soils are involved, this intrusion of moisture can cause the soilto swell. This swell is ultimately going to be rather uniform over the confined area. (This expansivesoil has a much greater lateral than vertical permeability.) Again no natural doming is likely to oc-cur. Refer to Sec. 1A.8.</p><p>Following paragraphs will provide further discussion concerning water migration in varioussoils as represented by several noted authorities.</p><p>1A.2 SOIL MOISTURE VERSUS WATER TABLE</p><p>Alway and McDole [1] conclude that deep subsoil aquifers (e.g., water table) contribute little, ifany, moisture to plants and, hence, to foundations. Upward movement of water below a depth of 12in (30 cm) was reportedly very slow at moisture contents approximating field capacity. Field capac-ity is defined as the residual amount of water held in the soil after excess gravitational water hasdrained and after the overall rate of downward water movement has decreased (zero capillarity).Soils at lower residual moisture content will attract water and cause it to flow at a more rapid rate.Water tends to flow from wet to dry in the same way as heat flows from hot to coldfrom higherenergy level to lower energy level.</p><p>Rotmistrov [1] suggests that water does not move to the surface by capillarity from depthsgreater than 10 to 20 in (25 to 50 cm). This statement does not limit the source of water to the watertable or capillary fringe. Richards [1] indicates that upward movement of water in silty loam can de-velop from depths as great as 24 in (60 cm). McGee [1] postulates that 6 in (15 cm) of water can bebrought to the surface annually from depths approaching 10 ft (300 cm). Again, the source of wateris not restricted in origin.</p><p>The seeming disparity among results obtained by these hydrologists is likely due to variation in</p><p>1.4 FOUNDATION AND CIVIL ENGINEERING SITE DEVELOPMENT</p><p>FREE WATERSURFACE</p><p>SOIL WATERBELT</p><p>INTERMEDIATEBELT</p><p>CAPILLARYFRINGE</p><p>AERATION ZONE</p><p>CAPILLARYWATER</p><p>PERCHEDGROUNDWATER</p><p>FIGURE 1.A1 Moisture regimes.</p><p>*The abbreviations of units of measure in this book are listed in Appendix C.Numbers in brackets indicate references at the end of the sections.</p><p>Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.</p><p>Any use is subject to the Terms of Use as given at the website.</p><p>WATER BEHAVIOR IN SOILS</p></li><li><p>experimental conditions. Nonetheless, the obvious consensus is that the water content of the surfacesoil tends to remain relatively stable below very shallow depths and that the availability of soil waterderived from the water table ceases when the boundary lies at a depth exceeding the limit of capil-lary rise for the soil. In heavy soils (e.g., clays), water availability almost ceases when the watersource is deeper than 4 ft (120 cm), even though the theoretical capillary limit normally exceeds thisdistance. In silts, the capillary limit may approximate 10 ft (300 cm), as compared to 1 to 2 ft (30 to60 cm) for sands. The height of capillary rise is expressed by Eq. (1A.1).</p><p>Tr2hc = Tst 2r cos </p><p>or (1A.1)</p><p>hc = cos </p><p>where hc = capillary rise, cmTst = surface tension of liquid at temperature T, g/cm</p><p>r = radius of capillary pore, cm = meniscus angle at wall or angle of contactT = unit weight of liquid at temperature T, g/cm</p><p>2</p><p>For behavior in soils, the radius r is difficult, if not impossible, to establish. It is dependent uponsuch factors as void ratio, impurities, grain size and distribution, and permeability. Since the capil-lary rise varies inversely with effective pore or capillary radius, this value is required for mathemat-ical calculations. Accordingly, capillary rise, particularly in clays, is generally determined by exper-imentation. In clays, the height and rate of rise are impeded by the soils swell (loss of permeability)upon invasion of water. Fine noncohesive soils will create a greater height of capillary rise, but therate of rise will be slower. More information on soil moisture, particularly that dealing with claysoils, will be found in Parts 6, 7, and 9 of this volume.</p><p>1A.3 SOIL MOISTURE VERSUS AERATION ZONE</p><p>Water in the upper or aeration zone is removed by one or a combination of three processes: Transpi-ration, evaporation, and gravity.</p><p>1A.3.1 Transpiration</p><p>Transpiration refers to the removal of soil moisture by vegetation. A class of plants, referred to asphreatophytes, obtain their moisture, often more than 4 ft (120 cm) of water per year, principallyfrom either the water table or the capillary fringe. This group includes such seemingly diversespecies as reeds, mesquite, willows, and palms. Two other groups, mesophytes and xerophytes, ob-tain their moisture from the soil water zone. These include most vegetables and shrubs, along withsome trees. In all vegetation, root growth is toward soil with greater available moisture. Roots willnot penetrate a dry soil to reach moisture. The absorptive area of the root is the tip, where root hairsare found. The loss of soil moisture by transpiration follows the root pattern and is generally some-what circular about the stem or trunk. The root system develops only to the extent necessary to sup-ply the vegetation with required water and nutrition. Roots not accessible to water will wither anddie. These factors are important to foundation stability, as will be discussed in following sections.</p><p>In many instances, transpiration accounts for greater loss of soil moisture than does evaporation.</p><p>2TstrT</p><p>1.5WATER BEHAVIOR IN SOILS</p><p>Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.</p><p>Any use is subject to the Terms of Use as given at the website.</p><p>WATER BEHAVIOR IN SOILS</p></li><li><p>In another process, interception, precipitation is caught and held by foliage and partially evaporatedfrom exposed surfaces. In densely planted areas, interception represents a major loss of rainfall, per-haps reaching as high as 10 to 25% of total precipitation [1].</p><p>1A.3.2 Gravity and Evaporation</p><p>Gravity tends to draw all moisture downward from the soil within the aeration zone. Evaporationtends to draw moisture upward from the surface soil zone. Both forces are retarded by molecular,adhesive, and cohesive attraction between water and soil as well as by the soils capacity for capil-lary recharge. If evaporation is prevented at the surface, water will move downward under the forcesof gravity until the soil is drained or equilibrium with an impermeable layer or saturated layers is at-tained. In either event, given time, the retained moisture within the soil will approximate the fieldcapacity for the soil in question.</p><p>In other words, if evaporation were prevented at the soil surface, as, for example, by a foundation,an excessive accumulation of moisture would initially result. However, given sufficient time, eventhis protected soil will reach a condition of moisture equilibrium somewhere between that originallynoted and that of the surrounding uncovered soil. The natural tendency of covered soil is to retain amoisture level above that of the uncovered soil, except, of course, during periods of heavy inundation(rains) when the uncovered soil reaches a temporary state at or near saturation. In this latter instance,the moisture content decreases rapidly with the cessation of rain or other sources of water. </p><p>The loss of soil moisture from beneath a foundation caused by unabated evaporation might tendto follow a triangular configuration, with one leg vertical and extending downward into the bearingsoil and the other leg being horizontal and extending under the foundation. The relative lengths ofthe legs of the triangle would depend upon many factors, such as the particular soil characteristics,foundation design, weather, and availability of moisture (Fig.1A.2).</p><p>Davis and Tucker [2] reported the depth as about 5 ft (1.5 m) and the penetration approximately10 ft (3 m). In any event, the affected distances (legs of the triangle) are relatively limited. As withall cases of evaporation, the greatest effects are noted closer to the surface. In an exposed soil, evap-oration forces are ever present, provided the relative humidity is less than 100%. The force of gravi-ty is effective whether soil is covered or exposed.</p><p>1.6 FOUNDATION AND CIVIL ENGINEERING SITE DEVELOPMENT</p><p>FIGURE 1A.2 Typical loss of soil moisture from beneath a slab founda-tion during prolonged drying cycle.</p><p>PERIMETER BEAM</p><p>GROUND</p><p>INTERIOR SLAB</p><p>PENETRATION</p><p>AREA OF PRINCIPALLOSS OF MOISTURE</p><p>DEPTH</p><p>Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.</p><p>Any use is subject to the Terms of Use as given at the website.</p><p>WATER BEHAVIOR IN SOILS</p></li><li><p>1A.4 PERMEABILITY VERSUS INFILTRATION</p><p>The infiltration feature of soil is more directly related to penetration from rain or water at the sur-face than to subsurface vertical movement. The exceptions are those relatively rare instances inwhich the ground surface us within the capillary fringe. Vertical migration or permeation of the soilby water infiltration could be approximately represented by the single-phase steady-state flow equa-tion, as postulated by Darcy [3].</p><p>Q = + gc sin (1A.2)</p><p>where Q = rate of flow in direction LA = cross-sectional area of flowk = permeability = fluid viscosity</p><p>= pressure gradient in direction L</p><p>L = direction of flow = fluid density = meniscus angle at wall or angle of contact = angle of dip ( &gt; 0 if flow L is up dip)gc = gravity constant</p><p>If = 90, sin = 1, and, simplified, Eq. (1.2) becomes</p><p>Q = (P + gc h)</p><p>where h = L sin and gch is the hydrostatic head.If H = P + gch, where H is the fluid flow potential, then</p><p>Q = When flow is horizontal, the gravity factor gc drops out. Any convenient set of units may be used inEq. (1A.2) so long as the units are consistent. Several influencing factors represented in this equa-tion pose a difficult deterrent to mathematical calculations. For example, the c..</p></li></ul>