Chapter 6 - Essentials of Soil Mechanics by David McCarthy

8/12/2019 Chapter 6 - Essentials of Soil Mechanics by David McCarthy

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Essentials of Soil Mechanics by David McCarthy (Answers to questions)

Chapter 6

6-1. A canal is located in an area where its route parallels that of a nearby river. The elevation of thewater surface in the canal is +375 ft. In the river, the water surface is at elevation +345 ft. A stratum ofcoarse-grained soil 10 ft thick is sandwiched between a relatively impermeable fine-grained soil, andextends between the canal and the river. The distance between the river and the canal is 500 ft, and thecoefficient of permeability of the buried coarse stratum is 0.01 ft per hour.

a. Make a sketch of the described conditions.

b. Calculate the seepage loss from canal to river, gallons per day per mile of river canal length.



6-2 . What are flow nets and why are they used?

Flow nets are a pictorial method of studying the path that moving water follows. They are usedbecause it may be necessary to use flow nets to evaluate flow where the zones of flow or directions offlow are irregular, where water enters and escapes from permeable zone of soil by traveling a short

distance, or where the flow boundaries are not well defined (a boundary being the separation betweenwhere flow does and does not occur).

6-3. A concrete gravity dam is 100 ft long from upstream to downstream end. It is constructed on astratum of sand-silt soil 40 ft thick. The coefficient of permeability for this soil is 0.1ft/day. Rockunderlines the rock stratum. The bottom of the dam is 6 feet below the soil surface.

a. For the condition where the height of water against the upstream face of the dam is 45 ft abovethe soil surface, and the water level at the downstream end is just at the soil surface, draw theflow net for seepage beneath the dam and calculate the quantity of seepage in gallons per dayper foot of width of dam.



b. For the condition where the height of water against the upstream face of the dam is 35 ft abovethe soil surface, and the water level at the downstream end is 5 ft above the soil surface, drawthe flow net for seepage beneath the dam and compute the seepage in gallons per day per footof width of dam.

6-4. The vertically upward flow of groundwater (an artesian condition) occurs through a sand depositwhere the void ratio is 0.60 and specific gravity of soil particles is 2.65. What hydraulic gradient isnecessary for a quicksand condition to develop?

6-5. The hydraulic gradient for an occurrence where there is a vertically upward flow of water through asand mass is 0.95. If the specific gravity of soil particles is 2.75 and the void ratio is 0.65, determine if a

quicksand or erosion condition could develop.6-6. Well points are to be used to dewater a large trench excavation that extends a 35 ft below thesurface of the water table. Assume that the bottom width of the trench excavation of 70 ft.

a. Develop a simple sketch of the necessary well-point system. Use separate diagrams to show theprogress of the necessary stages of installation.



b. Assume that half the necessary lowering of the water table elevation is achieved with the outerrow of well points. If the soil being dewatered is a fine to medium sand, approximately at whatdistance beyond the well-point location is the original groundwater table not affected by well-point pumping.

6-7. Indicate the intended functions for foundation drains, and the major requirements for design andinstallation so as to achieve a properly operating system.

The intended function for foundation drains is controlling the depth below the water table whennot to great where groundwater will be flowing in the vicinity of the structure, provision can be made sothat the water will be quickly carried away from the building area, and at worse only a limited height ofgroundwater buildup against the exterior will occur. With adequate provisions, the large hydrostaticpressures that tend to force seepage entry thus will not develop. The major requirements for design andinstallation so as to achieve a properly operating system are pipes provided with perforations orinstalled with open joints so that the groundwater can enter into the pipe, gravel and/or sand filtermaterial must surround the drain pipe so that the soil particles tending to be carried along by theinflowing water are prevented from clogging the drain pipe opening or causing erosion and an outlet forcollected water, preferably disposal will be by gravity flow.

6-8. What are the intended functions and limitations of interceptor drains as conventionally used forhighway and airfield construction?

In paved highways and airfield runways, the design frequently calls for interceptor trench drainslocated parallel to the shoulder. The purpose of the drains is to lower the groundwater table to a levelbeneath the pavement and to permit easy lateral drainage (escape) for water finding its way into thecoarse base material provided beneath the pavement (water resulting from upward migration from

capillarity and from infiltration through cracks, joints or voids in the pavement).

6-9. What are the primary reasons for having limitations on the sizes of particles used to constructdrainage filters?

Water flowing towards a drainage structure must be able to enter the drain easily and quicklybut must not be permitted to carry with it small soil particles that could eventually clog the drain, orcause erosion or cavitations in the areas surrounding the drain.

6-10. Briefly describe the conditions necessary and the resulting sequence of occurrences that typicallytake place in a frost heaving situation. Specify the soil type considered most susceptible to large frost

heaves.

The conditions necessary and the resulting sequence of occurrences that typically take place in afrost heaving situation are presence of frost-susceptible soils, presence of freezing temperatures, andpresence of a supply or source of water to help from and feed the ice lenses. The soil type consideredmost susceptible to large frost heaves is silt mixtures such as silty sands and silty clays.



6-11. What method are commonly use to prevent the occurrence of, or to offset the effects of, frostheave?

These methods include removing existing frost-susceptible soils to about frost depth, removingor cutting off the water that could feed ice-lens growth and protecting the susceptible soil from freezing

temperatures.

Chapter 6 - Essentials of Soil Mechanics by David McCarthy

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