Craig's Soil Mechanics 7th Edition
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Craig's Soil Mechanics 7th Edition

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equation.)

Figure 2.22 Example 2.5. (Reproduced from H.R. Cedergren (1989) Seepage, Drainage and Flow
Nets, ª John Wiley & Sons, Inc., New York, by permission.)

Seepage through embankment dams 65

2.9 GROUTING

The permeability of coarse-grained soils can be considerably reduced by means of
grouting. The process consists of injecting suitable fluids, known as grouts, into the
pore space of the soil; the grout subsequently solidifies, preventing or reducing the
seepage of water. Grouting also results in an increase in the strength of the soil. Fluids
used for grouting include mixes of cement and water, clay suspensions, chemical
solutions, such as sodium silicate or synthetic resins, and bitumen emulsion. Injection
is usually effected through a pipe which is either driven into the soil or placed in a
borehole and held with a packer.
The particle size distribution of the soil governs the type of grout that can be used.

Particles in suspension in a grout, such as cement or clay, will only penetrate soil pores
whose size is greater than a certain value; pores smaller than this size will be blocked
and grout acceptability will be impaired. Cement and clay grouts are suitable only for
gravels and coarse sands. For medium and fine sands, grouts of the solution or
emulsion types must be used.
The extent of penetration for a given soil depends on the viscosity of the grout and

the pressure under which it is injected. These factors in turn govern the required
spacing of the injection points. The injection pressure must be kept below the pressure
of the soil overburden, or heaving of the ground surface may occur and fissures may
open within the soil. In soils having a wide variation of pore sizes it is expedient to use a
primary injection of grout of relatively high viscosity to treat the larger pores, followed
by a secondary injection of grout of relatively low viscosity for the smaller pores.

2.10 FROST HEAVE

Frost heave is the rise of the ground surface due to frost action. The freezing of water
is accompanied by a volume increase of approximately 9%; therefore, in a saturated
soil the void volume above the level of freezing will increase by the same amount,
representing an overall increase in the volume of the soil of 21⁄2�5% depending on the
void ratio. However, under certain circumstances, a much greater increase in volume
can occur due to the formation of ice lenses within the soil.
In a soil having a high degree of saturation the pore water freezes immediately below

the surface when the temperature falls below 0 �C. The soil temperature increases with
depth but during a prolonged period of subzero temperatures the zone of freezing
gradually extends downwards. The limit of frost penetration in the UK is normally
assumed to be 0.5m, although under exceptional conditions this depth may approach
1.0m. The temperature at which water freezes in the pores of a soil depends on the pore
size; the smaller the pores the lower the freezing temperature. Water therefore freezes
initially in the larger pores, remaining unfrozen in the smaller pores. As the tempera-
ture falls below zero, higher soil suction develops and water migrates towards the ice in
the larger voids where it freezes and adds to the volume of ice. Continued migration
gradually results in the formation of ice lenses and a rise in the ground surface. The
process continues only if the bottom of the zone of freezing is within the zone of
capillary rise so that water can migrate upwards from below the water table. The
magnitude of frost heave decreases as the degree of saturation of the soil decreases.

66 Seepage

When thawing eventually takes place the soil previously frozen will contain an excess of
water with the result that it will become soft and its strength will be reduced.
In the case of coarse-grained soils with little or no fines, virtually all the pores are

large enough for freezing to take place throughout the soil and the only volume
increase is due to the 9% increase in the volume of water on freezing. In the case of
soils of very low permeability, water migration is restricted by the slow rate of flow;
consequently, the development of ice lenses is restricted. However, the presence of
fissures can result in an increase in the rate of migration. The worst conditions for
water migration occur in soils having a high percentage of silt-size particles; such soils
usually have a network of small pores, yet, at the same time, the permeability is not too
low. A well-graded soil is reckoned to be frost-susceptible if more than 3% of the
particles are smaller than 0.02mm. A poorly graded soil is susceptible if more than
10% of the particles are smaller than 0.02mm.

PROBLEMS

2.1 In a falling-head permeability test the initial head of 1.00m dropped to 0.35m in
3 h, the diameter of the standpipe being 5mm. The soil specimen was 200mm
long by 100mm in diameter. Calculate the coefficient of permeability of the soil.

2.2 A deposit of soil is 16m deep and overlies an impermeable stratum: the coefficient
of permeability is 10�6 m/s. A sheet pile wall is driven to a depth of 12.00m in the
deposit. The difference in water level between the two sides of the piling is 4.00m.
Draw the flow net and determine the quantity of seepage under the piling.

2.3 Draw the flow net for seepage under the structure detailed in Figure 2.23 and
determine the quantity of seepage. The coefficient of permeability of the soil is
5:0� 10�5 m/s. What is the uplift force on the base of the structure?

Figure 2.23

Problems 67

2.4 The section through a long cofferdam is shown in Figure 2.24, the coefficient of
permeability of the soil being 4:0� 10�7 m/s. Draw the flow net and determine
the quantity of seepage entering the cofferdam.

2.5 The section through part of a cofferdam is shown in Figure 2.25, the coefficient
of permeability of the soil being 2:0� 10�6 m/s. Draw the flow net and determine
the quantity of seepage.

2.6 The dam shown in section in Figure 2.26 is located on anisotropic soil. The
coefficients of permeability in the x and z directions are 5:0� 10�7 and
1:8� 10�7 m/s, respectively. Determine the quantity of seepage under the dam.

2.7 An embankment dam is shown in section in Figure 2.27, the coefficients of
permeability in the horizontal and vertical directions being 7:5� 10�6 and
2:7� 10�6 m/s, respectively. Construct the top flow line and determine the
quantity of seepage through the dam.

Figure 2.24

Figure 2.25

68 Seepage

2.8 Details of an excavation adjacent to a canal are shown in Figure 2.28. Determine
the quantity of seepage into the excavation if the coefficient of permeability is
4:5� 10�5 m/s.

2.9 Determine the quantity of seepage under the dam shown in section in Figure
2.29. Both layers of soil are isotropic, the coefficients of permeability of the
upper and lower layers being 2:0� 10�6 and 1:6� 10�5 m/s, respectively.

Figure 2.26

Figure 2.27

Figure 2.28

Problems 69

REFERENCES

1 British Standard 5930 (1981) Code of Practice for Site Investigation, British Standards

Institution, London.

2 Casagrande, A. (1940) Seepage through dams, in Contributions to Soil Mechanics

1925–1940, Boston Society of Civil Engineers, Boston, MA, pp. 295–336.

3 Cedergren, H.R. (1989) Seepage, Drainage and Flow Nets, 3rd edn, John Wiley & Sons,

New York.

4 Harr, M.E. (1962) Groundwater and Seepage, McGraw-Hill, New York.

5 Hvorslev, M.J. (1951) Time Lag and Soil Permeability in Ground-Water Observations,

Bulletin No. 36, Waterways Experimental Station, US Corps of Engineers, Vicksburg, MS.

6 Ischy, E. and Glossop, R. (1962) An introduction to alluvial grouting, Proceedings of the

ICE, 21, 449–74.

7 Sherard, J.L., Dunnigan, L.P. and Talbot, J.R. (1984) Basic properties of sand and gravel

filters, Journal of the ASCE, 110, No. GT6, 684–700.

8 Sherard, J.L., Dunnigan,