A wide excavation is made on a site which has the following soil conditions:0-2 m Gravel: gsat = 21.8 kN/m3, gdry = 18.5 kN/m32-6 m Silty sand: gsat = 19.6 kN/m3, gdry = 18.4 kN/m36-21 m Heavy Clay: gt = 20 kN/m321 m and below: PERVIOUS sandstoneThe water table is at a depth of 1.5 m below the ground level. The piezometric pressure head at the top of the pervious sandstone is 5 m above the ground surface.If an excavation of depth 10 m is required, and a factor of safety of 1.5 against heave at the bottom of the clay layer is set, calculate the pressure head reduction required in sandstone. Filter Design Criteria (USBR 2011)Base Soil Categories:Percent finer than No. 200sieve (0.075 mm) (afterregrading where applicable)Base soilcategoryBase soil description1> 85Fine silts and clays40 - 85Silts, clays, silty sands, and clayey sandsSilty and clayey sands and gravelsSands and gravels315 - 394.< 15Note: mm = millimeterParticle Retention Requirement:Base soilcategoryFiltering - Maximum D15FThe maximum D15F should be s 9 xunless the soils are dispersive. Dispersive soils require a maximumD15F that is s 6.5 x D85B size, but not less than 0.2 mm.B, but not less than 0.2 mm,1The maximum D15F should be s0.7 mm unless soil is dispersive, inwhich case the maximum D15F should be < 0.5 mm.A. For nondispersive soils, the maximum D15F should be:40- A[(4xDs5B)-0.7mm] + 0.7mm85425where:A = Percent passing No. 200 sieve.When 4 x DasB is less than 0.7 mm, use 0.7 mmB. For dispersive soils, use 0.5 mm.4The maximum D15F should be s 4 x D85B of base soil after regrading.Permeability Requirement:D15F > 5 x D15B, but not less than 0.1 mmMaximum and minimum particle size criteria:Base soil categoryMaximum D100FMinimum D5FALL categories<2 inches0.075 mm(51 mm)(No. 200 sieve)3. El = 5 mEl = 0 mGravelWater TableEl =-1.5 mEt=-2 mSilty SandEl = -6 mExcavation Line El=-10 mHeavy ClayEl = -21 mSandstoneSegregation criteria:If D10F is:(mm)Then, maximum D90F is:(mm)Base soil category20< 0.50.5-1.01.0-2.02530ALL categories2.0-5.05.0–10405060> 10

Given data - For - 0-2 m Gravel: gsat = 21.8 kN/m3, gdry = 18.5…

Answered: A wide excavation is made on a site…

Structural Analysis

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ISBN:9781337630931

Author:KASSIMALI, Aslam.

Publisher:KASSIMALI, Aslam.

Chapter2: Loads On Structures

Section: Chapter Questions

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A wide excavation is made on a site which has the following soil conditions:

0-2 m Gravel: g_sat = 21.8 kN/m³, g_dry = 18.5 kN/m³

2-6 m Silty sand: g_sat = 19.6 kN/m³, g_dry = 18.4 kN/m³

6-21 m Heavy Clay: g_t = 20 kN/m³

21 m and below: PERVIOUS sandstone

The water table is at a depth of 1.5 m below the ground level. The piezometric pressure head at the top of the pervious sandstone is 5 m above the ground surface.

If an excavation of depth 10 m is required, and a factor of safety of 1.5 against heave at the bottom of the clay layer is set, calculate the pressure head reduction required in sandstone.

Filter Design Criteria (USBR 2011)
Base Soil Categories:
Percent finer than No. 200
sieve (0.075 mm) (after
regrading where applicable)
Base soil
category
Base soil description
1
> 85
Fine silts and clays
40 - 85
Silts, clays, silty sands, and clayey sands
Silty and clayey sands and gravels
Sands and gravels
3
15 - 39
4.
< 15
Note: mm = millimeter
Particle Retention Requirement:
Base soil
category
Filtering - Maximum D15F
The maximum D15F should be s 9 x
unless the soils are dispersive. Dispersive soils require a maximum
D15F that is s 6.5 x D85B size, but not less than 0.2 mm.
B, but not less than 0.2 mm,
1
The maximum D15F should be s0.7 mm unless soil is dispersive, in
which case the maximum D15F should be < 0.5 mm.
A. For nondispersive soils, the maximum D15F should be:
40- A
[(4xDs5B)-0.7mm] + 0.7mm
854
25
where:
A = Percent passing No. 200 sieve.
When 4 x DasB is less than 0.7 mm, use 0.7 mm
B. For dispersive soils, use 0.5 mm.
4
The maximum D15F should be s 4 x D85B of base soil after regrading.
Permeability Requirement:
D15F > 5 x D15B, but not less than 0.1 mm
Maximum and minimum particle size criteria:
Base soil category
Maximum D100F
Minimum D5F
ALL categories
<2 inches
0.075 mm
(51 mm)
(No. 200 sieve)
3.

El = 5 m
El = 0 m
Gravel
Water Table
El =-1.5 m
Et=-2 m
Silty Sand
El = -6 m
Excavation Line El=-10 m
Heavy Clay
El = -21 m
Sandstone
Segregation criteria:
If D10F is:
(mm)
Then, maximum D90F is:
(mm)
Base soil category
20
< 0.5
0.5-1.0
1.0-2.0
25
30
ALL categories
2.0-5.0
5.0–10
40
50
60
> 10

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