Field Work Procedure
Field
works involved heavy percussion boring mechanism at all two boreholes around the proposed
building location to
the maximum depth of 15.0m from
the ground level and SPT/DCPT at every 1.5m interval is recorded according as
the ground condition. Borehole logs were prepared at the site on the basis of
the visual observation of the soil obtained from the boreholes.
Insitu Tests
Standard Penetration
Test (SPT): Standard penetration test (SPT)
had been conducted by means of driving standard split spoon sampler to count
the number of blows per 15cm of penetration. The equipment and hammer in which
the SPT were conducted according to IS 21311 963.
The test consisted of driving a
splitspoon sampler with and outside diameter of 50mm into the soil at the base
of borehole. Driving was accomplished by a trip hammer weighing 65kg falling
freely through a height of 75cm onto the drive head which was fitted at the top
of rods. Procedure of driving SPT sampler tube consisted of driving the entire
split spoon to 15cm into the soil at the bottom of the borehole. It was then
driven 30cm further. The number of blows required to drive each 15cm distance
was recorded. The blow count for driving last 30cm penetration was referred to
as SPT ‘N’ value. The split spoon sampler was used to collect the disturbed
sample of soil for visual classification, analysis and determination of soil
characteristics.
The value of the standard
penetration number N depends on the relative density of the cohesion less soil
and the unconfined compressive strength of the cohesive soil. If the soil is
compact or stiff, the penetration number is high. The angle of shearing
resistance (Φ) of the cohesion less soil depends upon the number N. In general,
the greater the Nvalue, the greater is the angle of shearing resistance.
Sampling
(i)
Disturbed Sample:
Before any sample was taken, the
borehole was cleaned up of loose disturbed soil deposited during boring
operation. The samples which were obtained from bailer and in the SPT tube were
preserved as representative disturbed samples for finding out index properties.
The samples thus obtained were placed in airtight double plastic bags, labeled
properly for identification and later transported to the lab for analysis.
(ii)
Undisturbed Sample:
Undisturbed Sample was extracted
by means of thin wall tube (Shelby tube). The tube was pushed into the ground
and the sample recovered manually. The tube was sealed with wax and wrapped
with airtight polythene sheets and then bound by adhesive tapes and properly
labeled. The tube was properly packed in a wooden box so as to minimize the
disturbances during transportation to the laboratory and avoided the changes of
moisture content of sample. This sample was used for the determination of
strength and consolidation parameters.
Laboratory Tests
Disturbed samples were collected
in plastic bags and transported to Kathmandu for the following tests.
a)
Grain size analysis
b)
Direct Shear Test
c)
Moisture Content
d)
Specific Gravity
e)
Atterberg’s Limit Test
f)
Hydrometer Analysis
g)
Unconfined Compression Test
The natural water content was determined
from samples recovered from the split spoon sampler.
Grain size distribution was
determined from sieve analysis for fine grained particles and coarse grained
particles, respectively. The results
were combined to obtain the grain size distribution curves of the soil as can
be seen in the attached figures. The
grain size composition of the soils is also presented in terms of the
percentage of each particle size in the Test Result Summary Sheets.
Direct shear tests were conducted
on disturbed samples collected from the single boreholes. The samples were carefully extruded from the
sampling tubes and molded using standard moulds of 6.0 x 6.0 cm²
crosssectional areas and trimmed to 2.5 cm high. Solid metal plates were placed on both
surfaces of the samples to prevent the dissipation of pore water during
shearing. The direct shear equipment is
mechanicallyoperated and shearing will be applied at more or less constant
strain rate. If the samples are cohesive they will be sheared at a relatively
fast rate (duration of tests less than 10 minutes) to maintain undrained condition. The samples were sheared at three different
normal stresses. The direct shear test
results is presented in terms of the failure envelops to give the angle of
internal frictions () and the cohesion
intercepts (c).
The primary purpose of Unconfined
Compressive Strength of a cohesive soil test is to determine the unconfined
compressive strength, which is then used to calculate the unconsolidated
undrained shear strength of the clay under unconfined conditions. According to the ASTM standard, the
unconfined compressive strength (qu) is defined as the compressive stress at
which an unconfined cylindrical specimen of soil will fail in a simple
compression test. In addition, in this test method, the unconfined compressive
strength is taken as the maximum load attained per unit area, or the load per
unit area at 15% axial strain, whichever occurs first during the performance of
a test.
For soils,
the undrained shear strength (s_{u}) is necessary for the
determination of the bearing capacity of foundations, dams, etc. The undrained
shear strength (s_{u}) of clays is commonly determined from an
unconfined compression test.
The unconfined compressive
strength (q_{u}) is the load per unit area at which the cylindrical
specimen of a cohesive soil fails in compression
Where,
P= axial load at failure;
A = (Corrected area)
Where,
A_{0 }= initial area of the specimen;
e = axial strain = change in length / original length.
The undrained shear strength (s_{u}) of the
soil equal to one half of the unconfined compressive strength
Strata
Both the boreholes of the
proposed building were drilled up to a depth of 15.0 m. The percussion drilling
operation had been carried out on 10^{th} & 11^{th} January,
2018 for BH1 & BH2 respectively from the surface level of the ground. Black
coloured silty clay had been observed throughout the all borehole. Three
undisturbed samples (2 samples from BH1 & 1 sample from BH2) had been
collected during the drilling operation for the laboratory Unconfined
Compression Test.
Field Tests
Altogether 17 SPT tests were carried out for BH1 & BH2. Water table at each hole was noted. The minimum of the converted SPT values in all boreholes is adopted as the SPT number for the analysis of the bearing capacity.Field SPT Summary:
When dynamic loads are
applied on silty gravel and sandy soils in saturated state the pore pressure in
such soil will not be in a position to get dissipated due to low permeability.
Hence, during dynamic loading (i.e. application of blows) the pore water
will offer a temporary resistance to dynamic loads. This leads to higher value
of Nvalue which is unsafe. Therefore, when SPT is performed in saturated silts
and fine sands and if the observed Nvalue is more than 15, a correction has to
be applied to reduce the observed values. This correction is applied on the
Nvalue corrected for over burden pressure (N’).
If the stratum (during
testing) consists of fine sand & silty gravel below water table, the
corrected Nvalue (N’) has to be further corrected to get the final corrected
value N”.
N” = 15+1/2(N’15)
Therefore, the final
average adopted N value used for bearing capacity calculation is 12.
Sampling
Disturbed
samples recovered from SPT are collected from the borehole for the necessary
laboratory tests.
 Disturbed Sample
Before sampling,
the borehole was cleaned up of loose disturbed soil deposited during drilling
operation. The samples obtained from bailer and in the SPT tube were preserved
as representative disturbed samples for finding out geotechnical properties.
The samples thus obtained were placed in airtight double plastic bags and
labeled properly for identification.
Collected
samples are transported to the laboratory carefully for a series of laboratory
testing: grain size analysis, natural moisture content, direct shear and
specific gravity.
 Undisturbed Sample
Undisturbed
samples were extracted by means of thin wall tube (Shelby tube). The tube was
pushed into the ground and the samples were recovered mechanically. The tube
was sealed with wax and wrapped with airtight polythene sheets and then bound
by adhesive tapes and properly labeled. The tube was properly packed in a
wooden box to minimize the disturbances during transportation to the laboratory
as well as to avoid the changes in moisture content of samples.
Ground Water Table
Ground water table (GWT) was
observed obseved at 4.0m in BH1 and 4.1m in BH2 during the drilling period.
Laboratory Investigation Results
Index Properties
The result of physical and index
properties of soil samples collected from various depths are presented in the
attached summary sheet.
The soil is classified as fined
grained CL i.e. silty clayey as per USCS Soil Classification System based on the
grain size analysis.
Natural Moisture Contents of the
soil ranged from 14.08 to 71.253 for BH1 & 14.44 to 75 for BH2.
Similarly, Specific Gravity determination on selected soil samples is in the
range of 2.51 to 2.60 for BH1 & 2.52 to 2.61 for BH2.
The above result verifies that
the soil falls as silty clayey below the consideration of foundation level to
the depth of investigation.
Bearing capacity analysis
Allowable Bearing Pressure
The allowable
bearing pressure (qa) is the maximum pressure that can be imposed on the
foundation soil taking into consideration the ultimate bearing capacity of the
soil and the tolerable settlement of the structure. Analysis to determine the
ultimate bearing capacity and the pressure corresponding to a specified maximum
settlement were performed and the minimum pressure obtained from two analyses
were adopted as the allowable bearing pressure.
Allowable Bearing Pressure Based on Ultimate Bearing Capacity
Since the soil
in the vicinity of the foundation level has been found to be CL i.e. silty
clayey at the proposed building site, the allowable bearing capacity has been
analyzed using the NValues from SPT results. Empirical formula of Teng (1988)
applicable for this type of soils has been used to obtain the allowable bearing
pressure with safety factor equal to 3.
For open foundation
q_{ns} =
0.02N^{2}BR_{W1} + 0.06(100 + B^{2})D_{f}R_{W2}
Where,
q_{ns} = net safe bearing
pressure, t/m^{2}
N = SPT value corrected
with respect to overburden and dilatancy
B = width of footing, m
D = depth of footing, m
RW & RW’ = correction
factors for position of water level
Allowable Bearing Pressure Based on Tolerable Settlement
The maximum
allowable settlement for footings in sandy gravel is generally 40 mm and for
Mat foundation in sand the allowable settlement is 65 mm (Skempton and
MacDonald, 1955).
The method of
Teng (1988) has been employed for the analysis. This method is a modification
of the method of Terzaghi and Peck (1948) such that the allowable bearing
pressure could be directly obtained from the SPT values.
For Mat foundation:
q_{ns} =
35(N3){(B+0.3)/2B}^{2}R_{W}R_{d}
Where,
q_{ns} =_{ }Net
safe bearing pressure, kN/m^{2} for maximum settlement of 25 mm.
N = SPT
value corrected for overburden pressure
B = width of footing, m
D = depth of footing, m
R_{w} = Water table
correction factor
R_{d} = Depth correction factor = 1 + D/B
The
minimum average SPT values from the boreholes in each structure have been
selected for the analysis of bearing capacity of the relevant structure.
The
allowable bearing pressure for a limiting settlement other than 25 mm (e.g. x
mm) can be linearly interpolated from the allowable bearing pressure for 25 mm
settlement.
q_{a} (x mm) = q_{a}
(25 mm)(x/25)
Analysis of the Results
Evaluation of safe bearing capacity for Borehole 1 & 2:
A.
For Isolated
Footing:
Based on Ultimate Bearing Capacity
q_{ns} =
0.02N^{2}BR_{W1} + 0.06(100 + B^{2})D_{f}R_{W2}
Depth
of footing, D = 1.5 m
Width
of footing, B = 1.0 m
Adopted average SPT value after
correction, N = 12 for the entire depth
Q_{ns}
= 117.426 KN/m^{2}
Therefore,
Net Safe Bearing Capacity (Q_{a}) = 117.426 KN/m^{2}
Summary of Results of the Analysis:
·
The safe bearing capacity for Isolated footing with different width
and resting at different depth is presented as follows;
Summary
Result of Isolated Footing
Depth, m

Foundation Width, m

B=1.0m

B=1.5m

B=2.0m

B=2.5m

B=3.0m

B=3.5m

1.5

Qall
KN/m^{2}

117.426

132.656

148.327

164.440

180.995

197.990

2.0

147.150

162.748

178.934

195.710

213.073

231.026


2.5

176.874

192.840

209.542

226.979

245.152

264.061


3.0

206.599

222.932

240.149

258.248

277.231

297.096


3.5

236.323

253.024

270.756

289.518

309.309

330.131

B.
For Mat
Footing:
Based on Tolerable Settlement
q_{ns} =
35(N3){(B+0.3)/2B}^{2}R_{W}R_{d}
Depth
of footing, D = 1.5 m
Width
of footing, B = 5.0 m
Adopted average SPT value after
correction, N = 12 for the entire depth
For a
settlement of 25 mm = 176.967 KN/m^{2}
So, Safe Bearing Capacity for a settlement
of 40 mm = 283.947 KN/m^{2}
·
The safe bearing capacity for Mat footing with different width and
resting at different depth is presented as follows;
Summary
Result of Mat Footing
Depth, m

Foundation Width, m

B=5.0m

B=10.0m

B=15.0m

B=20.0m

B=25.0m

B=30.0m

1.5

Qall
KN/m^{2}

283.147

267.347

262.181

259.617

258.084

257.065

2.0

288.490

269.942

263.894

260.896

259.104

257.914


2.5

293.832

272.538

265.608

262.175

260.124

258.762


3.0

299.174

275.134

267.322

263.453

261.145

259.610

C.
Computation
of Pile Capacity
The pile capacity has
been computed using soil mechanics method for both sandy as well as clayey
soil. The pile type analyzed is underreamed bored and cast in place pile.
Also, the ultimate
bearing capacity of drilled shaft of 0.8m, 1.0m and 1.2m diameter at various
depths are mentioned below. A factor of safety of 2.5 is shall be adopted for
estimation of allowable side shear capacity, and 3 for estimation of allowable
base bearing capacity from their respective ultimate
values. If the
foundation is submerged adopt a factor of safety of 6 for estimation of
allowable base bearing capacity from their respective ultimate values.
All Boreholes
Qs = ultimate side resistance;
Qb = ultimate base resistance;
Qall = total allowable load using
factors of safety applied to the ultimate side resistance and the ultimate base
resistance.
Dia. 0.8m

Dia. 1.0m


Depth, m

Qs, kN

Qb, kN

Qall KN

Qs, kN

Qb, kN

Qall KN

1.5

103.508

722.531

275.346

129.375

903.095

344.157

3.0

214.979

702.539

305.839

268.703

878.107

382.270

4.5

329.103

664.371

331.158

411.348

830.400

413.916

6.0

440.574

607.330

349.301

550.675

759.105

436.593

7.5

549.390

559.697

369.696

686.685

699.569

462.085

9.0

663.514

557.585

407.033

829.330

696.928

508.753

10.5

793.563

606.351

466.638

991.878

757.881

583.253

12.0

926.266

592.592

506.286

1157.744

740.683

632.809

13.5

1061.623

580.598

547.407

1326.928

725.693

684.207

15.0

1204.942

591.912

598.951

1506.063

739.833

748.632

Note: The pile
length also incorporates pile upto the scour depth which need to be deducted. So,
if x is the total pile length, then total allowable bearing capacity (Q_{all})of
the pile equals to the value at (x1.0)metre minus the value at scour depth.
When fine or medium fine,
saturated, loose sand deposit is subjected to a sudden shock (generated by an
earthquake) the mass will temporarily liquefy. This phenomenon is known as liquefaction.
When liquefaction takes place in a particular soil, the bearing capacity of the
soil is disappeared and the structure built on it, tilts or even sinks.
The past big earthquakes, have
shown that saturated sandy soils in a loose to medium dense condition were
liquefied during earthquakes varying in magnitude from 5.5 to 8.5 (Richter
scale) and epicenter distance from several miles to hundreds of miles.
From the case studies,
liquefaction potential characteristics of the soil depend on:
1) The soil contains less than
10 percent fines (silt and clay sizes)
2) D60 is between 0.2 mm and
1.0 mm
3) Cu = (D60/D10) is between 2
and 5; and
4) The blow count per 30 cm
standard penetration tests is less than 15.
Where:
D_{60 }=
60 percent of the soil grains
smaller than that size.
D_{10} = 10 percent of
the soil grains smaller than corresponding size
C_{u} = Coefficient of uniformity = D_{60}/D_{10}
Liquefaction Analysis
According to
borehole log and lab tests, both the boreholes BH1 & BH2 have mostly the all
the soil layers of cohesionless nature, which requires liquefaction analysis.
So liquefaction analysis is carried out for both boreholes.
Calculation of Reduction Factor, R_{d}
Another option is to assume a
linear relationship of rd versus depth and use the following equation
(Kayen et
al., 1992): rd = 1 – (0.012) (z)
Estimation of cyclic stress ratio (CSR)
It is the stress developed during earthquake of magnitude, M_{w }resulting
in soil liquefaction and it is based on the simplified procedure based on Seed
and Idriss (1971) equation.
where a_{max, }the
peak horizontal acceleration at the ground surface generated by an earthquake;
g, acceleration due to gravity; σ_{vo }and σ_{vo}’
are total and effective vertical over burden stresses, respectively; and r_{d }, stress
reduction coefficient, a_{max} is estimated using Boore et
al. 1993 relation.
Computation of cyclic resistance ratio (CRR)
Computation of cyclic resistance
ratio (CRR) based on corrected “N” (for Clean sand condition), (N_{1})_{60}.
The above equation gives goods approximation ≤30for (N_{1})_{60}≤30. So, the maximum value
of (N_{1})_{60}=30 is taken for CRR calculation.
Computation of factor of safety (FS)
Computation
of factor of safety (FS) for soil liquefaction based on ratio of stresses, (CRR/CSR) as
_{}
Reduction factor R_{d} versus depth
below level or gently sloping ground surfaces. (From Andrus and Stokoe 2000)
Depth
(m)

Reduction
Factor, R_{d}

Overburden
Stress (kPa)

Corrected
SPT Test

N_{1(60)}

CSR

CRR7.5

Safety
Factor, CRR/CSR

Effective

N


1.5

0.982

13.04

13

36

0.133





3.0

0.964

26.07

12

24

0.131

0.265

1.99

4.5

0.946

39.11

17

27

0.128

0.344

2.62

6.0

0.928

52.14

14

19

0.126

0.208

1.62

7.5

0.910

65.18

11

14

0.125

0.147

1.16

9.0

0.904

69.52

16

19

0.121

0.206

1.64

10.5

0.874

91.25

16

17

0.118

0.178

1.47

12.0

0.856

104.28

17

17

0.120

0.177

1.49

13.5

0.838

110.57

17

16

0.117

0.172

1.44

15.0

0.820

122.85

17

15

0.115

0.163

1.40

16.0

0.808

131.04

13

11

0.112

0.124

1.08

Based on the
factor of safety against liquefaction, it is concluded that during the
anticipated earthquake the in situ soil is not susceptible to liquefaction.
Seismic Design for Foundation
The following
relation expresses the seismic inertia force.
S_{if} = α_{c}
M
Where,
a_{c} = seismic
acceleration
M = mass
of the body,. We know the weight of the body,
W = Mg
Where, g is the
acceleration due to gravity (g = 9810 mm/sec2)
S_{if} = α_{c}W/g
Let us introduce
a new index characteristics α_{o, }which is basic horizontal seismic
coefficient.
S_{if} = α_{c}W
Each mass in a structure is
assumed to be subjected to an equivalent force given by α_{o} times its
weight at its own center of gravity in one horizontal direction at a time.
Earthquake
Intensity, M 7 8 8.5 9
Seismic
Coefficient, α_{o }0.025 0.05 0.08 0.1
Max.
Ground Accelerations 0.37 g 0.50 g 0.50 g 0.50
g
It shows that
seismic acceleration is a fraction of acceleration due to gravity. To arrive at
design coefficients, the basic values given above are multiplied by suitable
factors β to take care of foundation conditions and importance of structures.
The factor β for different soil conditions are given below.
Values of β for
Type of soils

For raft
foundation

Type I – rock
or hard soils

1.0

Type II –
Medium soils

1.0

Type III –
Soft soils

1.0

Nepal is located
on high seismic zone. An earthquake intensity M, not less than 8.5 is to be
adopted and accordingly the seismic coefficient for an earthquake structure.
Further, in this site, take type
II of medium soils for the determination of β value.
No comments:
Post a Comment