NEPAL ENGINEERING SERVICES

Wednesday, July 10, 2019

Soil Test Field Works


 
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.

    

 In-situ 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 2131-1 963.

The test consisted of driving a split-spoon 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 N-value, 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 cross-sectional 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 mechanically-operated 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 un-drained 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 un-drained shear strength (su) is necessary for the determination of the bearing capacity of foundations, dams, etc. The un-drained shear strength (su) of clays is commonly determined from an unconfined compression test.
The unconfined compressive strength (qu) 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,
A0 = initial area of the specimen;
e = axial strain = change in length / original length.
The undrained shear strength (su) of the soil equal to one half of the unconfined compressive strength


     Field Investigation Results

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 10th & 11th January, 2018 for BH-1 & BH-2 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 BH-1 & 1 sample from BH-2) had been collected during the drilling operation for the laboratory Unconfined Compression Test.

Field Tests

Altogether 17 SPT tests were carried out for BH-1 & BH-2. 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 N-value which is unsafe. Therefore, when SPT is performed in saturated silts and fine sands and if the observed N-value is more than 15, a correction has to be applied to reduce the observed values. This correction is applied on the N-value corrected for over burden pressure (N’).
If the stratum (during testing) consists of fine sand & silty gravel below water table, the corrected N-value (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 BH-1 and 4.1m in BH-2 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 BH-1 & 14.44 to 75 for BH-2. Similarly, Specific Gravity determination on selected soil samples is in the range of 2.51 to 2.60 for BH-1 & 2.52 to 2.61 for BH-2.

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 N-Values 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
qns = 0.02N2BRW1 + 0.06(100 + B2)DfRW2    
Where,
qns = net safe bearing pressure, t/m2
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:
qns = 35(N-3){(B+0.3)/2B}2RWRd
Where,
qns = Net safe bearing pressure, kN/m2 for maximum settlement of 25 mm.
N = SPT value corrected for overburden pressure
B = width of footing, m
D = depth of footing, m
Rw = Water table correction factor
Rd = 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.
qa (x mm) = qa (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
qns = 0.02N2BRW1 + 0.06(100 + B2)DfRW2
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
Qns = 117.426 KN/m2
 Therefore, Net Safe Bearing Capacity (Qa) = 117.426 KN/m2

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/m2
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
qns = 35(N-3){(B+0.3)/2B}2RWRd
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/m2
So, Safe Bearing Capacity for a settlement of 40 mm = 283.947 KN/m2
·               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/m2
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 under-reamed 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 (Qall)of the pile equals to the value at (x-1.0)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:
D60          =         60 percent of the soil grains smaller than that size.
D10       =          10 percent of the soil grains smaller than corresponding size
Cu        =          Coefficient of uniformity = D60/D10

Liquefaction Analysis
According to borehole log and lab tests, both the boreholes BH-1 & BH-2 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, Rd
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, Mw resulting in soil liquefaction and it is based on the simplified procedure based on Seed and Idriss (1971) equation.
where amax, the peak horizontal acceleration at the ground surface generated by an earthquake; g, acceleration due to gravity; σvo and σvo’ are total and effective vertical over burden stresses, respectively; and rd , stress reduction coefficient, amax 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), (N1)60.
The above equation gives goods approximation ≤30for (N1)60≤30. So, the maximum value of  (N1)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 Rd versus depth below level or gently sloping ground surfaces. (From Andrus and Stokoe 2000)


Depth (m)
Reduction Factor, Rd
Overburden Stress (kPa)
Corrected SPT Test
N1(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.
Sif        =          αc M

Where,
ac         =          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)
Sif        =          αcW/g

Let us introduce a new index characteristics αo, which is basic horizontal seismic coefficient.

Sif        =          αcW
Each mass in a structure is assumed to be subjected to an equivalent force given by αo 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, αo               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.


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