geotechnical engineering

ALMA MATER STUDIORUM • UNIVERSITY OF BOLOGNA FACULTY OF ENGINEERING

International Master Course Civil Engineering

GEOTECHNICAL ENGINEERING Project # 1: Site Investigation Report

Student: Student number:

Amar Topu 0000710129

Academic Year 2014/201

1. Introduction A geotechnical site investigation was carried out in the city of Parma, Italy with the aim of providing of geotechnical data for the design and construction of sixteen metal silos. This project seeks to analyze all the site data including in-situ and laboratory test. A geotechnical model of the site was then obtained from the campaign test data.

The project is divided into two parts; the first part involves report of analysis of ground investigation and mechanical properties. The second part includes the design of foundation; shallow and deep foundation.

2. Site description

Site descriptions of the area under investigation are shown in figure 1 to 3. The location of boreholes and CPT verticals are shown in figure 3.

Figure 1: General layout

Figure 2: Plan and elevation view

Figure 3: Planimetry and location of in situ investigations

3. Stratigraphy

Figure 5: soil stratigraphy with CPTS

Figure 6: soil stratigraphy from borehole

4. Methods and Test results

To provide a geotechnical model of the site the following site investigations were carried out. Independent observations from different test were then used in identifying and characterize the deposits of soil in the ground and produce a ground model for geotechnical analyses. 

3 Core rotary boreholes (S1, S2, S3)

  

4 Piezocone penetrations tests (CPTU1, CPTU2, CPTU3, CPTU4) 2 Seismic piezocone penetration tests (SCPTU1, SCPTU2) 8 Standard penetration tests (1 SPT in the borehole S1, 3 SPT in the borehole S2, 4 SPT in the borehole S3)

4.1 In – situ test analysis

Due to the many limitations associated with laboratory tests data in obtaining strength and stiffness parameters of soil, in-situ test are developed to overcome these limitations. For the purpose of this project two types of in-situ testing techniques were considered to provide rapid assessment of key parameters that can be conducted during ground investigations. The techniques employed are; Standard penetration tests (SPT) and cone penetration (CPT) test

4.1.1 Cone piezocone penetration tests (CPTU) and SCPTU

Cone piezocone penetrations test is one of the versatile tools available for soil exploration. These tests are mainly used in identifying and profiling the different strata within the ground, it can be used to reliably estimate soil strength, stiffness and consolidation parameters. CPTU are commonly used to assess undrained conditions. Seismic piezocone penetration tests allow for descrete seismic sound to be made inorder to determine shear wave velocity and strain shear modulus (Go).

Figure 7: Soil behaviour type classification using the Ic method

The OCR, Cu and qt plots of each penetration tests were performed from ground investigation data obtained. A comparison of the CPT test is shown in the below figures Where, the OCR and CU were computed using the following formula; 𝑞𝑡 − 𝜎𝑣0 𝑂𝐶𝑅 = 0.33 × ( ) ′ 𝜎𝑣0

𝑐𝑢 =

𝑞𝑐 − 𝜎𝑣0 𝑁𝑘

Whereby Nk is the correction factor; for normally consolidated soil, Nk = 15 recommended. Lab test most often need to be calibrated with a correction factor in order to refine the data to ensure reliability in the results.

Overconsolidation ratio, OCR 0

5

10

15

20

25

30

35

0 5 Cpt1

10

Depth (m)

Cpt2

15

Cpt3

Cpt4

20

Scpt1

25

Scpt2

30 35

40

Figure 8: OCR versus depth from CPTU test

Cu VALUES 0

0.1

0.2

0.3

0.4

0.00 -5.00 -10.00

CTP1

DEPTH [M]

CPT2

-15.00 -20.00 -25.00 -30.00 -35.00 -40.00

Figure 9: Cu versus depth from CPTU test

CPT3 CPT4 SCPT1 SCPT2

Cone resistance, qt [Mpa] -40 -35

Depth (m)

-30 CPTU1

-25

CPTU2 CPTU3

-20

CPTU4 SCPTU 1

-15

SCPTU 2

-10 -5 -0.5

0.5

1.5

2.5

3.5

4.5

0

Figure 10: qt versus depth from CPTU test

The OCR values reduces with depth, thus the deeper you go the higher is the vertical stress resulting in a decreasing trend in OCR values. From the data obtained one can conclude that during cone penetration test, excess pore water around the cone increases this in return influence pore water pressure. Lower pore water pressure is observed in coarse grained material due to high permeability, while high pore water pressure depicts fine grained material. A plot of cone resistance against depth clearly show low qt value in fine grained soil (i.e clay) in comparison to coarse grained sand or gravel. The CPT test results illustrate that different soil types exhibit different sleeve friction resistance (fs) and qt; gravel have low fs and high qc while clay have high fs and low qt.

5. Standard penetration tests (SPT) SPT is a rapid and cost effective in-situ testing techniques used in the investigation of the coarse-grained soils. This method uses a drop hammer to determine the number of blows required for the sampler penetration.

Due to the variation in rod hammer system used, amount of energy transferred to the standard sampler is varied. Therefore blow counts (N) are corrected to a standardized (reference) free fall hammer energy ratio of 60% (N60). Blow counts also depend on the size of the borehole. Penetration resistance increase with stress level, therefore corrected N SPT values were normalized using a depth correction factor (CN). Angle of shear resistance was evaluated using Schmertmann and Bolton the latter show slightly high value compared to Schmertmann method.

ER

N60 = NSPT. 60 ;

Where ER (%) =

Emeas Etheo

Thus, (N1)60 = CN.N60 According to Schemertmann NSPT values are related to the friction angle, vertical stress and relative density, such that NSPT = f (σv’ , ϕ’) and NSPT = f (σv’ , DR)

Different friction angle values were evaluated by Schmertmann method using the below equations and figure:

Figure 11: ϕ’ versus DR graph

Table 1: Evaluation of friction angle (ϕ’) according to Schmertmann method Bore Holes

Depth [m]

S1

S2

S3 Avg

σ'vo

CN

N60

NSPT

(N1)60

DR

ɸ'1

ɸ'2

ɸ'3

ɸ'4

Average ɸ'

36

0.329

1.498

77.76

72

116.45

139.31

47.50

47.52

48.43

49.15

48.15

35

0.320

1.498

108.00

100

161.74

164.19

50.99

50.38

50.92

51.13

50.86

54.5

0.495

1.496

108.00

100

161.60

164.11

50.98

50.37

50.91

51.13

50.85

35

0.320

1.498

66.96

62

100.28

129.28

46.10

46.37

47.43

48.34

47.06

38

0.347

1.497

88.56

82

132.61

148.67

48.81

48.60

49.37

49.89

49.17

51.5

0.468

1.496

92.88

86

138.99

152.20

49.31

49.00

49.72

50.18

49.55

56

0.509

1.496

108.00

100

161.59

164.11

50.98

50.37

50.91

51.13

50.85

0.398

1.497

92.88

86

139.04

151.70

49.24

48.94

49.67

50.14

49.50

The CN , (N1)60 , depth correction factor and DR are calculated according to the Skempton method.

CN =

3

; For coarse grained sands

σ′v

2+(100)

0.5

(N1)60 DR = ( ) 60

The Skempton method was necessary to obtain the above parameters used both in Schmertmann and Bolton

Table 2: Evaluation of friction angle (ϕ’) according to Bolton method Bore Holes

S1

S2

S3

Depth [m]

K0

σ'ho

p'

ɸ'1-ɸ'cv

ɸ'

36

0.305

100.457

529.915

12.578

50.58

35

0.305

97.709

515.419

15.496

53.50

54.5

0.305

151.144

797.288

13.340

51.34

35

0.305

97.709

515.419

11.563

49.56

38

0.305

105.954

558.907

13.386

51.39

51.5

0.305

142.900

753.800

12.410

50.41

56

0.305

155.419

819.838

13.202

51.20

Bolton suggested the best used equation to estimate peak friction angle from relative density. Where,

5.1 Determination of the Young’s modulus (Eu)

In fine-grained soils the corresponding over consolidated ratio (OCR) and undrained shear strength (Cu) at defined strata were used to evaluate the Young’s modulus (Eu). The below Figure used the interpolation of OCR value to obtain the ratio of the secant shear modulus at 25% (Eu25) of applied pressure (qf) and Cu at corresponding plasticity index (IP). Where,

Where v’ = 0.25 selected!

Figure 12: Relation between undrained Y. modulus & Undrained strength versus OCR

Table 3: Young’s modulus for the corresponding layers Layer (m)

IP

Cu

OCR

Eu/Cu

Eu(Kpa)

Eu(Mpa)

E'(Mpa)

0-8

41

46

4

300

13800

13.80

11.50

8-21

30

65

2.5

575

37375

37.38

31.15

22-34

26

72

1.4

620

44640

44.64

37.20

Organic

76

56

1.4

200

11200

11.20

9.33

Average

26.75

6 Laboratory test

Laboratory tests are necessary to determine relevant soil parameters, boreholes or trial pits is used to collect soil samples for data analysis. Two kinds of samples were collected; 13 undisturbed samples and 1 disturbed sample and subjected to laboratory tests. Two open standpipes were installed in the boreholes S1 (with perforated tube between 33 and 36 m b.g.l.) and S3 (with perforated tube between 6 and 12 m b.g.l.). Undisturbed samples are mainly required for shear strength and consolidation test and disturbed samples are used in soil classification and visual compact test. In this project two test were conducted, namely Triaxial and oedometer test.

6.1 Triaxial test

Triaxial test is widely used for measuring soil behavior in shear and is suitable for all types of soil. The main advantage it has over direct shear test is that, its drainage condition can be controlled. Three undisturbed samples were collected and triaxial lab test was performed on each of these individual samples where by friction angle (φ’) and cohesion (c’) of the soil are computed.

A general Failure envelope curve for obtaining strength parameters; friction angle (φ’) and cohesion (c’) using the stress invariant (q - p’) is shown in figure xxx below

Where;

Sample: S1 B

p versus q (maximum)

600

y = 0.9395x + 43.621 500

q400 300

p versus q (maximum) Linear (p versus q (maximum))

200 100 0 0.00

100.00

200.00

p'

Figure 13: Sample S1_B failure envelope

300.00

400.00

500.00

Table 4: Sample_1 summary results

Sample 2_C

p versus q (maximum) 450

y = 0.6316x + 128.42

400 350

q

300 250

p versus q (maximum)

200

Linear (p versus q (maximum))

150 100 50 0 0.00

100.00

200.00

p'

Figure 14: Sample S2_C failure envelope

300.00

400.00

500.00

Table 5: Sample 2 summary results Sample S2_C

σ'a Kpa q (max)

σ'r Kpa

p' (max)

Specimen 1

221.09

152.70

300.09

100

Specimen 2

295.34

255.45

452.34

200

Specimen 3

422.76

468.92

750.76

400

Sample 3

φ' (deg)

c'

16.60

60.62


y = 0.7558x + 55.12

350 300 250

q

200


Linear (p versus q (maximum))

100 50 0 0.00

100.00

200.00

300.00

400.00

500.00

p'

Figure 15: Sample S3 failure envelope

Table 6: Sample 3 summary table σ'a Kpa

Sample S3_C q (max)

σ'r Kpa

p' (max)

Specimen 1

145.97

125.66

222.97

100

Specimen 2

207.14

194.05

332.14

200

Specimen 3

373.12

422.37

671.12

400

φ' (deg)

c'

19.61

25.98

Layer

Clay

Clay

Organic

Clay

z (m) Go 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 24.5 25.5 26.5 27.5 28.5 29.5 30.5

37 39.3 42.6 38.3 73.9 48.2 51.4 59.5 55.2 60.9 85.9 72.1 49.8 61.3 44.2 35.6 35 52.7 55.8 43.9 30.9 47.9 71.2 95.4 51.8 78 118 105 71.9 60.3 102

SCPTU-1 0.1% IP G/G0 G MPa V' 0.62 22.94 0.62 24.366 0.62 26.412 0.62 23.746 41 0.62 45.818 0.62 29.884 0.62 31.868 0.62 36.89 0.62 34.224 0.55 33.495 0.55 47.245 0.55 39.655 0.55 27.39 0.55 33.715 0.55 24.31 29.5 0.55 19.58 0.55 19.25 0.55 28.985 0.55 30.69 0.55 24.145 0.55 16.995 0.78 37.362 76 0.78 55.536 0.53 50.562 0.53 27.454 0.53 41.34 0.53 62.54 26.33 0.53 55.65 0.53 38.107 0.53 31.959 0.53 54.06

E' Mpa E' (avg) Mpa 0.25 57.35 0.25 60.915 0.25 66.03 0.25 59.365 0.25 114.545 76.70778 0.25 74.71 0.25 79.67 0.25 92.225 0.25 85.56 0.25 83.7375 0.25 118.1125 0.25 99.1375 0.25 68.475 0.25 84.2875 0.25 60.775 71.96979 0.25 48.95 0.25 48.125 0.25 72.4625 0.25 76.725 0.25 60.3625 0.25 42.4875 0.25 93.405 116.1225 0.25 138.84 0.25 126.405 0.25 68.635 0.25 103.35 0.25 156.35 113.0225 0.25 139.125 0.25 95.2675 0.25 79.8975 0.25 135.15

Layer

Clay

Clay

Organic

Clay

z (m) 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 24.5 25.5 26.5 27.5 28.5 29.5 30.5 31.5 32.5 33.5

Go 43.1 49.8 38.2 56.2 53.8 65.8 61 51.3 50.4 58 82.6 95.9 62.4 58.5 65.5 53.8 43.4 59.9 49.6 37.9 48 44.4 69.9 77 216 29.1 96.2 80.4 117 53 79.2 64.6 82.1 85.3

SCPTU-2 0.1% IP G/G0 G MPa 0.62 26.722 0.62 30.876 0.62 23.684 0.62 34.844 41 0.62 33.356 0.62 40.796 0.62 37.82 0.62 31.806 0.62 31.248 0.55 31.9 0.55 45.43 0.55 52.745 0.55 34.32 0.55 32.175 0.55 36.025 29.5 0.55 29.59 0.55 23.87 0.55 32.945 0.55 27.28 0.55 20.845 0.55 26.4 0.78 34.632 76 0.78 54.522 0.53 40.81 0.53 114.48 0.53 15.423 0.53 50.986 0.53 42.612 26.33 0.53 62.01 0.53 28.09 0.53 41.976 0.53 34.238 0.53 43.513 0.53 45.209

V' 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

E' Mpa E' (avg) Mpa 66.805 77.19 59.21 87.11 80.88 83.39 101.99 94.55 79.515 78.12 79.75 113.575 131.8625 85.8 80.4375 90.0625 81.98 73.975 59.675 82.3625 68.2 52.1125 66 86.58 111.44 136.305 102.025 286.2 38.5575 127.465 106.53 155.025 123.87 70.225 104.94 85.595 108.7825 113.0225

Table 7: A comparison of shear (G) and Young’s modulus (E) of laboratory samples

Sample

Cell pressure Depth [kpa] (z)

S1 - B

200

S2 - C

400

S3 - C

200

16.5 17 42 42.5 24.5 25

q/2 [Mpa]

εa

v

G [Mpa]

0.12975

0.00858

0.5

5.04

15.12

0.21138

0.00357

0.5

19.74

24.67

0.10357

0.003915

0.5

8.82

13.23

E [Mpa]

The shear and Young’s modulus are computed using the following equations; where v is the Poisson ratio (0.5) G=

𝑞 + 3 𝑥 ∗ ℰa 2

𝐸 = 2𝐺 𝑥 (1 + 𝑣)

Table 8: Comparison of Young's Modulus (E) from CPT and Triaxial test DEPTH(m)

CPT

0-8

11.5

8-21

31.1458333

15.25

22-34

37.2

13.25

34-44 Organic

Tri-axial

24.67 9.33333333

Given the stress - strain curve from the borehole data, a failure envelope curve (p’ – q) for each sample at three different cell pressure of 100, 200 and 400 kpa were obtained from which we analyze the key parameters of the Triaxial test. Strength parameters (ɸ’ and c’) and stiffness parameters (G and E) were obtained.

6.2 Oedometer test

This test is used to determine the characteristics of soil during one-dimensional consolidation and swelling. All parameters related to soil compressibility and settlements are obtained from the oedometer analysis. The compression index Cc and the expansion index Cs were obtained from the void ratio (e) – effective axial stress curve. The coefficient of primary (Cv) and secondary consolidation (Cα) were obtained from the log time method (Casagrande), see figures below.

Figure 16: Typical void ratio – effective stress relationship

Figure 17: Settlement versus time – log time method (casagrande)

Figure 18: 0.196 constant from average degree of consolidation

Where, 𝐶𝑐 /𝐶𝑠 = specimen.

𝑒0 −𝑒1 𝜎′ log( 1⁄ ′ ) 𝜎2

𝑐𝑣 =

0.196×𝑑2 𝑡50

, where d is half the average thickness of the

1-D elastic modulus (constrain modulus);

𝐸′𝑜𝑒𝑑 =

1 𝑚𝑣

1

𝑒0 −𝑒1

Coefficient of volume of compressibility; 𝑚𝑣 = × ( ′ ′) 1+𝑒0 σ1 −σ0

6.2.1 Samples and Results Sample S1_D Sample S1 D 0.8 0.75 0.7

e

0.65 0.6 0.55 0.5 0.45 0.4 0.35 10

100

Figure 19: e log curve for sample S1_D

P [kPa]

1000

10000

h [mm]

Sample S1 D 19 18.9 18.8

H0

18.7 18.6 H 50

18.5 18.4 18.3 18.2

18.1 18 1

10

100

1000

10000

100000

1000000

t50

t [s]

Figure 20: Settlement versus time – log time method (casagrande) for S1D

Sample S2_A

Figure 21: e log curve for sample S2_A

Sample S2 A

18.4

18.2

h [mm]

18

17.8

17.6

17.4

17.2 1

10

100

1000

10000

100000

1000000

t[s]

Figure 22: Settlement versus time – log time method (casagrande) for S2 A

Sample S2_B

Sample S2 B 0.95 0.85

e

0.75 0.65 0.55 0.45 0.35 10

100

1000 P [kPa]

Figure 23: e log curve for sample S2_B

10000

Sample S2 B

18.9 18.8 18.7 18.6 18.5

18.4 18.3 18.2 1

10

100

1000

10000

100000

1000000

Figure 24: Settlement versus time – log time method (casagrande) for S2 B

Sample S2_E

Sample S2 E 0.75 0.7 0.65

e

0.6 0.55 0.5 0.45 0.4 0.35 10

100

1000 P [kPa]

Figure 25: e log curve for sample S2_E

10000

Sample S2 E

19.3 19.25 19.2

h [mm]

19.15 19.1

19.05 19 18.95 18.9 18.85 1

10

100 Time [s] 1000

10000

100000

Figure 26: Settlement versus time – log time method (casagrande) for S2 E

Sample S3_D

Sample S3 D 0.65 0.6

e

0.55 0.5 0.45 0.4 0.35 10

100

1000 P [kPa]

Figure 27: e log curve for sample S3_D

10000

Settlement _ Time graph 18.6

18.5

h [mm]

18.4

18.3

18.2

18.1

18 1

10

100

1000

10000

100000

1000000

t[s]

Figure 28: Settlement versus time – log time method (casagrande) for S3_ D

Table 9: Comparisons of parameters obtained from Oedometer test Sample

Cs/Cc

Comment (Cs/Cc)

Cv

cα

cα/cc

Comment (cα/cc)

mV [m /MN]

Cc

Cs

S1 D

0.312

0.043

0.138 Ok, within range

6.913E-08

0.010


0.178

1.229E-07

5.625

S2 A

0.704

0.083


3.591E-08

0.027


0.504

1.809E-07

1.985

S2 B

0.322

0.027


5.810E-07

0.010


0.199

1.154E-06

5.035

S2 E

0.223

0.030


9.576E-08

0.008


0.134

1.285E-07

7.453

S3 D

0.246

0.027


1.800E-07

0.007


0.132

2.382E-07

7.555

2

k

Eoed

Table 10: Comparisons of parameters with corresponding stratum test Stratum

Cc

Cs

Cv

cα

Clay 0 - 8

-

-

-

-

Clay 8 - 21

-

-

-

-

Organic Clay 21 - 22

0.704

0.083

3.591E-08

0.02676

Clay 22 - 34

0.322

0.027

5.810E-07

0.00971

Gravel 34 - 38

-

Clay 38 - 44 Gravel 44 - 58 Clay > 58

0.312

-

0.043

0.223

6.913E-08 -

0.030

9.576E-08

0.01030 0.00772

7 Physical and mechanical properties table

Table 11: Summary of physical properties of the analysed data

Sample

Z(m)

A0

Acum(%)

Lcum(% Scum(% G L(%) ) S(%) ) G(%) cum(%) wP(%) wL(%)

W(%)

IP(%)

IC

e0

mv Eoed [m2/M [MN/m N] 2]

Cα

k

Cc

Cs

Cv

S1-1

5.4

0

28.46

28.46

71.31

99.77

0.23

100

S1-A

6.25

0

98.64

98.64

1.36

100

0

100

29

70

36.9

41 0.80732

17.95

S1-B

16.75

0

99.11

99.11

0.89

100

0

100

29

74

33.9

45 0.89111

18.44

S1-C

31.75

0

99.19

0.81

100

0

100

30

68

36.2

38 0.83684

18.02

S1-D

42.25

0

75.58

24.25

99.83

0.17

100

0

100

25

56

29.5

31 0.85484

18.96

25.51

0.753

0.178

5.625

0.103 1.23E-07

0.312

0.043 6.91E-08

S2-A

20.25

0

64.06

33.9

97.96

2.04

100

0

100

67

143

80.5

76 0.82237

15.2

23.48

1.890

0.504

1.985

0.027 1.81E-07

0.704

0.083 3.59E-08

S2-B

30.75

0

70.65

27.4

98.05

1.95

100

0

100

28

49

32.5

21 0.78571

18.34

25.54

0.830

0.199

5.035 0.0097 1.15E-07

0.322

0.027 5.81E-07

S2-C

42.25

0

70.44

29.18

99.62

0.38

100

0

100

25

70

28.5

45 0.92222

18.68

S2-D

60.25

0

70.73

29.02

99.75

0.25

100

0

100

27

69

25.7

42 1.03095

19.71

S2-E

69.25

0

83.16

16.82

99.98

0.02

100

0

100

31

77

27.3

46 1.08043

19.09

25.34

0.692

0.134

7.453 0.0077 1.28E-07

0.223

0.03 9.58E-08

S3-A

2.3

0

98.31

98.31

1

99.31

0.69

100

S3-B

12.25

0

99.11

99.11

0.89

100

0

100

29

43

32.6

14 0.74286

18.45

S3-C

24.75

0

73.3

26.66

99.96

0.04

100

0

100

30

50

32.4

20

0.88

18.72

S3-D

43.75

0

42.53

53.83

96.36

3.62

99.98

0.02

100

24

51

25.1

27 0.95926

19.54

25.52

0.641

0.132

7.55 0.0072 2.38E-07

0.246

0.027 1.80E-07

99.19

Ic =

28.9

ɣs ɣ (KN/m^ (KN/m^3) 3) 18.95

31.9

WL (%) − W(%) IP (%)

eo =

W(%) ɣs 𝑥 100 ɣ𝑤

Gs =

eo x 100 W

1.63

geotechnical engineering

geotechnical engineering

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