presentation subsoil investigation foundation recommendation geotechnical assessment geology

UNIVERSITY OF NIGERIA DEPARTMENT OF GEOLOGY

SIWES SEMINAR PRESENTATON ON:

GEOTECHINCAL ASSESSMENT (SUBSOIL INVESTIGATION)

BY:BASSEY DANIEL KUFRE

2010/170012SUPERVISOR: MR. I. OKONKWO

OUTLINE• COMPANY PROFILE• ACTIVITIES UNDERTAKEN• SUBSOIL INVESTIGATION

METHODOLOGYTEST RESULTSRESULT ANALYSIS INTERPRETATION AND RECOMENDATION

• SKILLS ACQUIRED• CHALLENGES• CONCLUSION

COMPANY PROFILE KDF KONSULT LIMITED is a geotechnical and engineering company with head office at No. 3, Balogun Street, Ikeja, Lagos State. KDF KONSULT LIMITED is a leading service provider in:

Geotechnical assessment (subsoil investigation) Mining Dredging Water and environmental engineering Soil stabilization Foundation design Geophysical/geological and hydrographic investigation

SUBSOIL INVESTIGATIONAPPARATUS

There are several methods of subsoil investigation or soil testing.

They are:

Cone Penetration Test (CPT) Standard Penetration Test (SPT) Vane Shear Test (VST)

Pressuremeter Test (PMT)

Flat Plate Dilatometer Test (DMT)


CONE PENETRATION TEST (2.5 TON CAPACITY) This is done using a DUTCH CONE PENETROMETER. The apparatus consists of: Sounding rods Push/pull adaptor Spiral anchor Handle and T-piece Mantle cone Wrenches set Testometer


The apparatus consists of a cylindrical

probe of 1000mm2 cross sectional area,

and a conic head of apex angle of 60o

attached to a rod enclosed by an outer

sleeve.

CONE

1M ROD

SUBSOIL INVESTIGATIONPROCEDURE

The probe is forced down through the soil at a steady rate of about

20mm/s in the closed position by exerting pressure manually for a 2.5 Ton

capacity machine or hydraulic pressure (for larger capacity machines).

The cone penetrometer is used to estimate soil bearing capacity and

settlement of foundations.

The results from CPT can be correlated with lab test results and

other field tests (usually Standard Penetrometer Tests) to build

empirical relationship for soil strength and deformation parameters.

SUBSOIL INVESTIGATION

SUBSOIL INVESTIGATIONTEST RESULTS

The resistance offered by the soil to the penetration of the cone is measured.

This is called Cone Resistance Value (or Qc Value).

A low Qc value indicates soft or weak soil strata and thus poor geotechnical

quality while a high Qc value is indicative of good geotechnical property.

The groundwater level/condition can also be assessed during testing.

Test is aborted when the machine anchor legs pull from the ground as this

indicates that the rod can penetrate no further.


Generally,

For clay:Qc value Remark

• 0 – 2

• 2 – 4

• 4 – 10

• 10 – 20

• >20

Very soft

Soft

Firm

Stiff

Stiff to hard

For sand:Qc value Remark

• 0 – 40

• 40 – 100

• >100

Loose sand

Medium dense

Dense


0 05 0.25

10 0.510 0.7513 111 1.2510 1.514 1.7517 223 2.2521 2.529 2.7535 343 3.2556 3.570 3.7581 4

100 4.25112 4.5131 4.75152 5

DEPTH (m)Qc VALUE (kgf/cm2)

SOIL PROFILE

DEPTH(m) DESCRIPTION

0 – 0.75

0.75 – 2.0

2.0 – 5.0

Dark-browncohesionless soil

Firm to stiffcohesive soil

Stiff to very stiffcohesive soil

SUBSOIL INVESTIGATIONTEST RESULTS (SPT)

DEPTH (m) N-VALUE U4 SAMPLE

0.00 – 0.75 -- --

0.75 – 6.00 -- Retrieved at 3.00m

6.00 – 12.75 14 – 16 Abortive

12.75 – 16.50

20 – 32 N/A

16.50 – 20.00

20 – 32 N/A

GEOTECHNICAL ENGINEERING PARAMETERS

SUBSOIL INVESTIGATIONTEST RESULTS (SPT)

The sampler is driven in three

successive 150mm (450mm) into

the soil before retrieval.

The “N” value is taken as the

number of blows required to drive

the sampler the last 300mm

through the soil.

SUBSOIL INVESTIGATIONTEST RESULTS ANALYSIS

From the test results shown, the following information can be obtained: Depth (m)

Geotechnical Properties

0.00 to –.0.75 Moderate geotechnical properties, moderate shear

strength and moderate compressibility potential.

0.75 to –2.00 Moderate to good geotechnical properties, moderate

to high shear strength and low compressibility

potential.

2.00 to –5.00 Good geotechnical properties, high shear strength

and low compressibility potential


BEARING CAPACITY ANALYSIS

ULTIMATE BEARING CAPACITY qult = CNc + ΨDNq + 0.5BΨNΨ

NOTE:For pure clay, Φ = 0Nc = 5.14Nq = 1NΨ = 0Thus, for the test above, the equation becomes:qult = 5.14Cu + ΨD

For pure sand,C = 0

Where;

C = cohesive strength of soil

D = depth of foundation

B = width of foundation

ΨD = overburden pressure

Ψ = unit weight of soil

Nc = BC factor (cohesion)

Nq = BC factor (surcharge and friction)

NΨ = BC factor (self weight and friction)*BC = Bearing Capacity


ALLOWABLE BEARING CAPACITY

Qa = Qult / FS

*FS = Factor of Safety

SETTLEMENT ANALYSISSettlement for the calculated Allowable Bearing

Capacity must not exceed 50mm.

S = 1.1Mv x 0.55qn x D

Where;

S = Total Settlement

Mv = volume of compressibility potential

qn = net foundation base pressure

D = depth of foundation


The table below shows the permissible settlement as per I.S. Code:

SOIL TYPE

PERMISSIBLE TOTAL SETTLEMENT

PERMISSIBLE DIFFERENTIAL SETTLEMENT

ISOLATED

FOOTINGS

RAFT FOOTINGS ISOLATED FOOTINGS RAFT FOOTINGS

Sand

4.0cm

4.cm to 6.5cm

2.5cm

2.5cm

Clay

6.5cm

6.5 to 10.0cm

4.0cm

4.0cm

SUBSOIL INVESTIGATIONTEST RESULTS INTERPRETATION

From the test result and analysis presented above, Shallow Foundation

(Stiffened Raft Slab) is considered adequate for the proposed development.

The Stiffened Raft Slab foundation design is a quite simple and common

slab Foundation design, can be used on most classes of soil, and helps to

Minimize differential settlement as it spreads structural load evenly.

SKILLS ACQUIRED

After my internship with KDF KONSULT LIMITED, :

i can effectively carry out soil testing using a Dutch Cone Penetrometer.

i can effectively carry out soil testing using a Standard Penetrometer.

i can accurately give recommendation on foundation design.

CHALLENGES

Work place procedures/routine.

Poor welfare of internship students.

Restriction from some information.

Time constraint.

CONCLUSION

The SIWES programme has made it possible to apply the theoretical

knowledge in real work situations especially as it concern geotechnical

assessment and engineering geology in general.

I was also able to develop good work ethics and cooperation.

THANKYOU

presentation subsoil investigation foundation recommendation geotechnical assessment geology

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