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316x Filetype PDF File size 2.59 MB Source: ced.ceng.tu.edu.iq
Prepared by: Dr. Farouk Majeed Muhauwiss
Civil Engineering Department – College of Engineering
CHAPTER2 Tikrit University
SUBSOIL EXPLORATION
2.1 SOIL EXPLORATION
All office, laboratory and field worksare done in order to explore the subsurface of soil or
rock conditions at any given site to obtain the necessary information required in design and
construction. Subsoil exploration is the first step in the design of a foundation system. Soil
exploration consists essentially of boring, sampling and testing.
Mainly, planning of subsoil exploration involves three phases; reconnaissance phase,
preliminary site investigation phase, and detailed site investigation phase.
2.1.1 RECONNAISSANCE PHASE
This phase consists of:
(a) Collection of all available information, and
(b) Reconnaissance of the site.
So that, it will indicate any settlement limitations and help to estimate foundation loads.
2.1.2 A PRELIMINARY SITE INVESTIGATION PHASE
This phase consists of:
(a) Preliminary design data that satisfy building code requirements, and
(b) Number and depth of boreholes.
So,it involves knowing of the distribution of structural loads which is required in the design
of foundations. Also, a few borings or tests pits are to be opened to establish the stratification
types of soil and location of water table. In addition to, one or more borings should be taken to
rock when the initial boreholes indicate that the upper soil is loose or highly compressible.
2.1.3 A DETAILED SITE INVESTIGATION PHASE
In this phase, additional boreholes, samples will be required for zones of poor soil at smaller
spacing and locations which can influence the design and construction of the foundation.
Foundation Engineering Chapter 2: Subsoil Exploration
2.2 DRILLING OR BORING
• Definition:It is a procedure of advancing a hole into ground.
• Drilling Methods:
(1) Test Pits
(2) Auger Drilling
(a) Hand-auger drilling.
(b) Power-auger drilling.
(3) Wash Boring
(a) Jetting.
(b) Sludging (reverse drilling).
(4) Rotary Drilling
(a) Rotary drilling with flush.
(b) Rotary-percussion drilling.
(5) Percussion Drilling
Each of these methods has its merits and its drawbacks. However, Table (2.1) gives a
guide for selecting the most appropriate drilling method.
Table (2.1): Drilling method selection.
Wash boring Rotary drilling
Hand Rotary Rotary Percussion
Type of soil auger Jetting Sludging drilling percussion drilling
drilling with drilling
flush
Gravel X X X X 9 ? 9 ?
Sand 9 9 9 9 9 ? 9 ?
Silt Unconsolidated 9 9 9 9 9 ? 9 ?
Clay formations 9 ? 9 9 9 slow 9 slow
Sand with pebbles X X X X 9 ? 9 ?
or boulders
shale Low to medium X X X 9 9 slow 9
Sandstone strength X X X 9 9 9
formations
Limestone X X X 9 slow 9 9 slow
Igneous (granite, Medium to high X X X X 9 9 slow
basalt) strength
Metamorphic (slate, formations X X X X 9 9 V slow
gneiss)
Rock with fractures or voids X X X 9 9 9 !
Above water-table 9 ? X 9 9 9
Below water-table ? 9 9 9 9 9
9 = Suitable drilling method 9 ? = Danger of hole collapsing 9 ! = Flush must be maintained to continue drilling
? = Possible problems x = Inappropriate method of drilling
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Foundatioon Engineeriing CChapter 2: Suubsoil Explorration
2.2.1 TEST PITSS
A ppit is dug eithher by hand or by a backkhoe. Probabbly in a test ppit, the enginneer can exaamine
in detail the subsoil sstrata and taake disturbedd or undisturrbed samplees at the desiired locationn (see
Fig. 2.1):
Advantaages:
• Innexpensive.
• Prrovide detailled informattion of stratiggraphy.
• Large quantities of disturbbed soils cann be obtainedd for testingg.
• Large blocks of undisturbbed samples can be carveed out from tthe pits.
• Fiield tests cann be conductted at the boottom of the ppit.
Disadvaantages:
• DDepth limitedd to about 6mm.
• DDeep pits uneeconomical ssuch as in caase of investtigationthat iinvolves bassement
coonstruction.
• Exxcavation beelow grounddwater (high water table)) and into rocck difficult aand costly.
• Tooo many pits may scar ssite and requuire backfill ssoils.
• WWhen the soill is unstablee and has a ttendency to collapse, thiis prevents tthe engineer from
enntering the ppit and accommpanied by ccertain risks.
• UUnsuitable inn granular soils beloowwater levvelor whenn the standdard penetrration
reesistancetest (N-value) iss required.
Walls of test pit indicate foour layers
(1) Clayeey silt (2) Sanddy silt (3) Clean sand (4) Sandy gravel
Fig. (2.1): Test pits.
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Foundatioon Engineeriing CChapter 2: Suubsoil Explorration
2.2.2 AAUGER DRRILLING
(a) HHand-Augers
Thhe auger off (10-20) cmm in diametter is rotatedd by turningg and pushiing down on the
handlebar. Then withhdrawing annd emptyingg the soil-ladden auger too remove thhe excavatedd soil.
Several nnew auger seections are aadded up to the required depth is rreached. Theese augers can be
available in different types such aas (see Fig. 22.2):
a. HHelical Augerr.
b. Shhort flight AAuger, and
c. Iwwan Auger.
Advantaages:
• Innexpensive.
• Siimple to opeerate and maaintain.
• NNot dependennt on terrain.
• Poortable.
• UUsed in uncassed holes, annd
• GGroundwater location cann easily be iddentified andd measured.
Disadvaantages:
• Sllow compareed with otheer methods.
• DDepth limitedd to about 6mm.
• Labor intensivve.
• UUndisturbed ssamples can be taken only for soft cllay deposit, and
• CCannot be useed in rock, sttiff clays, drry sand, or caaliches soils.
a. Helical (worm tyypes) Augers b. Shortt flight Augerr c.Iwann (posthole) AAuger
Fig. (2.2)): Hand-auugers.
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