soil testing and analysis

8/13/2019 soil testing and analysis

1/21

The primary objectives of soil exploration are: Determination of the nature of the deposits of soil.

Determination of the depth and thickness of the varioussoil strata and their extent in the horizontal direction. The location of ground water table (GWT). Obtaining soil and rock samples from the various strata. The determination of the engineering properties of the soiland rock strata that affect the performance of the structure. Determination of the in-situ properties by performing fieldtests. Selecting the type and depth of foundation suitable for a

given site Evaluating the load-bearing capacity of the foundation. Estimating the probable settlement of a structure. Determining potential foundation problems (for example,

expansive soil, collapsible soil, sanitary landfill, and so on).Predicting lateral earth pressure for structures such as


2/21

Collection of data about the project

Geologic study of the site

Site inspection

Site Reconnaissance

Preliminary Ground Investigation

Detailed Ground Investigation

Field Investigations

Boring

Sampling

Laboratory Testing


3/21

Test pits, trialpits or

trenches

Borings

Soundings orpenetration

tests

Geophysicalmethods

Methods for soilexploration

Soil samples can be lifted

from deeper depths bydrilling bore holes byusing mechanicaldevices called samplers.

Permits visual inspection of soil

strata in place. Especially usefulfor gravelly soil whereboreholes may be difficult.Sampling done on exposedsurfaces.

Standard Penetration Test(SPT)Cone penetration test (CPT)

Electrical resistivity methodSeismic refraction method


4/21

Auger boring

Wash boring

Percussionboring

Rotary boring

The process of boring consists ofi. Drilling a hole and visually examining the cuttingscoming out from different depths.ii. Lifting the soil samples from different depths by using

mechanical devices called samplers.


5/21

AUGER BORING

Hand operated augers Power driven

augersUsed for boring holesupto a maximum of 10m and suitable for alltypes of soils above

water table butsuitable only belowwater table in claysoils.

Used for greaterboring depths wherehard or stiff soil strataare encountered.


6/21

Wash boring:this is a simple and fastest method, used for making

holes in all types of soils except boulders and rocks.

Percussion boring:This method is used to make hole in all types of soils

including boulders and rocks.

Rotary boring (Mud rotary drilling):This method is used to advance hole in rocks and

soils. Rotating core barrels which are provided withcommercial diamond bits or a steel bit with slots areused for rotary drilling. This method is used toobtain the rock cores, so this method is called as core

boring or core drilling.


7/21

Disturbed samples UndisturbedsamplesNatural soil structure

gets modified ordestroyed during thesampling operation.

Original soil structure ispreserved and thematerial properties havenot undergone any

modification.

Representative samples Non-representative samples

samples those the naturalmoisture content and theproportion of mineralconstituents can be preservedwith suitable precautions.

Where in addition to

alteration, soils from otherlayers get mixed up or theproportion of mineralconstituents get altered.


8/21

The standard penetration test is the most commonly used

in-situ test, especially for cohesionless soils which can notbe easily sampled. Generally used to determine the bearingcapacity of sand and gravel soil.

A split spoon sampler is used.It is a sampler tube which canbe split open longitudinally aftersampling which consists of adriving shoe, a split-barrel of

circular cross section and acoupling.


9/21

65 kg hammer

dropping from750mm

SPT Resistance(Nvalue) is total no ofblows to drivesampler 2ndand 3rd

150 mm increments

The no of blows required for the last 300 mm of penetration isadded together and recorded as the N- value at that particulardepth of the borehole. The no of blows required to effect the first150 mm penetration, called the seating drive, is discarded.


10/21

The boring log shows refusal and the test is halted if-a) 50 blows are required for any 150 mm penetrationb) 100 blows are required for 300 mm penetrationc) 10 successive blows produce no advance

Corrections in N value.:

SPT values obtained in the field for sand have to becorrected before they are used in empirical correlations anddesign charts.


11/21

Overburden correction:

In granular soils, the overburden pressure affects the penetrationresistance. If the two soils having same relative density but different

confining pressures are tested, the one with a higher confiningpressure gives a higher penetration number. As the confiningpressure in cohesionless soils increases with the depth, thepenetration umber for soils at a shallow depth is underestimated andthat at greater depth is overestimated. For uniformity, the N values

obtained from the filed tests under different effective overburdenpressures are corrected to a standard effective overburden pressure.

N= N * CWhere, N = N value after overburden correction

N = observed value

C = overburden correction factorThe correction factor proposed by Peck, Hanson and Thornburn is given

by the equationC = 0.77 log(20/P)

Where, P = effective overburden pressure at the depth at which N

value is measured (kg/cm^2)


12/21

Dilatancy correction:

Silty fine sands and fine sands below the water table develop porewater pressure which is not easily dissipated. The pore pressureincreases the resistance of the soil and hence the penetration number(N).Terzaghi and Peck (1967) recommend the following correction in the

case of silty fine sands when the observed value is N exceeds 15.N = 15 + (N15)/2

Importance of SPT N value :

N value represents compression and shear strength parameters of thesoil. N value is correlated to shear strength parameters (Cohesion andangle of internal friction). Design of foundations can be done directlyusing N value.


13/21

The sieve analysis is carried out by sieving 1000 g of soil

sample through the nest of sieves consists of 4.75 mm, 2.36mm, 1.18 mm, 600 micro meter, 425 micro meter, 300 micrometer, 150 micro meter .The sieves are placed one below the other so that theopening decrease in size from the top sieve downwards with apan at the bottom of the stack.10 minutes of shaking by a mechanical shaker is given.The amount of soil retained on each sieve is weighed andthe percentage of the total weight of soil passing through

each sieve is calculated.

% Retained = (wt of soil retained/total weight of soiltaken)*100

Cumulative % retained = sum of % retained on all sieves oflar er sizes and the % retained on that articular sieve


14/21

The property of soil which permits water to percolate through its

continuously connected voids is called permeability.The coefficient of permeability may be determined in the laboratoryby two methods- Constant head method Variable head method

Apparatus :Permeameter mould(internal diameter = 100 mm, height = 127.3 mm,capacity = 1000cc)Cover, base, detachable collar, porous stoneRound filter paper(100 mm dia)


15/21

Constant Head Method :

k = (Q*L)/(A*h*t)

Variable Head Method :

k = (2.3*a*L*Log(h1/h2))/(A*t)


16/21

Shear strength of a soil is the capacity of soil to resist shearing stress. Itcan be defined as the maximum value of shear stress that can bemobilised within a soil mass.

The shear parameters were calculated in laboratory by Direct ShearTest.

Apparatus:

Shear Box Assembly


17/21


18/21

Weight of empty mould, W1 = 10900 gmWeight of mould + sand, W2 = 14900 gmHeight of mould = 17 cm

Diameter of mould = 15 cmVolume of mould, V = (3.14 * 152* 17) /4

= 3002.62 cm3

Minimum density = (W2-W1)/V= (14900-10900)/3002.62

= 1.33 g/ccHeight of sand after compaction = 17 3 = 14 cmVolume of sand = (3.14 * 152* 14)/4

= 2472.75 g/ccMaximum density = (W2-W1)/Volume of sand

= (14900-10900)/2472.75= 1.62 g

17 cm

15 cm


19/21

The dynamic cone test is a quick test and helps to cover a large area

under investigation rather economically. It helps in identifying theuniformity or the variability of the subsoil profile at the site. Thetest is much less expensive and much quicker than the SPT.

The equipment consists of a

cone, driving rods, drivinghead, hoisting equipment anda hammer. The driving rodsshould be rods of suitablelength with threads for joining

A rod coupling at either end.The rods should be marked atevery 100 mm.The driving head shall be ofmild steel with threads at

either end for a rod coupling. Itshall have a diameter of 100


20/21

The cone with threads (recoverable) shall be of steel with tiphardened. The cone without threads (expendable) may be of mildsteel. For the cone without threads, a cone adopter shall be

provided.

The hammer used for driving the cone shall be of mild steel or cast-iron with a base of mild steel. It shall be 250 mm high and of suitablediameter. The weight of the hammer shall be 640 N (65 kg). The coneshall be driven into the soil by allowing the hammer to fall freelythrough 750 mm each time. The number of blows for every 100 mm

penetration of the cone shall be recorded. The process shall berepeated till the cone is driven to the required depth. To save the


21/21

Number of blows for 300mm penetration is called Conepenetration value denoted by N cbr.

When the depth of investigation is more than 6 m, bentonite slurrymay be used for eliminating the friction on the driving rods. Thecone used in this case is of 62.5 mm size.

Documents

soil testing and analysis