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Any simple test for estimation of Soil Bearing capacity
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elina.dutta2010
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PostPosted: Mon Mar 22, 2010 1:02 pm    Post subject: Reply with quote

Dear sirs,

The method suggested by Mr. Harsoda and Mr. Suraj Singh looks simple and practical. As pointed out by Mr. Suraj Singh "Too small house owner may not be in a capacity to afford soil testing but engineer may assist self to understand the soil & conclude based on ones working experience predicted conduct of soil." These methods will be quite useful in such cases. Thanks to Mr. AK Jha, Mr. amarjeet singh, Mr. Manish a for sharing valuable informations.

"It is agreed in principle that there is no alternative without soil testing for the required parameters". The exceptions of using simple methods to find the bearing capacity of soil and the precautions to be taken are well explained by Mr. Vikram Jeet.

Thanks

Cordially
Elina
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Dr. N. Subramanian
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PostPosted: Tue Mar 23, 2010 3:21 pm    Post subject: Re: Any simple test for estimation of Soil Bearing capacity Reply with quote

Dear All,

It is a good question by Er Elina. In fact we faced similar problems, when we started the consultancy. If we are not getting soil report, we used to write in the notes like this: "The SBC of Soil is based on the value provided by the Builder". Even in Chennai many builders were reluctant to spend money on Soil consultancy. It changed after a building failed(I do not remember the year- may be in the 1990s), which was constructed on a former lake bed and a couple people died(although in this case the builder had taken soil test but built four floors without increasing the column or footing size, which were designed for two floors only).

I have heard that Prof. P. C. Varghese used to visit sites and give the app.value of SBC by just pushing his finger or feet into the soil(He has M.S. Degrees in Soil Mechanics as well as Structural Engg). I was not fortunate to attend his lectures at IITM, as when I did my Ph.D. there, he had gone to Srilanka as an UN Expert. Of course now I know him well.

For small building with up to two floors we may use visual observation. For this we may have to dig a pit deeper than the foundation level and look at the soil(one may also look at the nearby well, if the soil strata is seen). As informed by Er Vikaramjeet, when such a visual method is adopted, the designer will definitely adopt conservative value for SBC, which
may offset the costs saved by avoiding the soil tests.

As rightly pointed out by Er Vikramjeet, Soil testing is a must for the following types of soils:
a) Filled -up-soil area
b) Strata in the vicinity of a water body like canal, Nallah , River,
Marshy area
c) Black cotton soil /expansive soils area.
d)Areas susceptible to flooding

We should also know the Max. water table level at the site during Monsoon season, as it may affect SBC. For clayey soils, one may assume 15 tonne/m2 . One has to be careful about sandy soils as the SBC for loose sands may be very low compared to compacted Sandy soils (under-reamed piles are not advisable in loose sandy soils, esp. when the water table is high).

Finally a note about Soil reports: Most of the soil reports we have received only give soil profile in bore holes, SBC, depth at which the foundation can be made, whether the soil can be used for backfilling and the water table depth at he time of investigation. Sometimes we may need more details such as Angle of repose, wet density, etc.

Best wishes,
NS
vikram.jeet wrote:
SBC without performing geo-tech investigatioin for small buildings

I think this is very pertinent querry because in remote areas/Villages, far away
from cities, the facilities of soil investigation ,that too for small works, may cost
the low cost project very dearly(owner'thinking) and tendency of the owners
is to skip this by relying on the judgement of the engineer/designer.

National Building Code,though, provides sbc value for soils based on  
observation for small load bearing works, but confidence of a designer
always lies with the soil report.

The methods given by the highly experienced sefi members like
P Eng Sh Suraj Singh ji,Sh AK Jha ji , Sh BV Harsoda ji for having
judgement of sbc are quite informative and definately helpful.

I think in situations where soil reports are not available , the tendency  
of designer will definately be on conservative assumption, which
sometimes offsets the costs saved by avoiding the soil tests.

However keeping in view the practicality of situation in subject cases,there
is no option but to assume net sbc and precutions to be provided in form of
Notes in foundation drawings , such as under:
(though a scape goat option but something better than nothing)

1 Foundations are designed for a net SBC= - - - Tonne/sqm at - - -
meters below EGL (Assumed as per site information)
2 Foundations must rest on Firm strata capable of above stated sbc.
If firm strata is not met at indicated founding level, the foundations shall
be placed deeper under intimation to the designer.

3 Foundation level shall be approved by an experienced site engineer
(Civil)
4 The Foundation details are NOT valid for following soil stratas and
separate foundation details shall be needed
a) Filled -up-soil area
b) Strata in the vicinity of a water body like canal, Nallah , River,
Marshy area
c) Black cotton soil /expansive soils area.
d)Areas susceptible to flooding

Net Sbc in the range of 6.0 tonne/m2 to 8.0 tonne/m2 could be assumed  
based on informations for areas having low water table,
otherwise 60% to 75% of above values in high water table areas in absence
of soil reports .---may be conservative but could be at par also.

STILL, Soil Testing , a must for any project ,big or small as that
removes the guess work involved apart from providing informations
of strata beneath ,upto pressure bulb influence zone.

regards

vikramjeet












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uhvaryani
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PostPosted: Tue Apr 06, 2010 10:30 am    Post subject: Any simple test for estimation of soil bearing capacity Reply with quote

Dear Er.Elina Dutta,
This is a very useful and pertinent question and some of our
experienced Sefians have already given their useful ideas.
Small buildings in rural and semi-urban areas are being built by
mistries based on their experience only.These are non-engineered
structures.Where engineers become involved,owners of small buildings
can not afford cost of soil investigation.So,this question posed by
Er.Elina becomes important.
One way is to follow the local practice.The soil capacity and
foundation depth are to be ascertained from other engineers practising
in the area.The other way to get dug a pit of 0.9mx0.9mx1.2m(depth)
and get a soil sample from the bottom of the pit.The type of soil
should then be compared to the soil types given in a table given in
many books(also in code IS:1904) and then one may decide the value of
soil bearing capacity.A foundation depth for small buildings may be
0.9m to 1.2m.The third way is that to go in for a soil investigation
by means of a single borehole.These days cost of buildings has gone
up.Say,Rs.20,000 may be spent in this effort of soil investigation.The
client should be persuaded to spend this amount as it will lead to
more savings in the cost of foundations.
In regions where a lot of construction activity is going on,it is
comparatively easy to decide these parameters.In Delhi region,10 t/mxm
soil capacity with 0.9m to 1.2m foundation depth is assumed in
design.When the soil strata is harder,we may consider 12t/mxm soil
capacity and when it is softer,8t/mxm soil capacity may be assumed.
The important poin is that SE should inspect the soil in footing
trenches before the lean concrete is laid.This requirement is a must
even when the soil investigation of the site has been got done.This
site visit by the SE is required for the safety of structure.This
topic,being interesting,caught my attention and I have given my views
a little later than other friends.
with best wishes and regards,
uhvaryani

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umeshrao
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PostPosted: Wed Aug 25, 2010 9:53 am    Post subject: Re: Any simple test for estimation of Soil Bearing capacity Reply with quote

elina.dutta2010 wrote:
Dear Sefians,

Determination of bearing capacity of soil and depth of foundation for not so important building (two storey load bearing residential) in remote areas is a matter of concern especially for practising engineers starting their careers. The unfactored maximum pressure at the foundaion level due design load in the foundation for two storey brick masonry wall building may be around 5t/m2 (derived from 25kN/m on 0.5m wide strip footing). Soil experts may be able to suggest the bearing capacity by just seeing the site soil conditions.  Identify the type of soil and get the corresponding standard values will also be a crude method.

Laboratory testing of soil is possible, but may not be a feasible solution in this case. Is there any simple tests which can be done at site to determine the bearing capacity of soil? Some people say that wells in the neighbourhood of the site can give an idea of the soil stratum, and the depth of foundation.

If there are residential buildings (two storey) exist in the vicinity of 10-15 metres around the site (with foundation depth of around 0.6-0.7m), can we take that as an additional data for getting the confidence to fix the foundation depth?

Regards
Elina

Miss Elina,
One of the simplest method of assessing probable SBC is
1--- Take crow bar, which is normally used on all construction sites - this is 16mm dia bar about 1200mm long and has a chiseled tip.
2--- Lift the crow bar by one hand holding approx at center of the bar. i.e. you grip will be holding the bar at the center having 600mm projections below and above the grip.
3--- Raise bsr so that the grip is in front of your eyes.i.e. the tip of bar will be about 750 to 850 mm from the surface of excavated pit.
4--- Release the bar without applying any force so that bar will fall vertically by gravity.
5--- The bar will penetrate the soil. Hold at the line of penetration of the crow bar and measure the distance from tip of the bar and line of penetration.
6--- Probable bearing capacity of the soil at the excavated surface level is = 25/ measured distance in cms.
7--- For example if the penetration is 1.5 cms then the probable bearing capacity is 25/1.5=17 T per Sqm or 170 kN per Sqm.
Word of caution
This is a very crude method of assesing soil SBC. This should not be used in fine sandy soil since the intrepretion will be erroneous.
But very effective at the site to infer the strength of soil.
Regards Umesh Rao
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spsvasan
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PostPosted: Sat Aug 28, 2010 11:28 am    Post subject: Reply with quote

Dear Elina Dutta

Trial pits and a few useful approximate methods have been suggested for the case where soil investigation is not feasible.

One other point - You must also check the soil region well below the footing to ensure that there is no weak soil below. Atleast one trial pit should be taken to a depth of atleast "width of foundation" below the proposed foundation level

There are quite a few cases where footings have rested on a good soil layer but the buildings have cracked because of weak layers beneath.

Best wishes
S.P.Srinivasan
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ajsingh
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PostPosted: Sat Aug 28, 2010 12:16 pm    Post subject: SBC of soil Reply with quote

I would like to state that, C-Phi test of soil will give a fair idea about the SBC of soil.
Some of the experts mentioned very good views but as far as my opinion is concerened Plate-Bearing test cost here in Chhattisgarh around INR 12000-15000 and C-Phi test cost INR 2500-3500 which is very small compare to building cost . A small housing unit cost around INR 10-12 Lacs and it is good if some one get SBC verify.Because, according to Terzaghi (Father of Soil Mechanics) Soil has comples nature as human beings.

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Amarjeet Singh
Structural and Vaastu Designer
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Raipur
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badamsundararao
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PostPosted: Sat Aug 28, 2010 5:00 pm    Post subject: S0IL TESTING Reply with quote

Dear friend


     For smaller size buildings and where lower loads are there especially in remote areas personal field inspection will give good idea,but the soil particulars beneath the footing up to pressure bulb depth is most critical ,hence one should certainly get the nature of soil up to this depth. Particularly in sea coast towns marshy soil layers of substantial depth will present and they will cause settlements,hence  one shold be very careful about soil strata up to a depth of depth of foundation +pressure bulb depth.


Badam Sundara RAO.
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sspawar
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PostPosted: Sun Aug 29, 2010 6:55 am    Post subject: Reply with quote

Dear All,

If you dont want to go SBC PLATE LOAD TEST, you may refer cheaper method
NBC 7.1.2.1 - STRUCTURAL DESIGN SECTION 2 SOILS AND FOUNDATIONS
(as eralier stated by Er. AJSingh)
Where
The safe bearing capacity on the basis of shear strength characteristics of soil, or
The allowable bearing pressure that the soil can take without exceeding the permissible settlement (see 7.1.3)


Bearing capacity by calculation the engineering properties of the soil are available, that is, cohesion, angle of internal friction, density, etc the bearing capacity shall be calculated from stability considerations of shear; factor of safety of 2.5 shall be adopted for safe bearing capacity. The effect of interference of different foundations should be taken into account. The procedure for determining the ultimate bearing capacity and allowable bearing pressure of shallow foundations based on shear and allowable settlement criteria shall be in accordance with good practice [6-2(7)].

In this Context you may refer following table also - which give direct idea of SBC of soil witth it properties as per NBC 1983 and IS 1904 -1961 (this table ommited from NBC2005 and also from 1904-1986)


NBC(Clause 7.1.2.2) 1983
SnTYPE OF Rocks/SoilSAFE B EARING CAPACITY R EMARKS
1234
kN/m2
a) Rocks
1Rocks (hard) without lamination and defects. for example, granite, trap and dioritc 3 240
2Laminated rocks, for example, stone and limestone in sound condition I 620
3Restdual deposits of shattered and broken bed rock and hard shale. cemented material 880
4Soft rock 440
b) Non-cohesive soil
5Gravel. sand and gravel, compact and offering high resrstance to penetratron when excavated by tools 440(see Note 2)
6Coarse sand, compact and dry 440Dry means that the ground water level is at a depth not less than the width of foundation below the base of the foundation
7Medium sand, compact and dry 245
8fine sand, silt (dry lumps easily pulverized by the fingers) 150
9Loose gravel or sand-gravel mixture, loose coarse IO medium sand, dry 245(See Note 2)
10Fine sand. loose and dry 100
c) Cohesive soils
11Soft shale. hard or stiff clay m deep bed. dry 440This group is suceptible to long term consolidation settelement
12Medium clay, readily indented with a thumb nail 245
13Moist clay and sand-clay mixture which can be indented with strong thumb pressure 150
14Soft clay indented with moderate thumb pressure 100
15Very soft clay whtch can be penetrated several ccntimctrcs with the thumb 50
16Black cotton so11 or other shrinkable or cxpanstvc clay in dry condition (50 percent saturatton) Sn Note 3. TO be determined after invetigation
d) Pear
17Peat See Nores 3 and 4. To be determined after investigation
e) Made-up ground
18Fills or made-up ground See Notes 2 and 4. To be dcter-mined after investigation


Note 1 - Values Listed in the table are from shear consideration only.

Note 2 -Values arc very much rough for the following reasons:

a) Effect of characteristics of foundations (that IS, effect of depth, width, shape. roughness, etc) has not been considered.

b) Effect of range of soil properties (that is, angle of frictional resistance. cohesion. water table, density, etc) has not been considered.

c) Effect of eccentricity and indication of loads has not been considered.

Note 3 for non cohesive soils the values listed in the table shall be reduced by 50% if the water, table is above or near the base of footing

Note 4  -Compactness of non-cohesive soils may be determined by driving a cone of 65 mm dia and 60 apex angle by a hammer of 65 kg falling from 75 cm. If corrected number of blows (N) for 3O cm penetration is less than 10, the soil is called loose; if N is high between 10 and 30, It is medium and if more than 30, the soil is called dense.

Regards
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PostPosted: Fri Sep 03, 2010 6:07 pm    Post subject: Re: Any simple test for estimation of Soil Bearing capacity Reply with quote

Hi All,

Very interesting method and quite practical! Hats off Er Umesh!

But I always suggest a soil testing, as the cost involved is nothing compared to the project cost.

Regards,
Subramanian
umeshrao wrote:

Miss Elina,
One of the simplest method of assessing probable SBC is
1--- Take crow bar, which is normally used on all construction sites - this is 16mm dia bar about 1200mm long and has a chiseled tip.
2--- Lift the crow bar by one hand holding approx at center of the bar. i.e. you grip will be holding the bar at the center having 600mm projections below and above the grip.
3--- Raise bsr so that the grip is in front of your eyes.i.e. the tip of bar will be about 750 to 850 mm from the surface of excavated pit.
4--- Release the bar without applying any force so that bar will fall vertically by gravity.
5--- The bar will penetrate the soil. Hold at the line of penetration of the crow bar and measure the distance from tip of the bar and line of penetration.
6--- Probable bearing capacity of the soil at the excavated surface level is = 25/ measured distance in cms.
7--- For example if the penetration is 1.5 cms then the probable bearing capacity is 25/1.5=17 T per Sqm or 170 kN per Sqm.
Word of caution
This is a very crude method of assesing soil SBC. This should not be used in fine sandy soil since the intrepretion will be erroneous.
But very effective at the site to infer the strength of soil.
Regards Umesh Rao
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