Subsurface investigations:
Subsurface investigations:
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• Subsoil conditions are examined using test Subsoil conditions are examined using test borings, provided by soil engineer (geotechnical).borings, provided by soil engineer (geotechnical).
•
• Number of borings and l Number of borings and location of borings docation of borings depends on buildinepends on building type and site conditg type and site conditions.ions.
•
• TyTypically for uniform soil pically for uniform soil conditions borings are spaced 1001!0" apart, conditions borings are spaced 1001!0" apart, for more detailed for more detailed #or$,#or$, #here soil footings are closely spaced and soil conditions are not even borings are
#here soil footings are closely spaced and soil conditions are not even borings are spaced !0" apart.spaced !0" apart.
•
• %arger open #arehouse type spaces, #here fe#er columns are present (long %arger open #arehouse type spaces, #here fe#er columns are present (long span) re&uired lessspan) re&uired less boring samples.
boring samples.
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• Borings must extend to firm StrataBorings must extend to firm Strata (go through unsuitable foundation soil) and (go through unsuitable foundation soil) and then extend atthen extend at least 20 feet more
least 20 feet more into bearable soil. into bearable soil.
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• %ocation of borings samples are indicated on engineer plan.%ocation of borings samples are indicated on engineer plan.
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• 'orings are not ta$en directly under proposed columns.'orings are not ta$en directly under proposed columns.
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• 'orings indicate depth, soil classification (according to the 'orings indicate depth, soil classification (according to the unified soil system), and moistureunified soil system), and moisture content and sometimes ground #ater level is sho#n as #ell. (h
content and sometimes ground #ater level is sho#n as #ell. (hysical properties particle si*e, moistureysical properties particle si*e, moisture
content, density).
content, density).
•
• Soil report recommendation should be based on testing of materials obtained from on site Soil report recommendation should be based on testing of materials obtained from on site boringsborings and to include and to include 1 1.. ''eeaarriinng g ccaappaacciitty y oof f ssooiill.. + +.. oouunnddaattiioon n ddeessiiggn n rreeccoommmmeennddaattiioonnss.. --.. aavviinng g ddeessiiggn n rreeccoommmmeennddaattiioonnss.. .. //oommppaaccttiioon n oof f ssooiill.. ! !.. %%aatteerraal l ssttrreennggtth h ((aaccttiivvee, , ppaassssiivvee, , aannd d ccooeeffffiicciieennt t oof f ffrriiccttiioonn)).. .. eerrmmeeaabbiilliittyy.. .. rroosst t ddeepptthh..
Surface investigations:
Surface investigations:
•• 2igh 3ater Table.2igh 3ater Table.
•
• resence of trouble soils eat, soft claresence of trouble soils eat, soft clayy, loose silt, or fine #ater bearing sands., loose silt, or fine #ater bearing sands.
•
• 4oc$ close to the surface 4oc$ close to the surface (re&uire blasting for excavations).(re&uire blasting for excavations).
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• 5umps or ills.5umps or ills.
•
Factors Affecting Selection of Foundation are:
1. Loads from Building:
The first factor considered is loads from building on the foundation. This load is a combination of dead load and imposed loads on the buildings. 7ther loads such as #ind loads, earth&ua$e loads, sno# loads etc. are also considered based on location.
The &uantity of loads depends on the type of structure, number of floors and material of construction. 8s the number of floors increases, the dead load and imposed loads also increase. /hoice of material for construction such as reinforced concrete or steel construction also has impacts on foundation. 4einforced concrete buildings exert more loads on the foundation compared to steel structures.
'ased on the safe bearing capacity of structure and &uantity of loads on foundation, type of foundation and its base area is calculated.
2. T!e of Soils:
Soil is a mixture of solid particles, moisture and air. Soil can be of many t ypes such as clayey soil or
expansive soil, sandy soil or loose soils etc. The soil near surface is called as top soil and belo# a depth of -00mm is called as sub soil. 9enerally subsoil is used as base for foundation for small buildings.
2o#ever, soil investigation should be carried out to $no# the nature of soil, depth of #ater table, t ype of soil, depth of different layers of soil and to $no# the bearing capacity of soil at different levels for large structures.
3hen the load is transferred from the structure to soil through foundations, the soil tends to consolidate and settlement of foundation occurs. This consolidation process can be &uic$ in case of noncohesive soils such as sands and can even ta$e years for other soils. The complete settlement of foundation in sandy soil may occur even before the building construction has been completed. /layey soil can hold the #ater for longer time and thus settlement is very slo# and can ta$e years. Soil clayey holds large amount of #ater, and thus settlement of foundation is large in such soils.
The settlement of foundation causes crac$s in building #alls, beams, slabs etc. and building can even fail in case of large settlement.
The soil investigation is necessary #hen the loads from the building are large and the bearing capacity cannot be estimated based on type of soil condition at site.
The soil investigation should be carried out for follo"ing information:
• The nature and thic$ness of madeup ground:top soil above the subsoil • The nature, thic$ness and stratum depth of subsoil
• 8n assessment of allo#able bearing pressure • 9round#ater levels, chemicals in the ground, etc. • 6xisting structures or ha*ards in the ground.
#. T!e of Structure in $eighborhood:
The selection of foundation for building construction can also be done based on the t ype of foundation selected for the buildings in the neighboring buildings for the same types. Based on the success
or failure of foundations for such buildings, decision can be ta$en for the selection of foundation. %. T!es of Foundations:
Types of foundation such as isolated foundations, combined footings, pile foundations and raft or mat foundations etc. based on the type of soils and loads from the buildings can be selected based on suitability and re&uirement.
T&'($)*+&S +S&, F- )/-)$ B&A)$ 'AA')T3 -F S-)L
The follo#ing techni&ues can be used for improving bearing capacity of soil as per the site condition.
• ;ncreasing depth of foundation
• 5raining the soil
• /ompacting the soil
• /onfining the soil
• 4eplacing the poor soil
• <sing grouting material
• Stabili*ing the soil #ith chemicals
1. )$'&AS)$ ,&T( -F F-+$,AT)-$
8t deeper depths, the over burden pressure on soil is higher= hence the soil is more compacted at deeper depth. 8s a result it sho#s higher bearing capacity. This is applicable only for cohesionless soils such as sandy and gravelly soils. This method of improving bearing capacity of soil is not applicable if the subsoil material gro#s #etter as depth increase. This method has a limited use because #ith increase in depth, the #eight and cost of foundation also increases.
2. ,A)$)$ T(& S-)L
3ith increase in percentage of #ater content in soil, the bearing capacity decreases. ;n case of sandy soil, the bearing capacity may reduce as much as !0> due to presence of #ater content. /ohesionless soils
(i.e. sandy ? gravelly soils) can be drained by laying the porous pipes to a gentle slope, over a bed of sand and filling the trenches above the pipes #ith loose boulders. These trenches subse&uently should lead to the nearest #ell or any #ater body.
#. '-/A'T)$ T(& S-)L
;f #e compact soil using appropriate method, then there #ill be increase in its density and shear strength. 8s a result the bearing capacity of soil also increases. There are many methods of co mpacting soils on site. e# of them are mentioned belo#.
• 'y spreading bro$en stones, gravel or sand and thereafter ramming #ell in the bed of trenches.
• <sing an appropriate roller as per the soil type to move at a specified speed.
• 'r driving concrete piles or #ood piles and #ithdra#ing piles and subse&uently filling the holes
#ith sand or concrete.
%. '-$F)$)$ T(& S-)L
;n this method, the soils are enclosed #ith the help of sheet piles. This confined soil is further compacted to get more strength. This method is applicable for shallo# foundations.
4. &LA')$ T(& -- S-)L
;n this method the poor soil is first removed and then the gap is filled up by superior material such as sand, stone, gravel or any other hard material. ;n order to do this, first excavate a foundation trench of about 1.! m deep, and then fill the hard material is stages of -0 cm. Then compact the hard material at every stage. This method is useful for foundations in blac$ cotton soils.
5. +S)$ -+T)$ /AT&)AL
This method is applicable for soils #here there is presence of pores, fissures or crac$s etc underneath the foundation. ;n this method, poor soil bearing strata is hardened by in@ecting the cement grout under
pressure, because it scales off any crac$s or pores or fissures etc. or proper distribution of the cement grout, the ground is bored and perforated pipes are introduced to force the grout.
6. STAB)L)7)$ T(& S-)L 8)T( '(&/)'ALS
This method of improving bearing capacity of soil is costly and applied in exceptional cases. ;n this method, chemical solutions, li$e silicates of soda and calcium chloride is in@ected #ith pressure into the soil. These chemical along #ith the soil particles form a gel li$e structure and develop a compact
mass.This is called chemical stabili*ation of soil and used to give additional strength to soft soils at deeper depths.
