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Geotechnical Issues in Tall Buildings - Note from Jaydeep Wa

 
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PostPosted: Mon Nov 19, 2012 12:27 pm    Post subject: Geotechnical Issues in Tall Buildings - Note from Jaydeep Wa Reply with quote

Dear All: 


Foll is Initial note received from Geotech Consultant Mr Jaydeep Wagh, one of the subject experts for the Econference. 


regards,
Moderators


***************************************




SITING OF TALL BUILDINGS
High rise buildings can generally be constructed on almost any site with any subsurface conditions.  However, certain difficult or complex subsurface conditions at some sites may render construction of high rise buildings impractical with respect to time and cost.  These difficult subsurface conditions may include presence of competent strata at relatively deep depths (>60m) or due to presence of miscellaneous fill with boulders, which make it difficult for piling equipment to penetrate to competent strata.  A comprehensive geotechnical engineering program combined with extensive geotechnical experience of high rise buildings is essential in arriving at safe and cost effective foundation design.
 
GEOTECHNICAL INVESTIGATION PROGRAM:
A complete geotechnical investigation program is an essential pre-requisite to arrive at design spring constants and/or foundation design of high rise buildings.  A proper geotechnical investigation program may also help in significantly reducing foundation costs along with ensuring safe foundations.   
 
An universal detailed geotechnical investigation program cannot be prescribed for a high rise building site as it will obviously depend on the type of building, number of floors, number of basements (if any), and subsurface conditions.  Typically, boreholes for a high rise building are spaced at every 25m to 30m and are extended to 1.5 times the width of foundation or 5m into relatively incompressible strata and atleast 5 times pile diameters in case of pile foundations.  At a minimum SPT tests are generally conducted in soil and soft rock, while rock coring operations are conducted in hard rock.  A detailed custom geotechnical program can be chalked out after initial boreholes are available and a foundation system type has been selected.  Detailed tests including pressuremeter tests, plate load tests, or pile load tests can then be conducted.  If local experience is available, a detailed geotechnical investigation program may be specified right from the beginning.  However, it is prudent to still study initial borehole logs to see if any modification to the geotechnical investigation program is required.
 
PILED RAFT FOUNDATION :
A piled raft foundation can be considered in lieu of a complete pile foundation, if relatively competent strata but not totally adequate, is encountered at raft founding level.  The efficiency of a piled raft foundation would depend on the relative stiffness of the soil/rock at raft founding level in comparison to the soil/rock near pile tip level.  It may be important to note that some degree of resistance is always provided by the soil/rock beneath the pile cap or the piled raft.  However, the effort required in analyzing this available resistance may not be justified if the soils at pile cap/raft level are relatively much softer.  A typical example of this is a building constructed in soft marine clay areas when the pile tip is extended to underlying hard rock.  The contribution of a pile cap or raft portion of the piled raft in carrying loads may range from negligible (as mentioned above) to even 90% in some cases.
 
Typical allowable deformations are generally governed by the permissible differential settlement of the structure.  Differential settlements limits as specified in several country codes for raft foundations can also be applied to piled rafts.  Typical permissible limits range from L/660 for spread foundations to L/400 for raft foundations.  Some country codes also specify a permissible for total maximum deformations, for eg. 75mm in IS1904.
 
A more aggressive form of piled raft design, which has become quite popular in recent times due to development of geotechnical finite element programs is settlement reducing piles.  This specific application can be used where a conventional raft only foundation would have adequate factor of safety against shear failure, but cannot be utilized as it does not meet the permissible total/differential settlement criteria.  Few piles are then added beneath the raft at strategic locations solely to reduce the total/differential settlements to within permissible limits.
 
A pile load test is absolutely essential for design of a piled raft system.  A pile load test not only provides ultimate pile capacity, but also provides the pattern of pile load v/s. pile settlement, which is essential in deformation analysis of a piled raft.  Test pile should be conducted using same equipment and methods as for final working piles.      
 
Manual design methods can be utilized for geotechnical analysis of conventional buildings with relatively uniform loading patterns and uniform homogeneous typically single layer soil type.  However, finite element softwares are often a must for high rise building piled raft analysis, and are also considered more accurate.              
 


Jaydeep Wagh 

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PostPosted: Mon Nov 19, 2012 4:55 pm    Post subject: Soil Investigation & Reporting brief Reply with quote

Soil Investigation & Reporting brief<xml><o></o>
1.     Tall buildings require higher level of soil investigation <o></o>
2.     True that structure rests on hard rock which is available much deeper than other normal buildings, which rest lesser deep<o></o>
3.     Exploration requires value of N/ SPT, TCR, SCR, RQD<o></o>
4.     N applies to 300 mm penetration for standard measurement pertinent number of blows during standard penetration testing<o></o>
5.     Higher value of N for lesser penetration indicates a better soil type<o></o>
6.     But, it applies to cohesive soils & not to rocks<o></o>
7.     Rock samples are taken as core drill extracts<o></o>
8.     TCR stands for total core recovery<o></o>
9.     SCR stands for solid core recovery<o></o>
10.                        Core pieces more than 100 mm are counted/measured to arrive to total solids recoveries<o></o>
11.                        Core pieces so summed up are compared to total core recoveries<o></o>
12.                        RQD which stands for rock quality designation is arrived by computing ratio SCR divided by TCR<o></o>
13.                        Higher RQD indicates better rock<o></o>
14.                        In some cases, more than 80% RQD has been reported<o></o>
15.                        Soft rocks give lesser values<o></o>
16.                        Other tests are thermal conductivity & resistivity tests<o></o>
17.                        Electrical conductivity as well as resistivity testing<o></o>
18.                        Where soil is involved around raft over piles, soil parameters related to Atterberg’s limits are investigated<o></o>
19.                        Other miscellaneous tests are also conducted, but prominent investigation in case of deep or pile foundation that may rest on hard rock, have been named in foregoing<o></o>
20.                        Reporting on soil investigation & testing should be descriptive giving complete testing procedures adopted with relevant tabulated results & relevant recommendations to enable design engineer work out foundation preliminaries keeping abreast all indicated parameters<o></o>
21.                        All assumptions should explicitly be included for clear communication<o></o>
22.                        All limitations on report should be elaborately defined<o></o>
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PostPosted: Tue Nov 20, 2012 1:54 pm    Post subject: Re: Geotechnical Issues in Tall Buildings - Note from Jaydee Reply with quote

Econf_Moderator wrote:
Dear All:


Foll is Initial note received from Geotech Consultant Mr Jaydeep Wagh, one of the subject experts for the Econference.


regards,
Moderators


***************************************




SITING OF TALL BUILDINGS
At a minimum SPT tests are generally conducted in soil and soft rock, while rock coring operations are conducted in hard rock.



Jaydeep Wagh

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Respected Sir,
How do we define soft rock and hard rock from soil investigation point of view ?
Warm Regards.

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PostPosted: Wed Nov 21, 2012 6:15 am    Post subject: Rock Strength Reply with quote

Rocks/Compressive strength/BS 5930

  1. Extremely strong/Rocks ring on hammer blows/>200MN/m^2
  2. Very srong/Core chipped only by heavy hammer blows/100-200MN/m^2
  3. Strong/Broken by heavy hammer blows/ 50-100MN/m^2
  4. Moderately strong/Broken by hammer blows while hand held/12.5-50MN/m^2
  5. Moderately weak/Thin slabs or edges broken by heavy hand pressure/5-12.5MN/m^2
  6. Weak/Gravel size lumps broken by heavy hand pressure/1.25-5MN/m^2
  7. Very weak/Gravel size lumps crushed between fingers & thumb/<1>
  8. RQD 70/75 may be considered hard range

Granular

  1. # of blows 50/Very dense
  2. # of blows 30-50/Dense


Cohesive soil


  1. Undrained shear strength>300 KN/m^2/Hard

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PostPosted: Tue Nov 27, 2012 10:55 am    Post subject: Geotechnical Engineering Reply with quote

Geotechnical Engineering

  1. Though it is a repetition of comments as were made long back on general forum, yet useful to suggest all engineers not to avoid soil exploration interpretation
  2. It has been noticed that engineers do not with interest, study soil reports submitted by material testing laboratories
  3. It is true that soil reporting documents are voluminous page wise, yet very informative if issued by some professional laboratory
  4. Without clearly understanding report, engineer cannot decide designs philosophy as well as, prosecution site engineer cannot understand handling soil engineering
  5. There could be many masters duly qualified on soil mechanics, yet not many topics pertinent that subject have been conferred in this conference
  6. I think that soil engineering can help a lot all engineer to do good foundation design & carry put works on site
  7. Even design engineer do not bother about thorough understanding reports, but move on to carry out designs, which situation leads to altering design in some cases, when work starts on sites
  8. Soil parameters should clearly be understood prior to designing & work execution

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