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Responses to Geotech Issues raised in Econference

 
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PostPosted: Mon Mar 26, 2018 6:59 am    Post subject: Responses to Geotech Issues raised in Econference Reply with quote

Dear All:  



As promised, we will try to respond to the backlog of queries. Here is response to some queries responded to by geotech engineer Jaydeep Wagh.  



Comment: Clause 9.7.1 clarification on zoned spring constants is required. It should be clearly mentioned that "in plan area of raft variation of strata can be considered by using appropriate spring constants."



Response: Zoned spring constant is required not only due to variation of strata, but also since the settlements of raft are variable. Spring constant by definition is pressure divided by settlement. Typically even for a raft subjected to uniformly distributed pressure, maximum settlement at raft centre is twice the settlement at raft edges. Thus spring constant at centre of the raft will be half the spring constant at edges.



Comment: Clause 9.3.1- This clause is more applicable to uniform soil strata. But when Non Uniform or Structures Sensitive to Settlements & Heavy load is considered requires extensive subsurface knowledge and closer spacing of Boreholes and no. of boreholes requirement can increase from min 3. This should have been highlighted that requirement of min 3 borehole is specifically for uniform soil strata.



Response: The comment made is correct. However, the clause only specified minimum requirements. Additional investigation can be conducted on a case to case basis.



Comment: ASCE- Unless Bedrock is encountered first, Boring should be carried to such Depths that the Net increase in soil stress under the weight of the structure is less than 10% of the Average load of the structure, or less than 5% of the effective overburden stress in the soil at that Depth , whichever is the lesser Depth. This was the better criteria for Bore Hole Depth.  



But 9.31. (c) - 1,5 Times of estimated width of foundation in soil is mentioned. Code should avoid simplification based on Rule of Thumb.



Response: The 1.5 times width of foundation depth is the depth at which dissipated foundation pressures reduce to less than 20% to 15% of bearing pressures. This minimum criteria of 1.5 times width of foundation is the same as recommended in IS1892. This is only the minimum recommended. Deeper boreholes can be conducted on a case to case basis.  



Comment: Clause 9.7.1- Till date no concrete procedure has been highlighted by the code for finding the Modulus of sub grade reaction. Zoned springs also called pseudo coupled method is never provided by in the Geo technical Reports because Structural engineers don't share column loads initially at time of Geo technical reports are prepared. This is more a practical challenge and practice procedure missing.



Response: Zoned spring constants can always be derived and utilized for design after the geotechnical report has identified the type of foundation. Use of zoned spring constants can be specified in the geotechnical report if a raft foundation is envisaged.  



Pseudo coupled Method produced computed moments are 18-25 % higher then those determined from the Wrinkler method. Which to Follow code must define.  



Response: For raft foundation, zoned spring constants should be utilized for the very fact that this method is realistic, and the moments are higher.  



Soil Structure interaction is mentioned but for buildings height more than 150 m, but what happens when soil structure becomes significantly important in buildings with height less than 150 m. Procedure for carrying out the soil structure interactions and caution for adopting such a process is also not highlighted. Soil structure interaction should be jointly done with Structural consultant & Geo-technical consultant knowledge and approval, because by performing the analysis there will be decrease in the Base shear, Lateral forces but may increase the lateral displacement in association with P-delta. Soil Structures procedures for Foundation Deformations, Inertia SSI effects, Kinematic SSI effect should have been mentioned.



Response: Soil structure interaction is currently mentioned for building height more than 150m. This may be used for buildings of lesser heights also. Soil structure interaction using actual building column loads and locations has been mentioned. Further refinements are possible ​​in future versions of the code.  

Econf Moderators.

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