what is the considerations for foundation design and construction in civil technology ?

CONSIDERATIONS FOR FOUNDATION DESIGN AND CONSTRUCTION

 

1.1             GENERAL

 

The master plan of the proposed project site envisages the construction of multistoried structure (2B+LG+G+M+5F).

 

The considerations for the foundation design have been made keeping in view the topography of the area, type of structure, types of loads, settlement in foundation and the subsoil characteristics. A safe and an economical design of foundations of the structures have to be ensured.

Soil Excevation

 

1.2             GEOTECHNICAL DESIGN CRITERIA

 

Keeping in view the load from the structure and subsurface soil characteristics, the analysis for mat/raft foundations are considered on following criteria:

 

a.       For the design purposes, the guidance is taken from “Foundation Analysis and Design” by Josephe E Bowles.

b.      The Allowable Bearing Capacity is calculated at shear failure and settlement analysis on the basis of subsurface profile of Borehole profile as enclosed at Appendix-B.

c.       Factor of Safety is taken as 3.0 for both shear and settlement analysis.

 

d.      The maximum allowable settlement of 25 mm is considered for strip/isolated footing and 50 mm for mat/raft footing. The angular distortion between two adjacent foundations should not exceed 1/500.

e.      For settlement analysis Boussinesq Equation, “Foundation Analysis and Design” by Josephe E Bowles is used.

f.        For shear failure analysis, Terzaghi bearing capacity formula used for calculation of net bearing capacity of foundation:

 

qU  = cNcsc + q’Nq + 0.5BNs

 

 

Whereas;

c         = average cohesion for the soil stratum interest q          = effective average vertical stress

B         = width of footing

Nc’ = coefficient of Bearing Capacity Factor Nq’   = coefficient of Bearing Capacity Factor N      = coefficient of Bearing Capacity Factor sc & s = foundation shape factors

 

1.3             SOIL PARAMETERS

 

Engineering analyses for the determination of allowable load carrying capacity for various  subsoil conditions have to be based on carefully selected representative soil parameters on the basis of field observation and thereafter laboratory test results.

 

1.4             FOUNDATION TYPE

 

Keeping in view the high-rise structure and subsurface strata, it is recommended to use Mat/Raft foundation with bearing capacity at different levels as stipulated in subsequent section.

Foundations

 

1.5             ALLOWABLE LOAD BEARING CAPACITY OF FOUNDATION

 

The allowable load carrying capacity of substrata at different depths of both plots is stipulated hereunder:

 

S. No	(Plot No.1 &2) Depth below adjacent road/street (feet)	Allowable Bearing Capacity (T/ft2) (sand/gravel blend cushion 2 ft thick)
a.	34.0	2.80
b.	36.0	3.00
c.	38.0	3.15

 

 

1.6             SPRING STIFFNESS/MODULUS OF SUBGRADE REACTION

 

The Spring Stiffness/Modulus of Subgrade Reaction “Ks” is calculated keeping in view the soil condition encountered at site is 35,000 KN/m3. The source for correlation is from “Foundation Analysis and Design” by Josephe E Bowles (5th Edition).

 

1.7             GRAVEL/SAND BLEND CUSHION

 

It is recommended to use well graded gravel/sand blend cushion under the footing to increase net allowable bearing capacity, to avoid any pore water generation and to provide uniform modulus of subgrade reaction. The non-plastic gravel/sand blend cushion should be provided in

2.0 feet thickness in three layers and compacted up to 95% of maximum dry density (MDD). The following gradation may be adopted as stipulated hereto:

 

Size/Sieve No.	Percent passing
mm                              Inch

 

75.00	3”	100
19.00	¾”	70-80
4.75	No. 4	60-70
2.00	No. 10	40-60
0.425	No. 40	20-40
0.075	No. 200	0-5

 

5.8             RETAINING STRUCTURE

 

Approximately 32-38 feet of excavation is envisaged for the proposed structure in sandy silty clayey strata. It is recommended to provide some shoring system to provide safe working environment at level of excavated basements.

 

5.9             COEFFICIENTS OF LATERAL EARTH PRESSURE

 

The material encountered at site below existing ground level shall not be used as backfill material behind the retaining walls. It is recommended to use non-cohesive granular material blend with sand as the backfill material behind the retaining walls. The sand/gravel blend should be compacted to at least 95% Modified Proctor density. Heavy compaction equipment should not be employed for backfill behind retaining walls.

The following earth pressure coefficients are recommended to be adopted:

 

 

 

Soil Condition	Lateral Earth Pressure Coefficients
	Ka                        Ko                         Kp

 

Compacted Granular Material

0.33                        0.50                       3.00

 

 

 

The lateral earth pressures to be used in design should be increased for the additional residual earth pressures to be induced by the effect of compaction.

 

The backfill shall be provided throughout the wall back as shown in typical figure of retaining wall below so that the pore water shall not be generated.

 

 

 

5.10        COLLECTION OF SEEPAGE FLOW BEHIND RETAINING WALL
 

As stated in Section 5.9 of the Report that the backfill shall be provided throughout the wall back so that the pore water will not be generated. The seepage water accumulated in the granular backfill shall be collected in perforated pipe duly covered with geotextile membrane and drainage out either in any sump tank or drain in nearby river. The phenomenon is shown as hereunder: