SOILS AND FOUNDATIONS BNBC 2020
PARTHA SAHA
SENIOR RESEARCH ENGINEER
HOUSING AND BUILDING RESEARCH INSTITUTE
WHY DO WE NEED SOIL TEST??
CONTENT
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SITE INVESTIGATION
Sub-soil investigation includes;
a) Collection, b) Recording and c) Interpretation of geotechnical information
Geotechnical information includes;
a) Ground conditions, b) Geology, c) Geomorphology, d) Seismicity and e) Hydrology
***A competent graduate engineer having experience in supervising sub-soil exploration works shall be employed during sub-soil investigation.***
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ACTIVITIES OF SUBSOIL EXPLORATION
Subsoil exploration conists of three activities;
a) Reconnaissance: Gives us an overall idea about the site that helps preparing site investigation design.
b) Site investigation plan / design: To select the number and depth of field investigation points.
c) Field investigation: Includes a detailed subsoil investigation of the construction site
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STEPS OF FIELD INVESTIGATION
Field investigation comprises of;
a) Drilling / excavation
b) Ground water measurement
c) Field testing
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LOCATION OF INVESTIGATION POINTS
The following conditions should be considered for the selection of the bore hole points.
a) A detailed soil stratification should be achieved
b) Points should be close to the critical points of the structure
c) The boreholes should be drilled close to the proposed foundations
d) At least 67% of the required number of borings should be placed under the construction
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NUMBER OF BORINGS
a) At least 3 (three) not in one line for small building
b) At least 5 (five) for large building, one at the middle and four at the corners.
c) For large industrial and residential colonies the following chart may be followed
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Types of building | Spacing of Boring (m) | ||
Types of soil in horizontal stratification | |||
Uniform | Average | Erratic | |
Small Building | 60 | 30 | 15 |
Large building | 45 | 30 | 15 |
| | |
| | |
| | |
DEPTH OF BORINGS
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B
D
W
L
Plot
DEPTH OF BORINGS
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b
d
l
Maximum dimension of pile group or shaft
CONTENT OF A GEOTECHNICAL INVESTIGATION REPORT
a) Purpose and scope the investigation including site description, topography and the planned structure.
b) The name of all consultants and contractors.
c) Actual date and time duration for the whole investigation work.
d) The reconnaissance of the project site and surrounding area should be noted
e) Documentation of the methods, procedures and results
**The results of the field and laboratory investigation shall be presented and reported according to the requirements defined in ASTM or equivalent standards**
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SOIL CLASSIFICATION
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SOIL CLASSIFICATION BASED ON PARTICLE SIZE
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Soil type | Particle size range (mm) | Retained on mesh size/sieve no | |
Boulder | | > 300 | 12” |
Cobble | | 75 - 300 | 3” |
Gravel | Coarse | 19 - 75 | 3/4” |
| Medium | 9.5 - 19 | 3/8” |
| Fine | 4.75 - 9.5 | No 4 |
Sand | Coarse | 2.0 - 4.75 | No 10 |
| Medium | 0.425 - 2 | No 40 |
| Fine | 0.075 - 0.425 | No 200 |
Silt | | 0.002 - 0.075 | --- |
Clay | | < 0.002 | --- |
Ref: Table 6.3.1- Particle size ranges of soils (BNBC 2020)
ENGINEERING CLASSIFICATION OF SOIL
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Ref: Table 6.3.2- Engineering classification of soils (BNBC 2020)
ENGINEERING CLASSIFICATION OF SOIL
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Ref: Table 6.3.2- Engineering classification of soils (BNBC 2020)
Ref: Figure 6.3.1- Plasticity chart (BNBC 2020)
FOUNDATION DESIGN
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TYPES OF FOUNDATION
a) Shallow foundation
b) Deep foundation
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Pile less than 600 mm
Pile greater than 600 mm
Box type foundation
DESIGN OF SHALLOW FOUNDATION
Minimum depth “D” for shallow foundation
a) In cohesive soil: at least 1.5 m
b) In cohesionless soil: at least 2.0 m
c) For temporary structures: at least 0.4 m
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B
D
DESIGN OF SHALLOW FOUNDATION
Design load for foundation
a) Foundation should be designed to support design load with tolerable settlement
b) The location of the resultant pressure should be maintained within B/6 from the center of the footing
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b
b
b/3
b/3
Which one is the most unfavorable
DESIGN OF SHALLOW FOUNDATION
Bearing capacity of shallow foundation
a) Established bearing capacity equation can be used to calculate bearing capacity of foundation
b) A factor of safety 2 – 3 should be used depending on the engineering judgment for dead load and live load
c) 33% overstressing above the allowable bearing pressure is allowed in case of design considering wind or seismic loading
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PERMISSIBLE SETTLEMENT OF SHALLOW FOUNDATION
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| Sand | Clay |
Total settlement | 25 | 40 |
Differential settlement | 20 | 20 |
General rule of permissible settlement for Isolated footing in mm
However, the tolerable settlement is very much subjective to the type of structure, foundation and soil. Table 6.3.8 (BNBC 2020) can be used as guideline in this regard.
DYNAMIC GROUND STABILITY OR LIQUEFACTION OF SOIL
Liquefaction: Loss of strength due to the generation of excess pore water pressure
Factor of safety FL = CRR/CSR
Fine grained soils are susceptible to liquefaction if (Finn et. al. 1994)
Fraction finer than 0.005 mm ≤ 10%
Liquid limit (LL) ≤ 36%
Natural water content ≤ 0.9 LL
Liquidity Index ≤ 0.75
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Cyclic Resistance ratio
Cyclic stress ratio
Liquefaction occurs if FL ≤ 1
DESIGN OF DEEP FOUNDATION
In terms of construction, piles are broadly classified as
a) Driven pile
b) Bored pile
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a) Friction pile
b) End bearing pile
c) Combined friction and end bearing
P
End bearing
Skin friction
LOAD CARRYING CAPACITY OF PILE
Ultimate axial load carrying capacity of a pile can be calculated as
For uplift loading
Allowable or working axial load
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P
End bearing
Skin friction
Skin friction
End bearing
Weight of pile
Factor of safety
ESTIMATION OF LOAD CARRYING CAPACITY OF PILE
The SPT (N) value can be used to determine the bearing capacity of pile
and
For cohesive soil For sand For non-plastic silt
Skin friction (kPa)
End bearing (kPa)
FS = 3.5 (while using SPT value for estimating allowable bearing capacity)
For bored pile
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P
End bearing
Skin friction
L/D ≤ 10
SPACING OF PILES
For end bearing pile
S > 750 mm
S ≥ 2.5*D
For friction pile
S ≥ 3.0*D
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S
S
D
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