Soils and Foundations as per BNBC 2020

Soils and foundation

Soils and Foundations as per BNBC 2020

#BNBC_2020 এর Soils and Foundations Chapter টিতে কিছু ভুল আছে এবং এক সেকসনের সাথে আরেক সেকসনের কিছু অসামাঞ্জস্য আছে । তারপর ও বলবো , বাংলাদেশের সিভিল ইঞ্জিনিয়ারদের জন্য এই চ্যাপ্টারটি অনেক ভাল হয়েছে । তবে সাবধান থাকবেন, এখানকার পাইল ক্যাপাসিটি ফরমুলা গুলো ব্যাবহার করবেন না । সিভিল ইঞ্জিনিয়ারদের নিত্য দিনের অনেক প্রশ্নের জবাব এখানে পাওয়া যাবে । settlement and differential settlement of footing and mat, factor of safety of pile foundation, quality control of pile foundation construction ইত্যাদি অনেক বিষয়ে এত ভাল গাইডলাইন পুরাতন কোড #BNBC_2006 এ ছিল না । যদিও BNBC 2007 এখনো অনুমোদিত নয়, যে কেঊ চাইলে এটি এখনি প্র্যাক্টিস করতে পারেন । বাংলাদেশে অনেক বড় বড় প্রজেক্টে BNBC 2017 ব্যাবহার হচ্ছে ।

Soils and Foundation as per BNBC 2017 will be a great short course for future Geotechnical Engineers of Bangladesh. There are some errors in the Soils and Foundations Chapter of BNBC_2020 and some inconsistencies between one section and another. Even Then I will say, this chapter is good for the civil engineers of Bangladesh. However, be careful not to use the pile capacity formulas here. The answers to many of the daily questions of civil engineers can be found here. The old code # BNBC_2006 did not have such proper guidelines on settlement and differential settlement of footing and mat, the factor of safety of pile foundation, quality control of pile foundation construction etc. Although BNBC 2020 is not yet approved, anyone can practice it now. BNBC 2020 is being used in many big projects in Bangladesh.

What you'll learn

  • Difference between safe bearing capacity and allowable bearing capacity
  • Settlement limits for different soil condition and Structures
  • Subsoil investigation guidelines
  • Exansive soil
  • Dispersive soil
  • Soft soil
  • Minimum requirements of foundations
  • Important criteria of foundation design
  • Guideline for excavations
  • Factor of safety of foundations

Prerequisite / Eligibility

  • BSc, MSc or PhD in Civil Engineering
  • Level 4 or 4th year student of bachelor’s in civil engineering
  • AMIEB students of IEB

Course Teacher

Professor Dr. Jahangir Alam, Department of Civil Engineering, BUET, Dhaka, Bangladesh

Brief Biography of Course Teacher

Education

  • PhD in Geotechnical Earthquake Engineering, the University of Tokyo, Japan, 2005
  • MSc in Civil and Geotechnical Engineering, BUET, Dhaka, Bangladesh, 2002
  • BSc in Civil Engineering, BUET, Dhaka, Bangladesh, 1998

Biography

Professor Dr. Engr. Md. Jahangir Alam is faculty member at the Department of Civil Engineering, BUET, Dhaka-1000, since 1999. He completed his PhD from University of Tokyo in Geotechnical Earthquake Engineering in 2005 as a Monbu-Kagaku-sho Scholar. He was research fellow in Ecole Centrale Paris in 2008. Professor Jahangir did his BSc in Civil Engg with major in Structure and MSc in Civil and Geotechnical Engineering from BUET.

Professor Jahangir has multidisciplinary research interests and has publications in international journals and conferences. His current research topics are “Risk Sensitive Land Use Planning of Mega City”, Physical and Numerical Modeling of Liquefaction Hazard, Mitigation against Seismic Liquefaction, Cyclic Behavior of Non-plastic silt, Reinforced Earth, Earthquake Resistant Foundation in Soft Soil, Climate Resilient Concrete, Climate Resilient Road.

Professor Jahangir was involved in many consultancy projects where he designed/checked high rise RCC buildings, Communication towers, Jetty, Shore Pile, Embankment, Container Terminal, Ground Improvement, Bridge Foundation etc. He supervised many MSc and PhD students. He was involved in pile load testing, pile integrity testing, concrete mix design and development of laboratory and field-testing equipment.

Professor Jahangir actively involved in National and International professional bodies. He is life member and was Treasurer of Bangladesh Society for Geotechnical Engineering (BSGE), which is Bangladesh Chapter of ISSMGE. He is life fellow of Institute of Engineers Bangladesh (IEB). He is life member of Bangladesh Earthquake Society (BES). He was Treasurer of Bangladesh Earthquake Society (BES).

Certificate of Attendance

Certificate of attendance will be awarded after completion of all video lessons and quizzes

Related Courses

  • Earthquake Load as per BNBC 2020
  • Wind Load as per BNBC 2020
  • Seismic Detailing of RCC Structure as per BNBC 2020

Free Courses

  • Design Issues for Large Earth Structures: Stress, Deformation, FOS and Liquefaction
  • Webinar on Dynamic Analysis

Features of ourPROFESSORs.com

Frequently Asked Questions (FAQ)

Course Preview

Detail Course Outline

Soils and Foundations

  • DIVISION A (Site Investigations, Soil Classifications, Materials and Foundation Types): Sub-Surface Survey, Methods of Exploration, Depth of Exploration, Geotechnical Investigation Report, USCS Classification, Organic soil, Expansive Soils, Collapsible Soils, Dispersive Soil, Exchangeable Sodium Percentage, Double Hydrometer Test, Soft Inorganic Soil.
  • DIVISION B (Service Load Design Method of Foundations): Bearing capacity of the soil, Dimension of footing, Depth of footing, Thickness of Footing, Scour, Mass Movement of Ground in Unstable Areas, Foundation Excavation, Design Considerations, Geotechnical design of shallow foundations, Settlement,
  • DIVISION C (Additional Considerations in Planning, Design and Construction of Building Foundations): Excavation, Dewatering, Slope stability of adjoining buildings.

Course contents

1
BNBC intensive outline

CONTENTS

  • BNBC Part 6, Chapter 1 – Definition and General Requirement
  • BNBC Part 6, Chapter 2 – Load (wind and earthquake load)
  • BNBC Part 6, Chapter 3 – Soils and Foundations
  • BNBC Part 6, Chapter 8 – Seismic Detailing of RCC Structures
  • Software implementation of wind load and earthquake load


2
All documents of Soils and Foundations as per BNBC 2020

Soils and foundations

1
Soils and foundations, Part-1

SOILS AND FOUNDATIONS


Contents of Chapter 3


The Soils and Foundations Chapter of the Code is divided into the following three distinct Divisions :


•     Division A: Site Investigations, Soil Classifications, Materials and Foundation Types

•     Division B: Service Load Design Method of Foundations

•     Division C: Additional Considerations in Planning, Design and Construction of Building Foundations






Ultimate // Safe // Allowable Bearing Capacity

1.      Ultimate bearing capacity = soil pressure at which failure occur

2.      Safe bearing capacity = ultimate/FS; so safety margin exist, no failure; but may have excessive settlement

3.      Allowable bearing capacity = soil pressure at which settlement of footing will be within permitted limit

4.      settlement governs for most of the soils

5.      Ultimate bearing capacity is determined using any formula = 600 kPa

6.      Safe Bearing Capacity = Ultimate / FS = 200 kPa

7.      Say footing size is 3 m x 3 m using safe bearing capacity and calculated settlement is 150 mm

8.      To limit the settlement within allowable limit (say 50 mm), footing size need to be increased. Say, 4 m x 4 m is ok at 100 kPa

9.      So, Allowable bearing capacity = 100 kPa



Bearing capacity vs. bearing pressure

Bearing capacity

1.     Ultimate bearing capacity is determined using any formula = 600 kPa

2.     Safe Bearing Capacity = Ultimate / FS = 200 kPa

3.     Say footing size is 3 m x 3 m using safe bearing capacity and calculated settlement is 150 mm

4.     To limit the settlement within allowable limit (say 50 mm), footing size need to be increased. Say, 4 m x 4 m is ok at 100 kPa

5.     So, Allowable bearing capacity = 100 kPa


Bearing pressure

1.      Bearing pressure = pressure applied on soil at the bottom of footing = soil pressure

2.      Applied load divided by footing area

3.      Bearing pressure may be less, equal or more than Allowable bearing capacity

4.      Bearing pressure should be less than Allowable bearing capacity





DIVISION A

Site Investigations, Soil Classifications, Materials and Foundation Types



3.4.1 Sub-Surface Survey


Depending on the type of project thorough investigations has to be carried out for identification, location, alignment and depth of various utilities, e.g., pipelines, cables, sewerage lines, water mains etc. below the surface of existing ground level. Detailed survey may also be conducted to ascertain the topography of existing ground.

  1. identification, location, alignment and depth of various utilities
  2. the topography of existing ground


3.4.2 Sub-Soil Investigations


  1. describing the character, nature, load bearing capacity and settlement capacity of the soil
  2. establish the soil, rock and groundwater conditions,
  3. determine the properties of the soil and rock
  4. gather additional relevant knowledge about the site.
  5. Careful collection, recording and interpretation of geotechnical information shall be made including ground conditions, geology, geomorphology, seismicity and hydrology, as relevant.
  6. Indications of the variability of the ground shall be taken into account.
  7. An engineering geological study may be an important consideration to establish the physiographic setting and stratigraphic sequences of soil strata of the area. Geological and agricultural soil maps of the area may give valuable information of site conditions.
  8. a competent graduate engineer having experiences in supervising sub-soil exploration works shall be employed by the drilling contractor.



3.4.3 Methods of Exploration


  1. Reconnaissance

a)     Geophysical measurement

b)     Sounding or probing (eg. DCP)

  1. Exploration and detail investigation

a)     Drilling and/or excavations for sampling

b)     Groundwater measurements

c)     Field tests (CPT, SPT etc)

d)     Laboratory tests



3.4.4 Number and Location of Investigation Points:

When selecting the locations of investigation points, it should be :

       i.           Arranged in such a pattern that the stratification can be assessed across the site.

     ii.           Placed at critical points relative to the shape, structural behavior and expected load distribution.

   iii.           Arranged at adequate offsets to the centre line for linear structures (road, railway).

   iv.           Arranged outside the project area for structures on or near slopes and steps in the terrain.

     v.           The locations and spacing of sounding, pits and boreholes shall be such that the soil profiles obtained will permit a reasonably accurate estimate of the extent and character of the intervening soil or rock mass.



For building structures, the following guidelines shall be followed:

On uniform soils, at least three borings, not in one line, should be made for small buildings and at least five borings one at each corner and one at the middle should be made for large buildings.

As far as possible the boreholes should be drilled closed to the proposed foundations but outside their outlines.

Spacing of exploration depends upon nature and condition of soil, nature and size of the project. In uniform soil, spacing of exploration (boring) may be 30 m to 100 m apart or more and in very erratic soil conditions, spacing of 10 m or less may be required.

Spacing of exploration depends upon nature and condition of soil, nature and size of the project. In uniform soil, spacing of exploration (boring) may be 30 m to 100 m apart or more and in very erratic soil conditions, spacing of 10 m or less may be required.



Number and Location of Investigation Points:

For large areas covering industrial and residential colonies, the whole area may be divided into grid pattern with Cone Penetration Tests performed at every 100 m grid points. The number of boreholes or trial pits shall be decided by examining the variation in penetration curves. At least 67% of the required number of borings or trial pits shall be located within the area under the building.


3.4.5 Depth of Exploration

The site investigation should be carried to such a depth that the entire zone of soil or rock affected by the changes caused by the building or the construction Change of effective stress is less than 10% of the average contact pressure of foundation or Less than 5% of the effective stress in the soil at that depth

At least 1 BH upto 30 m depth to define site class




  1. Where substructure units will be supported on spread footings, the minimum depth boring should extend below the anticipated bearing level a minimum of two footing widths for isolated, individual footings where length £ 2 times of width, and four footing widths for footings where length > 5 times of width. For intermediate footing lengths, the minimum depth of boring may be estimated by linear interpolation as a function of length between depths of two times width and five times width below the bearing level. Greater depth may be required where warranted by local conditions.
  2. For more heavily loaded structures, such as multistoried structures and for framed structures, at least 50% of the borings should be extended to a depth equal to 1.5 times the width of the building below the lowest part of the foundation.
  3. Normally the depth of exploration shall be 1.5 times the estimated width or the least dimension of the footing below the foundation level. If the pressure bulbs for a number of loaded areas overlap, the whole area may be considered as loaded and exploration shall be carried down to one and a half times the least dimension. In weak soils, the exploration shall be continued to a depth at which the loads can be carried by the stratum in question without undesirable settlement or shear failure.
  4. Where substructure units will be supported on deep foundations, the depth boring should extend a minimum of 6 m below the anticipated pile of shaft tip elevation. Where pile or shaft groups will be used, the boring should extend at least two times the maximum pile or shaft group dimension below the anticipated tip elevation, unless the foundation will be end bearing on or in rock.
  5. For piles bearing on rock, a minimum of 1.5 m of rock core should be obtained at each boring location to ensure the boring has not been terminated in a boulder.
  6. For shafts supported on or extending into rock, a minimum of 1.5 m of rock core, or a length of rock core equal to at least three times the shaft diameter for isolated shafts or two times the maximum shaft group dimension for a shaft group, whichever is greater, should be obtained to ensure that the boring had not been terminated in a boulder and to determine the physical properties of rock within the zone of foundation influence for design.
  7. The depth, to which weathering process affects the deposit, shall be regarded as the minimum depth of exploration for a site. However, in no case shall this depth be less than 2 m, but where industrial processes affect the soil characteristics, this depth may be more.
  8. At least one boring should be carried out to bedrock, or to well below the anticipated level of influence of the building. Bedrock should be ascertained by coring into it to a minimum depth of 3 m.




For structures supported on spread footings:

In weak soils, the exploration shall be continued to a depth at which the loads can be carried by the stratum in question without undesirable settlement or shear failure.

a)     For Deep foundations : D ≥ 6 m below the anticipated pile shaft tip elevation.

 

For piles bearing on rock :

 A minimum of 1.5 m of rock core should be obtained at each boring location to  ensure the boring has not been terminated in a boulder

a)     For shafts supported on or extending into rock,

b)     The depth, to which weathering process affects the deposit, shall be regarded as the minimum depth of exploration for a site. However, in no case shall this depth be less than

c)     At least one boring should be carried out to bed rock, or to well below the anticipated level of influence of the building. Bedrock should be ascertained by coring into it to a minimum depth of 3 m



3.4.7 Geotechnical Investigation Report


The results of a geotechnical investigation shall be compiled in the Geotechnical Investigation Report which shall form a part of the Geotechnical Design Report. The Geotechnical Investigation Report shall consist of the following:

1.     A presentation of all appropriate geotechnical information on field and laboratory tests including geological features and relevant data;

2.     A geotechnical evaluation of the information, stating the assumptions made in the interpretation of the test results.

3.     The names of all consultants and contractors;

4.     The dates between which field and laboratory investigations were performed;

5.     Evidence of groundwater;

6.     Behaviour of neighbouring structures;

7.     Exposures in quarries and borrow areas;

8.     Areas of instability;

9.     Difficulties during excavation;

10. History of the site;

11. Geology of the site,

12. Survey data with plans showing the structure and the location of all investigation points;

13. Local experience in the area;

14. Desk studies;

15. Field investigations

16. Laboratory tests and test standard followed


USCS Classification is adopted by BNBC 2017


3.5.4 Organic Soil


Table 6.3.3: Classification and Description of Organic Soils (after Edil, 1997) (ASTM D2974-07a)


Organic Content< 5 %=Little effect on behavior; considered inorganic soil.

Organic Content 6 ~ 20 %=Effects properties but behavior is still like mineral soils; organic silts and clays.

Organic Content 21 ~ 74 %=Organic matter governs properties; traditional soil mechanics may be applicable; silty or clayey organic soils.

Organic Content > 75 %=Displays behavior distinct from traditional soil mechanics especially at low stress.




Problems of organic soil


  1. Low bearing capacity
  2. High consolidation and creep settlement
  3. Swelling and shrinkage potential
  4. Vertical and lateral capacity of pile is low
  5. If it is PEAT>>> extremely high natural moisture content, high compressibility including significant secondary and even tertiary compression and very low undrained shear strength at natural moisture content.




How to determine organic content?


  1. Loss on ignition
  2. Mass loss on treatment with hydrogen peroxide (H2O2)



3.5.5 Expansive Soils


  1. Expansive soils are those which swell considerably on absorption of water and shrink on the removal of water.
  2. In monsoon seasons, expansive soils imbibe water, become soft and swell. In drier seasons, these soils shrink or reduce in volume due to evaporation of water and become harder.
  3. As such, the seasonal moisture variation in such soil deposits around and beneath the structure results into subsequent upward and downward movements of structures leading to structural damage, in the form of wide cracks in the wall and distortion of floors.
  4. For identification and classification of expansive soils parameters like
  5. liquid limit, plasticity index, shrinkage limit, free swell, free swell index, linear shrinkage, swelling potential, swelling pressure and volume change from air dry to saturate condition
  6. Various recommended criteria for identification and classification of expansive soils are presented in Appendix E.



Based on the values of plasticity index and shrinkage limit, United States Bureau of Reclamation (USBR) suggests the following classification criteria for expansive soil:


Plasticity Index------Shrinkage Limit-----Degree of Expansion

>35------------------------------<10---------------------Very High

25-41-------------------------------6-12-------------------High

15-28------------------------------8-18-----------------Medium

<18--------------------------------->13-----------------------Low




On the basis of previous data for linear shrinkage of Bangladesh soils, criteria for the degree of expansion proposed by Hossain (1983) is as follows:

Linear Shrinkage (%) ---------------Degree of Expansion

> 14--------------------------------------High 

10-14------------------------------------Medium

0-10-------------------------------------Low



On the basis of the values of free swell, Indian standard (IS: 1948, 1970) recommends criteria of expansion is as follows:



Based on the value of free swell index, Indian Standard (IS: 2911, Part III, 1980) suggests the following criteria for the degree of expansion of soils:



Based on the values of liquid limit, plasticity index and shrinkage limit, Indian Standard (IS: 2911, Part 3, 1980) suggests the following criteria for the degree of expansiveness of soils:



Based on the values of swelling potential, Seed et al. (1962) proposed the following four categories of expansion characteristics:



Based on the values of swelling pressure, Chen (1965) proposed the following criteria for degree of expansion:



Based on the values of volume change from air dry to saturated condition, Seed et al. (1962) proposed the following four categories of expansion characteristics:



Look (2007) reports that the plasticity index by itself can be misleading, as the test is carried out on the percent passing the 425 micron sieve, i.e. any sizes greater than 425 µm is discarded. There have been cases when a predominantly “rocky/granular” site has a high PI test results with over 75 percent of the material discarded. The weighted plasticity index (WPI) considers the percent of material used in the test, where WPI=PI×% passing the 425 micron sieve. Degree of expansion with weighted plasticity index is presented as under. 




Collapsible Soils

Unsaturated soils which collapsed upon wetting


Soil deposits most likely to collapse are; 

(i) loose fills, 

(ii) altered wind-blown sands, 

(iii) hill wash of loose consistency and 

(iv) decomposed granite or other acid igneous rocks.




sausage test for cohesive soil


Two undisturbed cylindrical samples (sausages) of the same diameter and length (volume) are carved from the soil. One sample is then wetted and kneaded to form a cylinder of the original diameter. A decrease in length as compared to the original, undisturbed cylinder will confirm a collapsible grain structure. 

Collapse is probable when the natural void ratio, e_i is higher than a critical void ratio, e_c that depends on void ratios e_L and e_P at liquid limit and plastic limits respectively. The following formula should be used to estimate the critical void ratio. 

e_c=0.85e_L+015e_P (6.3.1)


Collapsible soils (with a degree of saturation, S_r  0.6) should satisfy the following condition:

(e_L-e_i)/(1+e_i )≤0.10 (6.3.2)



Consolidation test to identify Collapsible Soil (clay or sand)


A consolidation test is to be performed on an undisturbed specimen at natural moisture content and to record the thickness, “H” on consolidation under a pressure “p” equal to overburden pressure plus the external pressure likely to be exerted on the soil. The specimen is then submerged under the same pressure and the final thickness H’ recorded. Relative subsidence, I_subs is found as:

I_subs =(H-H^')/H (6.3.3)

Soils having Isubs  0.02 are considered to be collapsible. 



Collapsible soil (sand)

Collapse is probable when the natural void ratio, e_i is higher than a critical void ratio

Critical void ratio can be determined by direct shear test



Dispersive Soil

dispersive soil can lead to catastrophic failures of earth embankment dams as well as severe distress of road embankments.

Clay soil which easily disperse in water

It is one of the major causes of soil erosion in Bangladesh

pinhole test

The pinhole test was developed to directly measure dispersive potential of compacted fine grained soils in which water is made to flow through a small hole in a soil specimen, where water flow through the pinhole simulates water flow through a crack or other concentrated leakage channel in the impervious core of a dam or other structure. The test is run under 50, 180, 380 and 1020 mm heads and the soil is classified as follows in Table 6.3.4.



Exchangeable Sodium Percentage (ESP)

Another method of identification is to first determine the pH of a 1:2.5 soil/water suspension. If the pH is above 7.8, the soil may contain enough sodium to disperse the mass. Then determine: (i) total excahangable bases, that is, K^+, Ca^(2+), Mg^(2+)and Na+ (milliequivalent per 100g of air dried soil) and (ii) cation exchange capacity (CEC) of soil (milliequivalent per 100g of air dried soil). The Exchangeable Sodium Percentage ESP is calculated from the relation:

ESP=N_a/CEC×100(%) (6.3.4)

EM_g P is given by: 

EM_g P=Mg/CEC×100(%) (6.3.5)

If ESP > 8% and ESP + EM_g P > 15, dispersion will take place.

ESP =7 to 10 are moderately dispersive

ESP > 15 have serious piping potential



Table 6.3.4: Classification of Dispersive Soil on the Basis of Pinhole Test (Sherard et. al. 1976)

Test Observation------------------------------------------------------Type of Soil---------------Class of Soil

Fails rapidly under 50 mm head------------------------------Dispersive soils---------------D1 and D2

Erode slowly under 50 mm or 180 mm head-------------Intermediate soils-----------ND4 and ND3

No colloidal erosion under 380 mm or 1020 mm head----Non-dispersive soils-----ND2 and ND1



Double Hydrometer Test

The test evaluates the dispersibility of a soil by measuring the natural tendency of the clay fraction to go into suspension in water. The procedure involves the determination of the percentage of particles in the soil that are finer than 0.005 mm using the standard hydrometer test.



Soft Inorganic Soil

Low shear strength, 

High compressibility and 

Severe time related settlement problems *****

Soft clays have undrained shear strengths between about 10kPa and 40kPa,


N-value---------------Consistency--------------Undrained Shear Strength (kN/m2)

Below 2----------------Very soft---------------------Less than 20

2 – 4------------------------Soft-------------------------20 - 40

However, SPT is not a good testing method for soft soil.

FVT and CPT are most suited for soft soil characterization




MATERIALS


Concrete :

All concrete materials and steel reinforcement used in foundations shall conform to the requirements specified in this chapter. For different types of foundation the recommended concrete properties are given in this table: 



Steel :

All steel reinforcement and steel materials used in foundations shall conform to the requirements specified in Chapter. Corrosion is the main dangerous thing for steel. For the purpose of calculations, a maximum corrosion rate of 0.015 mm per side per year may be used. In recent-fill soils or industrial waste soils, where corrosion rates may be higher, following protection systems should be considered.



Timber: 


Timber may be used only for foundation of temporary structure but not for concrete concrete structure.

Where timber is exposed to soil or used as load bearing pile above ground water level, it shall be treated in accordance with BDS 819:1975 (Code of practice for preservation of timber).



TYPES OF FOUNDATION

Shallow Foundations

Footing

Raft/Mat

Deep Foundations

Driven Piles

Bored Piles/Cast-in-Situ Piles

Drilled Pier/Drilled Shafts

Caisson/Well










DIVISION B

DESIGN OF FOUNDATIONS 



ULTIMATE, SAFE AND ALLOWABLE BEARING CAPACITY OF THE SOIL


Ultimate bearing capacity : Ultimate bearing capacity is the maximum pressure that a foundation soil can withstand without undergoing shear failure. Again the Ultimate Bearing Capacity of the soil is the max load which can be applied. It is also defined as the ultimate pressure per unit area of the foundation that can be supported by the soil in excess of the pressure caused by the surrounding soil at the foundation level.

Safe bearing capacity : Safe bearing capacity is the safe extra load the foundation soil is subjected to in addition to initial overburden pressure. Again Safe bearing capacity is the maximum Pressure that a soil bears without shear failure.

Allowable bearing pressure : Allowable bearing pressure is the maximum pressure where the foundation soil is subjected to considering both shear failure and settlement. Again Allowable bearing pressure is the net load intensity at which no failure occurs.



3.8.2 DIMENSION OF FOOTING


Dimension of Footings :

Footings shall generally be proportioned from the allowable bearing pressure and stress limitations imposed by limiting settlement. 

The angle of spread of the load from the wall base to outer edge of the ground bearing shall not exceed the following:



3.8.2 Depth of footing


Dimension of Footings :

A footing shall be placed to depth so that: 

Adequate bearing capacity is achieved,

In case of clayey soil , shrinkage and swelling due to seasonal weather change is not significant, 

It is below possible excavation close by, and

It is at least 500 mm below natural 

ground level unless rock or other 

weather resistant material is at the surface.

So, Df > 1.5 m



3.8.3 Thickness of Footing


The minimum thickness for different types of footing for light structures, shall be as follows :

Type of Footing Minimum Thickness Remark

Masonry 250 mm; twice the maximum projection from the face of the wall Greater of the two values shall be selected

Plain concrete 200 mm, or twice the maximum offset in a stepped footing

Reinforced concrete (depth above bottom reinforcement) 150 mm 300 mm Resting on soil Resting on pile



3.8.6 Minimum Depth of Foundation


The minimum depth of foundation shall be :

For permanent structures 1.5 m for exterior footing in cohesive soils and 2 m in cohesionless soils. 

For temporary structures the minimum depth of exterior footing shall be 400mm. 



3.8.7 Scour


Footings supported on soil shall be embedded sufficiently below the maximum computed scour depth or protected with a scour countermeasure



3.8.8 Mass Movement of Ground in Unstable Areas


In certain areas mass movement of ground may occur from causes independent of the loads applied to the foundation. These include 

Mining subsidence

Landslides on unstable slopes

Creep on clay slopes.




3.8.9 Foundation Excavation

Foundation excavation below ground water table shall be made such that the hydraulic gradient at the bottom of the excavation is not increased to a magnitude that would case the foundation soils to loosen due to upward flow of water. 

Footing excavations shall be made such that hydraulic gradients and material removal do not adversely affect adjacent structures

Seepage forces and gradients may be evaluated by standard flow net procedures. 

Dewatering or cutoff methods to control seepage shall be used when necessary. 



3.8.10 Design Considerations for Raft foundation


For raft supports structure consisting of several parts with varying loads and height, it is advisable to provide separate joints between these parts. ***** 

Joints shall also be provided wherever there is a change in the direction of the raft.

The minimum depth of foundation shall generally be not less than 1.5 m in cohesive soil and 2 m in cohesionless soils.



Geotechnical Design of Shallow Foundation


3.9.1 General : 

The location of the resultant pressure due to seismic and dynamic loads on the base of the footings should be maintained preferably within B/6 of the centre of the footing.



Design Load : 

Shallow foundation design considering bearing capacity due to shear strength shall consider the most unfavourable effect of the following combinations of loading:

D + L

D + L + E or W

0.9 D + Buoyancy Pressure


Shallow foundation design considering settlement shall consider the most unfavourable effect of the following combinations of loading:


SAND


D + L

D + L + E or W

 

CLAY


D + 0.5 L 



Bearing Capacity of Shallow Foundations :


Established bearing capacity equations shall be used for calculating bearing capacity. A factor of safety of between 2.0 to 3.0 shall be adopted to obtain allowable bearing pressure when dead load and normal live load is used.

Allowable increase of bearing pressure due to wind and earthquake forces : The allowable bearing pressure of the soil determined in accordance with this Section may be increased by 33 percent when lateral forces due to wind or earthquake act simultaneously with gravity loads. 

Presumptive bearing capacity for preliminary design : For lightly loaded and small sized structures and for preliminary design of any structure, the presumptive bearing values (allowable) as given in next slide may be assumed for uniform soil in the absence of test results.

Soil Type Soil Description Safe Bearing Capacity, kPa

1 Soft Rock or Shale 440


2 Gravel, sandy gravel, silty sandy gravel; very dense and offer high resistance to penetration during excavation (soil shall include the groups GW, GP, GM, GC)

400**

3 Sand (other than fine sand), gravelly sand, silty sand; dry (soil shall include the groups SW, SP, SM, SC) 200**

4 Fine sand; loose & dry (soil shall include the groups SW, SP) 100**


5 Silt, clayey silt, clayey sand; dry lumps which can be easily crushed by finger (soil shall include the groups ML,, SC, & MH)

150

6 Clay, sandy clay; can be indented with strong thumb pressure (soil shall include the groups CL, & CH) 150

7 Soft clay; can be indented with modest thumb pressure (soil shall include the groups CL, & CH) 100

8 Very soft clay; can be penetrated several centimeters with thumb pressure (soil shall include the groups CL & CH) 50

9 Organic clay & Peat (soil shall include the groups OH, OL, Pt) To be determined after investigation.

10 Fills To be determined after investigation.

Two stories or less (Occupancy category A, B, C and D)


** 50% of these values shall be used where water table is above the base, or below it within a

   distance equal to the least dimension of foundation


Settlement of Shallow Foundation : Foundations can settle in various ways and each affects the performance of the structure


a) Total settlement : Total settlement (ձ) is the absolute vertical movement of the foundation from its as-constructed position to its loaded position.


Secondary consolidation is due to particle reorientation, creep, and decomposition of organic materials.

Secondary compression is always time-dependent and can be significant in highly plastic clays, organic soils, and sanitary landfills, but it is negligible in sands and gravels.



Differential settlement :

Differential settlement is the difference in total settlement between two foundations or two points in the same foundation. This kind of settlement can occur due to the following circumstances: 


Non-uniformity in subsoil.*****

Non-uniform pressure distribution.*****

Ground water condition during and after construction.

Loading influence of adjacent structures.

Uneven expansion and contraction due to moisture migration, uneven drying, wetting or softening.


Notes: The values given in the Table may be taken only as a guide and the permissible total settlement, differential settlement and tilt (angular distortion) in each case should be decided as per requirements of the designer. 

L denotes the length of deflected part of wall/ raft or centre to centre distance between columns. 

H denotes the height of wall from foundation footing. 

* For intermediate ratios of L/H, the values can be interpolated



3.9.5 Liquefaction Potential


Soil liquefaction is a phenomenon in which a saturated soil deposit loses most, if not all, of its strength and stiffness due to the generation of excess pore water pressure during earthquake-induced ground shaking.


Sandy and silty soils tend to liquefy; clay soils do not undergo liquefaction except the sensitive clays.

Resistance to liquefaction of sandy soil depends on fines content. Higher the fines content lower is the liquefaction potential. ???

As a rule of thumb, any soil that has a SPT value higher than 30 will not liquefy. ???



Raft foundation reactions : 

For determining the distribution of contact pressure below a raft both analytical and numerical methods require values of the modulus of subgrade reaction (k) of the soil. 

       

          k=0.65×((E_s B^4)/EI)^(1⁄12) E_s/((1-μ^2 ) ) 1/B         

Where,  

E_s = Modulus of elasticity of soil 

EI = Flexural rigidity of foundation

B = Width of foundation

μ = Poisson’s ratio of soil



Raft foundation reactions :

 For use in preliminary design, indicative values of the modulus of subgrade reaction (k) for cohesionless soils are given below :

Stiff

Very Stiff

Hard

The values apply to a square plate 300 mm x 300 mm. The above values are based on the assumption that the average loading intensity does not exceed half the ultimate bearing capacity



Critical section for moment :

 External moment on any section of a footing shall be determined by passing a vertical plane through the footing and computing the moment of the forces acting over the entire area of the footing on one side of that vertical plane.


Reinforcement in band width/Total reinforcement in short direction=2/((β+1) 




DIVISION C

ADDITIONAL CONSIDERATIONS IN PLANNING, DESIGN AND CONSTRUCTION OF BUILDING FOUNDATIONS 



3.12 EXCAVATION:


Excavation for building foundation or for other purpose shall be done in a safe manner so that no danger to life and property prevails at any stage of the work or after completion. 

Permanent excavations shall have retaining walls of sufficient strength made of steel, masonry, or reinforced concrete to retain the embankment, together with any surcharge load. 

Excavations for any purpose shall not extend within 300 mm under any footing or foundation, unless such footing or foundation is properly underpinned or protected against settlement, beforehand. 

The design and construction of deep excavation work more than 6 m depth or excavation in soft soil or erratic soil must be checked by a competent Geotechnical Engineer.


Excavations for any purpose shall not extend within 300 mm under any footing or foundation, unless such footing or foundation is properly underpinned or protected against settlement, beforehand



Notice to Adjoining Property :

Prior to any excavation close to an adjoining building in another property, a written notice shall be given to the owner of the adjoining property at least 10 days ahead of the date of excavation. 

The protective measures shall incorporate the following: 


Where the level of the foundations of the adjoining structure is at or above the level of the bottom of the proposed excavation, the vertical load of the adjoining structure shall be supported by proper foundations, underpinning, or other equivalent means. 

Where the level of the foundations of the adjoining structure is below the level of the bottom of the proposed excavation, provision shall be made to support any increased vertical or lateral load on the existing adjoining structure caused by the new construction.



Excavation Work :


a) Method of Protection :

Shoring, Bracing and Sheeting 


With the exception of rock cuts, the sides of all excavations, including related or resulting embankments, 1.5 m or greater in depth or height measured from the level of the adjacent ground surface to the deepest point of excavation, shall be protected and maintained by shoring, bracing and sheeting, sheet piling, or other retaining structures. Alternatively, excavated slopes may be inclined not steeper than 1:1, or stepped so that the average slope is not steeper than forty five degrees with no step more than 1.5 m high, provided such slope does not endanger any structure, including subsurface structures. All sides or slopes of excavations or embankments shall be inspected after rainstorms, or any other hazard increasing event, and safe conditions shall be restored. Sheet piling and bracing needed in trench excavations shall have adequate strength to resist possible forces resulting from earth or surcharge pressure. Design of Protection system shall be checked by a qualified Geotechnical Engineer. 



Guard Rail :


A guard rail or a solid enclosure at least 1 m high shall be provided along the open sides of excavations, except that such guard rail or solid enclosure may be omitted from a side or sides when access to the adjoining area is precluded, or where side slopes are one vertical to three horizontal or flatter


Every excavation shall be provided with safe means of entry and exit kept available at all times.

When an excavation has been completed, or partly completed and discontinued, abandoned or interrupted, or the required permits have expired, the lot shall be filled and graded to eliminate all steep slopes, holes, obstructions or similar sources of hazard. 

Fill material shall consist of clean, noncombustible substances. 

The final surface shall be graded in such a manner as to drain the lot, eliminate pockets, prevent accumulation of water, and preclude any threat of damage to the foundations on the premises or on the adjoining property.


So that workers may escape if they see any danger or guess any failure of protection



Placing of construction material : 

Excavated materials and superimposed loads such as equipment, trucks, etc. shall not be placed closer to the edge of the excavation than a distance equal to one and one-half times the depth of such excavation, unless the excavation is in rock or the sides have been sloped or sheet piled (or sheeted) and shored to withstand the lateral force imposed by such superimposed load. When sheet piling is used, it shall extend at least 150 mm above the natural level of the ground. In the case of open excavations with side slopes, the edge of excavation shall be taken as the toe of the slope.



Safety regulations :

Whenever subsurface operations are conducted that may impose loads or movement on adjoining property, such as driving of piles, compaction of soils, or soil densification, the effects of such operations on adjoining property and structures shall be considered. The owner of the property that may be affected shall be given 48 hours written notice of the intention to perform such operations. Where construction operations will cause changes in the ground water level under adjacent buildings, the effects of such changes on the stability and settlement of the adjacent foundation shall be investigated and provision made to prevent damage to such buildings. When a potential hazard exists, elevations of the adjacent buildings shall be recorded at intervals of twenty four hours or less to ascertain if movement has occurred. If so, necessary remedial action shall be undertaken immediately. 

On excavation, the soil material directly underlying footings, piers, and walls shall be inspected by an engineer/architect prior to construction of the footing.


Except in cases where a proposed excavation will extend less than 1.5 m below grade, all underpinning operations and the construction and excavation of temporary or permanent cofferdams, caissons, braced excavation surfaces, or other constructions or excavations required for or affecting the support of adjacent properties or buildings shall be subject to controlled inspection. The details of underpinning, and construction of cofferdams, caissons, bracing or other constructions required for the support of adjacent properties or buildings shall be shown on the plans or prepared in the form of shop or detail drawings and shall be approved by the engineer who prepared the plans.



3.13 DEWATERING


All excavations shall be drained and the drainage maintained as long as the excavation continues or remains. Where necessary, pumping shall be used. No condition shall be created as a result of construction operations that will interfere with natural surface drainage. Water courses, drainage ditches, etc. shall not be obstructed by refuse, waste building materials, earth, stones, tree stumps, branches, or other debris that may interfere with surface drainage or cause the impoundment of surface water.



3.14 SLOPE STABILITY OF ADJOINING BUILDINGS


Overturning :

The possibility of overturning and sliding of the building shall be considered. 

The minimum factor of safety against overturning of the structure as a whole shall be 1.5. 

Stability against overturning shall be provided by the dead load of the building, the allowable uplift capacity of piling, anchors, weight of the soil directly overlying footings provided that such soil cannot be excavated without recourse to major modification of the building, or by any combination of these factors. 



Sliding : 

The minimum factor of safety against sliding of the structure under lateral load shall be 1.5. 



SLOPE STABILITY OF ADJOINING BUILDINGS Continue


Sliding : Resistance to lateral loads shall be provided by 

friction between the foundation and the underlying soil, 

passive earth pressure, 

batter piles or by plumb piles, 


But note that: 

The resistance to lateral loads due to passive earth pressure shall not be taken into consideration where the abutting soil could be removed inadvertently by excavation.

In case of pile supported structures, frictional resistance between the foundation and the underlying soil shall be discounted. 

The available resistance to friction between the foundation and the underlying soil shall be predicted on an assumed friction factor of 0.5. A greater value of the coefficient of friction may be used subject to verification by analysis and test.



Missing information for excavation in BNBC 2017


Allowable limits of lateral movement shore pile

Allowable limit of vertical subsidence of retained soil near excavation

Instrumentation options for deep excavation



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Soils and foundations, Part-2
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Soils and foundations, Part-3
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Soils and foundations, Part-4
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Soils and foundations, Part-5
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Soils and foundations, Part-6
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Soils and foundations, Part-7
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Soils and foundations, Part-8
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Soils and foundations, Part-9
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Soils and foundations, Part-10
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Soils and foundations, Part-11
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Soils and foundations, Part-12
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Soils and foundations, Part-13
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Soils and foundations, Part-14
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BNBC Quiz 7 : Soils and foundations
35 questions

Miscellenious

1
BNBC 2020 Provision of Pile Foundation, Part-1

SOILS AND FOUNDATIONS

BNBC 2017, PART 6, CHAPTER 3




Foundation Engineer vs. Geotechnical Engineer

FOUNDATION ENGINEER

A graduate Engineer with at least 5 (five) years of experience in civil engineering particularly in foundation design or construction.


GEOTECHNICAL ENGINEER

Engineer with Master’s degree in geotechnical engineering having at least 2 (two) years of experience in geotechnical design/construction or graduate in civil engineering/engineering geology having 10 (ten) years of experience in geotechnical design/construction.


Geotechnical Engineer > Foundation Engineer



Working Stress Load combinations

·        D + F  (F = fluid load)

·        D + H + F + L + T (H = earth pressure load, T = load due to temperature change)

·        D + H + F + (Lr or R)

·        D + H + F + 0.75(L + T ) + 0.75(Lr or R)

·        D + H + F + (W or 0.7E)

·        D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or R)

·        0.6D + W + H (needed to check uplifting)***

·        0.6D + 0.7E + H (needed to check uplifting)***



Load combinations for foundation design

Design Load :

a)      Shallow foundation design considering bearing capacity due to shear strength shall consider the most unfavorable effect of the following combinations of loading:

1.      D + L

2.      0.75×[D + L + (W or E)]   {0.75 = 1/1.33}

3.      0.9×D + Buoyancy Pressure

4.      0.6xD + W

b)     Shallow foundation design considering settlement shall consider the most unfavorable effect of the following combinations of loading:

1.      Sand: D + L

2.      CLAY: D + 0.5 L



Critical Depth for pile in sand

Dc=10D, for loose sand

Dc=15D, for medium dense sand

Dc=20D, for dense sand***

Applicable only in the method where no limiting value is mentioned



Important notes

Driven Precast Piles :

·        The minimum center-to-center pile spacing of 2.5B is recommended. The nominal dimensions and length of all the piles in a group should be similar. *****

·        All piles shall be braced to provide lateral stability in all directions.

·        Three or more piles connected by a rigid cap shall be considered as being braced (stable),

·        A two pile group in a rigid cap shall be considered to be braced along the axis connecting the two piles.

·        Piles supporting walls shall be driven alternately in lines at least 300 mm apart and located symmetrically under the centre of gravity of the wall load, unless effective measures are taken to cater for eccentricity and lateral forces, or the wall piles are adequately braced to provide lateral stability.

·        Individual piles are considered stable if the pile tops are laterally braced in two directions by construction, such as a structural floor slab, grade beams, struts, or walls. *****



Pile cap

1.      Pile caps shall be made of reinforced concrete.

2.      The soil immediately below the pile cap shall not be considered as carrying any vertical load.

3.      The tops of all piles shall be embedded not less than 75 mm into pile caps and the cap shall extend at least 100 mm beyond the edge of all piles.

4.      The tops of all piles shall be cut back to sound material before capping.

5.      The pile cap shall be rigid enough, so that the imposed load can be distributed on the piles in a group equitably.

6.      The cap shall generally be cast over a 75 mm thick levelling course of concrete.

7.      The clear cover for the main reinforcement in the cap slab under such condition shall not be less than 50 mm.



Structural capacity of driven precast pile section

Pile diameter/cross-section of a pile shaft at any level shall not be less than the designated nominal diameter/cross-section. The structural design of piles must consider each of the following loading conditions.

1.      Handling loads are those imposed on the pile between the time it is fabricated and the time it is in the pile driver leads and ready to be driven. They are generated by cranes, fork lifts, and other construction equipment.

2.      Driving loads are produced by the pile hammer during driving.

3.      Service loads are the design loads from the completed structures.



Maximum Allowable Stress on Driven Pile

RCC pile =0.33fc

Steel pile =0.25fy

Prestressed RCC pile =0.33fc-fpc

If Su < 10 kN/m2 Pile shall be designed As a long column



Minimum rebar in driven pile

Minimum Reinforcement in Driven Concrete Pile: The maximum bending stress is produced while handling if the pile is pitched at the head. To prevent whipping during handling, length/diameter ratio of the pile should never exceed 50. The following reinforcement provisions may not be valid for laterally loaded piles or piles for uplift resistance.

1.      Pile length < 30 times the least width : 1.00%

2.      Pile length 30 to 40 times the least width : 1.5%

3.      Pile length > 40 times the least width : 2%



Hoops in driven pile

The lateral reinforcement resists the driving stresses induced in the piles and should be in the form hoops or links of diameter not less than 6 mm. The volume of lateral reinforcement shall not be less than the following:

·        At each end of the pile for a distance of about three times the least width/diameter – not less than 0.6% of the gross volume of the pile.

·        In the body of the pile – not less than 0.2% of the gross volume of the pile.

·        The transition between closer spacing and the maximum should be gradual over a length of 3 times the least width/diameter.



Concrete of driven pile

Minimum Grades of Concrete: The minimum 28 days cylinder strength of concrete for driven piles is 21 MPa. Depending on driving stresses, the following grades of concrete should be used.

·        For hard driving – 28 MPa

·        For easy driving – 21 MPa

**** Durability must be considered to select concrete mix ratio



Bored Pile vs. Drilled Shaft?

·        Same thing

·        Sometimes large diameter piles called shaft or pier



Bored pile rebar

Minimum Reinforcement in Bored Concrete Pile

The longitudinal reinforcement shall be of high yield steel bars (min f_y = 420 Mpa) and shall not be less than:

               0.5% of A_c , for A_c ≤ 0.5 m2;

               2500 mm2 , for 0.5 m2 < A_c ≤ 1 m2;

               0.25% of A_c, for A_c > 1.0 m2;

Where, A_c is the gross cross-sectional area of the pile.

The minimum diameter for the longitudinal bars is 16 mm



Bored pile rebar detailing

1.      The assembled reinforcement cage should be sufficiently strong to sustain lifting and lowering into the pile bore without permanent distortion or displacement of bars

2.      bars should not be so densely packed that concrete aggregate cannot pass freely between them.

3.      Hoop reinforcement (for shear) is not recommended closer than 100 mm centres

4.      Minimum concrete cover to the reinforcement periphery shall be 75 mm.

5.      This guidance is only applicable for piles with vertical load.



Group capacity of bored pile or drilled shaft in cohesive soil

CAP IN FIRM CONTACT WITH GROUND

·        Reduction factor, ζ = 1.0

·        The group capacity may then be computed as the lesser of

o  the sum of the individual capacities of each shaft in group, or

o  the capacity of an equivalent pier defined in the perimeter area of the group

·        For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction


CAP IN FIRM CONTACT WITH GROUND OR GROUND IS SOFT

1.      Reduction factor, ζ = 0.67 for a center-to-center (CTC) spacing of 3B (B=pile dia)

2.      Reduction factor, ζ = 1.0 for a CTC spacing of 8B (B=pile dia)

3.      The group capacity may then be computed as the lesser of

               (i) the sum of the individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Group capacity of bored pile or drilled shaft in cohesionless soil

CAP CONTACT WITH GROUND is not considered here

·        Reduction factor, ζ = 0.67 for a center-to-center (CTC) spacing of 3B (B=pile dia)

·        Reduction factor, ζ = 1.0 for a CTC spacing of 8B (B=pile dia)

·        The group capacity may then be computed as the lesser of

               (i) the sum of the modified individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Group capacity of driven pile

·        Reduction factor, ζ = 1.0

·        The group capacity may then be computed as the lesser of

               (i) the sum of the individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Minimum pile spacing for bored pile

Center to Center Spacing:

Conflict with previous

If closer spacing is required, the sequence of construction shall be specified and the interaction effects between adjacent shafts shall be evaluated by the designer.



Longitudinal Bar Spacing:

·        The minimum clear distance between longitudinal reinforcement shall not be less than 3 times the bar diameter nor 3 times the maximum aggregate size.

·        If bars are bundled in forming the reinforcing cage, the minimum clear distance between longitudinal reinforcement shall not be less than 3 times the diameter of the bundled bars.

·        Where heavy reinforcement is required, consideration may be given to an inner and outer reinforcing cage.



Bentonite slurry specification

In drilling of holes for all piles, bentonite and any other material shall be mixed thoroughly with clean water.

Bentonite slurry shall meet the Specifications as shown below


Density during drilling to support excavation, greater than 1.05 g/ml, Mud density Balance (ASTM D4380)


·        Density prior to concreting, less than 1.25 g/ml, Mud density Balance (ASTM D4380)

·        Viscosity, 30 – 90 Seconds, Marsh Cone Method (ASTM D6910)

·        pH, 9.5 to 12, pH indicator paper strips or electrical pH meter (ASTM D4972)

·        Liquid limit, > 450%, Casagrande apparatus (ASTM D4318)



Drilling fluid level

Where a borehole is formed using drilling fluid for maintaining the stability of a boring, the level of the water shall maintain at a level not less than 2 m above the level of ground water.



Bored pile construction sequence

·        Augured cast-in-situ pile shall not be installed within 6 pile diameters centre to centre of a pile filled with concrete less than 24 hours old.

·        Bored cast-in-situ concrete piles shall not be drilled/bored within a clear distance of 3 m from an adjacent pile with concrete less than 48 hours old.

·        For concreting under water, the concrete shall contain at least 10 percent more cement than that required for the same mix placed in the dry.



“Rabbit” for bored pile

Successful placement of concrete under water requires preventing flow of water across or through the placement site. Once flow is controlled, the tremie placement consists of the following three basic steps:

·        The first concrete placed is physically separated from the water by using a “rabbit” or go-devil in the pipe, or by having the pipe mouth capped or sealed and the pipe dewatered.

·        Once filled with concrete, the pipe is raised slightly to allow the “rabbit” to escape or to break the end seal. Concrete will then flow out and develop a mound around the mouth of the pipe. This is termed as “establishing a seal”.

·        Once the seal is established, fresh concrete is injected into the mass of existing concrete.



Methods of tremie concreting

Tremie concreting method

·        The capped tremie pipe approach

·        The “rabbit” plug approach



Important notes

1.      Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3 to 5 ft) and maintained at that depth throughout concreting to prevent entry of water into the pipe.

2.      Rapid raising or lowering of the tremie pipe should not be allowed.

3.      All vertical movements of the tremie pipe must be done slowly and carefully to prevent “loss of seal”.



Number of tests?

1.      Integrity Test: In order to check the structural integrity of the piles Integrity tests shall be performed on the piles in accordance with the procedure outlined in ASTM D5882. For any project where pile has been installed, integrity tests shall be performed on 100% of the piles.

2.      Axial Load Tests for Compression : 

        I.           For a major project, at least 2% of pile's axial load test for compression shall be tested in each area of uniform subsoil conditions.

      II.           The load test on a pile shall not be carried out earlier than 4 weeks from the date of casting the pile. 

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BNBC 2020 Provision of Pile Foundation, Part-2
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BNBC 2020 Provision of Pile Foundation, Part-3
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BNBC 2020 Provision of Pile Foundation, Part-4
3.7
3.7 out of 5
3 Ratings

Detailed Rating

Stars 5
2
Stars 4
0
Stars 3
0
Stars 2
0
Stars 1
1

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