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
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
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
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.
- identification, location, alignment and depth of various utilities
- the topography of existing ground
3.4.2 Sub-Soil Investigations
- describing the character, nature, load bearing capacity and settlement capacity of the soil
- establish the soil, rock and groundwater conditions,
- determine the properties of the soil and rock
- gather additional relevant knowledge about the site.
- Careful collection, recording and interpretation of geotechnical information shall be made including ground conditions, geology, geomorphology, seismicity and hydrology, as relevant.
- Indications of the variability of the ground shall be taken into account.
- 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.
- 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
a) Geophysical measurement
b) Sounding or probing (eg. DCP)
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
- Low bearing capacity
- High consolidation and creep settlement
- Swelling and shrinkage potential
- Vertical and lateral capacity of pile is low
- 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?
- Loss on ignition
- Mass loss on treatment with hydrogen peroxide (H2O2)
3.5.5 Expansive Soils
- Expansive soils are those which swell considerably on absorption of water and shrink on the removal of water.
- 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.
- 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.
- For identification and classification of expansive soils parameters like
- 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
- 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
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
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.
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.
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 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
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:
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
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:
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 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
Bored Piles/Cast-in-Situ Piles
Drilled Pier/Drilled Shafts
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.
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
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:
D + L
D + L + E or W
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)
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)
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
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 :
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)
ADDITIONAL CONSIDERATIONS IN PLANNING, DESIGN AND CONSTRUCTION OF BUILDING FOUNDATIONS
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.
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
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.
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