Foundations

Soil exploration (soil test) is necessary for foundation design because:

1. It finds the type, thickness and sequence of soil layers and the depth of rock.
2. It gives the bearing capacity of soil, which decides the size and type of foundation.
3. It indicates probable settlement (total and differential) and so the safe design.
4. It locates the water table, which affects excavation, bearing capacity and dewatering.
5. It finds problem soils such as black cotton, peat, fill or liquefiable sand.
6. It helps to choose between shallow and deep foundations, or ground improvement.
7. It gives information for the economy and safety of the building, preventing failure and damage, and avoiding over-design.

Pile foundations are used when shallow foundations are not suitable or economical. The situations are:

1. Weak surface soil: A thick layer of soft clay, loose sand, peat or fill above, with hard stratum or rock at a lower depth; piles carry load to the firm layer (end-bearing piles).
2. Heavy loads: Tall buildings, bridges, towers and industrial structures whose loads shallow footings cannot carry.
3. Large settlement expected: Compressible soils, where settlement of shallow footings is excessive.
4. High water table or submerged site: Excavation of shallow foundation is hard (river, marine, bridge piers).
5. Uplift or overturning loads: Transmission towers, chimneys, basement under water pressure; piles resist tension.
6. Lateral loads: Wind, earthquake, or water pressure on retaining walls, jetties, and docks (raked piles).
7. Scour risk: River bed erosion around foundations of bridges.
8. Expansive soils (black cotton): Piles go below the zone of seasonal change.
9. Liquefaction-prone soil in earthquake areas.
10. Adjacent structures or very deep excavations, where shallow footings would disturb nearby buildings.
11. Compaction of loose sand (compaction piles).

Bearing capacity of soil is the ability of soil to carry load from the foundation without shear failure or excessive settlement. Ultimate bearing capacity ($q_u$) is the pressure at which the soil fails in shear; safe bearing capacity is $q_u$ divided by factor of safety (usually 2.5 to 3), and also limited by permissible settlement.

Methods of improving bearing capacity

1. Compaction: Rollers, rammers or vibrators raise the density of loose, granular soil in layers.
2. Drainage: Lowering the water table by sumps, well points or sub-soil drains increases effective stress and strength.
3. Replacement: Poor soil is excavated and replaced with compacted sand, gravel or murrum (sand cushion).
4. Increasing depth and width of the foundation: A deeper and wider footing gives better capacity and reduced pressure.
5. Confining the soil: Sheet piles or a ring wall keep loose soil in place.
6. Sand piles and stone columns: Holes bored and filled with sand or crushed stone, compacted; raise the density of soft soil.
7. Chemical treatment / grouting: Cement, lime or chemical grout injected into soil voids.
8. Stabilisation: Mixing cement, lime, bitumen or fly ash with soil.
9. Pre-loading and geosynthetics: Surcharge to pre-consolidate soft clay; geotextile and geogrids reinforce.
10. Vibroflotation for deep compaction of sand.
11. Pile foundations for transfer to a deeper stratum, if the above are not sufficient.

Bearing capacity of soil for design is determined by the following methods.

1. Presumptive values from codes

Safe bearing capacity (SBC) is read from tables in IS 1904 or the Nepal National Building Code (NBC 105 / 205): for example, hard rock 3300 kN/m² and above, soft rock 440, gravel 440, dense sand 245, medium clay 150, soft clay 100, as per codes. Used for small, light buildings.

2. Analytical (theoretical) methods

Using soil properties $c$, $\phi$ and $\gamma$ from lab tests. Terzaghi's equation for a strip footing:

and $q_{safe} = q_u / FS$. Other methods are Meyerhof, Hansen, and Skempton for clay.

3. Plate load test

A steel plate (300 to 750 mm square) is loaded in a pit at the foundation level in stages, with settlement measured by dial gauges, until failure or settlement of 25 mm; a load-settlement curve is plotted. Used for direct field estimate. It tests only a shallow depth (about 2 times the plate width).

   Load frame / kentledge
       |  jack  |
   ====[ plate ]====  ground
    dial gauges

4. Standard Penetration Test (SPT)

A split spoon sampler is driven 450 mm by a 63.5 kg hammer falling 750 mm; the number of blows for the last 300 mm is N. SBC is correlated with N (Terzaghi and Peck charts, Meyerhof).

5. Static / dynamic cone penetration test (SCPT, DCPT)

A cone is pushed or driven into soil and the resistance gives strength; good for continuous profile.

6. Other

Laboratory tests on undisturbed samples (triaxial, direct shear, unconfined compression), pile load test, and geophysical methods (seismic refraction, resistivity).

Foundation failure means the foundation can no longer support the structure safely, because of shear failure of soil or excessive (differential) settlement. Common causes are:

1. Unequal bearing capacity or non-uniform soil under different parts of the foundation, giving differential settlement.
2. Weak or overloaded soil: Bearing capacity less than the pressure applied; shear failure.
3. Settlement due to consolidation of clay, or compressible soil, and loss of soil by erosion.
4. Lateral movement of soil (sliding of sloping ground, squeezing of soft soil out from under the footing).
5. Variation of water table and flooding; withdrawal of ground water.
6. Seasonal swelling and shrinkage of expansive soils such as black cotton soil.
7. Tree roots shrinking soil moisture near the foundation.
8. Poor design and construction: Inadequate depth, size, or concrete quality; construction on filled ground, poor compaction; unequal loading.
9. Adjacent excavations or construction removing lateral support.
10. Chemical attack (sulphates) and corrosion on the foundation.
11. Earthquake and vibrations causing liquefaction or compaction.
12. Frost action in cold areas.
13. Underground voids (mines, caves, leaking pipes).

Existing foundations commonly suffer from the following problems.

1. Settlement: Uniform or differential settlement from consolidation of soil, weak strata, heavy loads or extra floors added; it causes cracks in walls and floors, tilting, and door and window jamming.
2. Cracks in the foundation, plinth and walls from settlement, soil movement or earthquake.
3. Dampness: Capillary rise because DPC has failed or was never provided; water table rise, leaking drains.
4. Deterioration of materials: Sulphate attack on concrete, corrosion of reinforcement, rotting of timber, and weathering of stone and brick.
5. Soil movement: Swelling and shrinking of expansive soil, erosion of soil from the base, scour near rivers and drains.
6. Overloading due to change of use or vertical extension without checking capacity.
7. Effect of nearby work: Excavation, construction of neighbouring building, pile driving, vibration, blasting.
8. Tree roots and water table lowering causing shrinkage settlement.
9. Landslide or slope failure on hill sites.
10. Earthquake damage and liquefaction.

Remedies

Underpinning (pit method, pile method), enlarging the footing, grouting, soil stabilisation, drainage improvement, repair of cracks and DPC, chemical injection, and strengthening by jacketing.

Classification of foundations

Foundation is the lowest part of the structure which transfers the load to the ground. They are classified into two main groups.

1. Shallow foundations (depth $D_f \le$ width $B$, or less than about 3 m)

• Spread (isolated) footing, wall (strip) footing, combined footing, strap (cantilever) footing, raft (mat) foundation, grillage footing, stepped footing.

2. Deep foundations (depth greater than width)

• Pile foundation (bearing, friction, bored, driven), pier foundation, well (caisson) foundation, drilled shaft.

 Shallow: strip   [|____|]     Deep: pile   | | |
 ground ====================     weak soil  | | |
                                hard stratum  ===

Common problems of existing foundations

1. Differential settlement causing cracks and tilting, as the soil is unequal or the load is increased.
2. Dampness due to capillary rise through walls where DPC is poor.
3. Cracks and deterioration of concrete, masonry and steel through sulphate attack, corrosion and weathering.
4. Soil movement: Swelling and shrinking of clay, erosion, scour.
5. Overloading by additional floors or change of use.
6. Effect of adjacent excavation, vibration, and water table change.
7. Earthquake damage and liquefaction.

Remedies: underpinning, grouting, soil stabilisation, drainage, widening of footing, and jacketing.

Excavation in water-logged ground or with a high water table is difficult because the sides collapse, the bottom becomes soft, the soil strength falls, and the work cannot be done in dry condition. Therefore the water must be removed or kept out of the excavation, and the sides supported.

Methods commonly used in Nepal

1. Sumps and ditches (open pumping)

• A trench (drain) around the excavation at its bottom collects seepage and leads it to a sump (a pit deeper than the foundation level).
• Water is pumped out from the sump by centrifugal or submersible pumps, so the excavation stays dry.
• Simple and cheap, used for shallow depth and low inflow. Not suited for fine sand (boiling and piping).

 GL
  \         /
   \_______/   <- foundation level
   ditch  [sump]-- pump

2. Well-point system (dewatering)

• Many small perforated pipes (well points) about 1 to 1.5 m apart are jetted into the ground around the trench and joined to a header pipe and a suction pump.
• The water table is lowered below the excavation level, so the soil is dry and stable.
• Good for sandy and silty soil with deeper trenches. It needs skilled labour and equipment.

3. Other methods (not in common use) Cofferdams with sheet piles, deep wells, and the freezing or chemical method.

In Nepal, the sump and ditch method is most common because it is cheap, and the well-point method is used for large works. Timber or steel shoring is added to support the sides, and the work is done in the dry season where possible.

Pile foundation is chosen when

• Top soil is weak, soft or loose to a large depth, and a hard layer lies deeper (end-bearing piles); or the soil is uniformly soft and friction piles are used.
• Building loads are heavy (high-rise, bridges, towers) and settlement is critical.
• The site has a high water table, is submerged, or near rivers where scour is expected.
• The foundation must resist uplift or lateral load (towers, retaining structures, jetties).
• Expansive, liquefiable or filled ground is present.
• Shallow excavation near existing structures is not possible.

Mat (raft) foundation is chosen when

• Soil has low bearing capacity, so that individual footings would cover more than 50 % of the plan area.
• Loads are heavy and columns are closely spaced, so footings would overlap.
• Differential settlement must be reduced, such as for compressible or non-uniform soils.
• The basement is below the water table, so the raft acts as a waterproof floor and resists uplift.
• Structures like silos, chimneys, tanks, and multi-storey buildings with a basement.
• Earthquake areas, where a rigid single slab gives better behaviour.

Pile gives deep transfer; mat spreads load over a large area at shallow depth, so it is cheaper if the soil is moderately firm at the surface.

Soil exploration

Soil exploration (site investigation) is the process of studying the soil and rock at a site by borings, sampling, field tests and laboratory tests, to find the type and thickness of strata, groundwater level and the engineering properties (bearing capacity, settlement, strength) for safe foundation design. Methods are test pits, auger boring, wash boring, percussion boring, rotary drilling, SPT, cone penetration, plate load test, geophysical methods.

Methods of improving bearing capacity

1. Compaction of loose soil by rolling, ramming and vibration.
2. Drainage of ground water (sumps, well points, sub-soil drains).
3. Replacement of weak soil with sand, gravel or murrum, compacted in layers (sand cushion).
4. Increasing size and depth of foundation.
5. Sand piles, stone columns, to densify soft soils.
6. Grouting and chemical injection (cement, lime, silicate).
7. Soil stabilisation with cement, lime, bitumen or fly ash.
8. Pre-loading and use of geosynthetics.
9. Confining the soil with sheet piles or sheeting.
10. Piles to reach firm strata.

Method commonly used in Nepal

The most common method is replacement with a compacted sand or gravel cushion, along with compaction and increasing the footing size, for ordinary buildings, because it is cheap, uses local materials and manual labour, and needs no special equipment. For larger works in soft soil, sub-soil drainage, stone columns and pile foundations are used. For many Kathmandu valley sites with soft clay and a high water table, these methods plus raft foundations are adopted.

Ultimate bearing capacity

Ultimate bearing capacity ($q_u$) is the maximum pressure per unit area that the soil can bear at the foundation level, at which it fails in shear.

Safe bearing capacity

Safe bearing capacity ($q_s$) is the maximum pressure that soil can carry safely without shear failure and with settlement within the permissible limit.

or simply $q_s = q_u/FS$ with a factor of safety of 2 to 3. The net safe bearing capacity is the net ultimate capacity divided by FS.

Methods to improve bearing capacity

1. Compaction: The soil is compacted in layers using rollers, rammers and vibrators to raise density; good for granular soils.
2. Drainage: Lowering the water table by sumps, well points and sub-soil drains gives higher effective stress.
3. Replacement: Removing weak soil and replacing it by sand, gravel or murrum compacted in layers (sand cushion).
4. Confining soil: Using sheet piles or retaining walls to prevent sideways flow.
5. Increasing depth and width of footing.
6. Sand piles and stone columns.
7. Chemical treatment and grouting: Cement, lime or silicate grout filling pores.
8. Soil stabilisation: Mixing cement, lime, fly ash or bitumen.
9. Preloading with surcharge to consolidate soft clay, and geotextiles for reinforcement.
10. Pile foundation if the above are not enough.

 footing
 [=======]
 |sand cushion|   <- replacement
 weak clay

Methods of soil exploration

1. Test pits (trial pits): Pits dug 1.5 to 3 m deep; soil is inspected and samples taken. Cheap, for shallow depth and small buildings.
2. Auger boring: Hand or power auger drills holes in soft soil up to about 10 m; disturbed samples.
3. Wash boring: Water jet cutting the soil in a casing; used in sand and clay.
4. Percussion boring: Chisel breaks hard soil or rock.
5. Rotary drilling: Core drilling for rock and hard strata.
6. Sampling: Disturbed and undisturbed samples (thin-wall tube, split spoon).
7. Field tests: Standard Penetration Test (SPT), static and dynamic cone penetration tests, vane shear test, plate load test.
8. Geophysical methods: Seismic refraction and electrical resistivity, for large areas and depth profile.
9. Laboratory tests: Grain size, Atterberg limits, shear strength, consolidation.

The number and depth of boreholes depend on the size of the building and the soil variation; depth is at least 1.5 times the width of the foundation (or 1.5 times to the depth of influence).

Safe bearing capacity

Safe bearing capacity is the maximum intensity of load per unit area that the soil can carry safely without shear failure and with permissible settlement:

where $q_u$ is ultimate bearing capacity and $FS$ is the factor of safety (2.5 to 3).

Methods of improving bearing capacity

• Compaction by rolling and ramming.
• Drainage of ground water.
• Replacement by compacted sand or gravel cushion.
• Increasing depth or width of foundation.
• Sand piles and stone columns.
• Grouting and chemical injection.
• Stabilisation with cement or lime.
• Confinement by sheet piles; preloading; geosynthetics; or pile foundation.

Sub-soil exploration

Sub-soil exploration (soil exploration) is the investigation of the ground below a site to find the soil layers, their thickness, strength, water table and bearing capacity, so as to choose and design the foundation. It includes reconnaissance, boring or pits, sampling, in-situ tests (SPT, plate load, cone penetration), and lab tests. The result is a soil report with borehole log, soil profile and recommended SBC.

Method of improving bearing capacity: Sand cushion (replacement)

This is the method commonly used for light buildings.

1. The weak soil under the footing is excavated to a depth of about 1 to 1.5 m (or until firm soil), and over a width larger than the footing (extending beyond it by the thickness of the cushion).
2. Clean, well graded sand, gravel or murrum is laid in layers of 200 to 300 mm.
3. Each layer is sprinkled with water and compacted thoroughly by rammers or rollers up to the required density.
4. The foundation is then constructed on the cushion.

The sand layer spreads the load over a larger area of the weak soil below, reducing pressure, and also drains the water.

        [ footing ]
     /               \
    /  compacted sand  \
   /     cushion        \
   weak soil below

Other methods: compaction, drainage, sand piles, stone columns, grouting, stabilisation, and increasing the footing depth.

Factors affecting the design of foundation

1. Bearing capacity of soil and the type of soil (clay, sand, rock).
2. Loads on the structure: Dead, live, wind, seismic and their eccentricity.
3. Depth of foundation: Depends on bearing strata, frost, shrinkage zone, scour, water table, and depth of nearby foundations. Rankine's formula gives minimum depth $D = \frac{q}{\gamma}\left(\frac{1-\sin\phi}{1+\sin\phi}\right)^2$.
4. Settlement: Total and differential settlement must be within limits.
5. Ground water table: Affects bearing capacity, excavation and uplift.
6. Type of structure: Sensitivity to settlement, e.g. framed or load-bearing.
7. Seismic zone and liquefaction risk.
8. Adjacent structures and underground services.
9. Soil problems: Expansive, collapsible or filled soils; sulphate in soil and ground water.
10. Site topography: Slope, drainage and erosion.
11. Construction methods, materials and economy.
12. Building codes: Nepal National Building Code, IS 1904, IS 6403.

Improving the bearing capacity of soil

1. Compaction by roller, rammer or vibration.
2. Drainage of water by sumps, well points, sub-soil drains.
3. Replacement of weak soil by a sand or gravel cushion.
4. Increasing foundation depth and width.
5. Sand piles or stone columns.
6. Grouting or chemical injection.
7. Stabilisation with cement, lime or fly ash.
8. Preloading with surcharge, and geotextile reinforcement.
9. Confining soil with sheet piles.
10. Using pile foundation.

Soil exploration (sub-soil or site investigation) is the process of investigating the soil and rock below a site, by pits, borings, sampling, in-situ and laboratory tests, to find the soil strata, ground water level and engineering properties such as bearing capacity, so that a safe and economical foundation can be designed.

For a foundation on hard rock, a shallow foundation is recommended: a simple strip or spread footing (or a footing directly on the cleaned, levelled rock), with the rock benched or stepped if the surface is sloping.

Justification

1. High bearing capacity: Sound rock has safe bearing capacity of 3300 kN/m² or more (IS 1904), higher than the building load, so no deep foundation is needed.
2. Negligible settlement: Rock is nearly incompressible, so differential settlement is not a problem.
3. Economy: Deep excavation, piles or rafts are not needed; the cost and time are lower.
4. Excavation: Only the top weathered layer is removed. If the rock is sloping, it is benched into steps, or anchor bars (dowels) are grouted into the rock to avoid sliding.
5. Bonding: Cleaned, roughened rock surface is bonded with a PCC bed or the footing concrete for a firm contact; any cracks or fissures are grouted.

   column
    | 
  [footing] 
 ///rock/// (benched, cleaned)

If the rock is weathered or fractured, the depth of excavation is increased until sound rock is reached, or the cracks are grouted.

Foundation

A foundation is the lowest part of a structure, in direct contact with the ground, which transfers the loads of the structure safely to the soil.

Types

• Shallow foundations: spread (isolated) footing, strip (wall) footing, combined footing, strap footing, raft (mat) foundation, grillage.
• Deep foundations: pile foundation, pier foundation, well (caisson) foundation.

 Shallow: [__|__]      Deep:  |   |   |   piles
 ----------------------     weak soil
                            ===hard layer===

Causes of foundation settlement

1. Consolidation of compressible clay and silt layers under load.
2. Weak, loose or filled soil and poor compaction.
3. Non-uniform soil strata, giving differential settlement.
4. Lowering of ground water table or seasonal shrinkage and swelling of clay.
5. Overloading or unequal loading of foundation.
6. Adjacent excavation and construction, and vibrations.
7. Tree roots drawing moisture.
8. Loss of soil by erosion or leakage, and underground voids.
9. Earthquake causing densification and liquefaction.
10. Faulty design or construction: footing too small or shallow.

Treatment of settlement

1. Underpinning: Pit method, pile method, or by extending the footings deeper to a firm stratum.
2. Enlarging the foundation to spread the load.
3. Soil improvement: Grouting (cement, chemical), compaction grouting, jet grouting.
4. Drainage: Control surface and ground water, repair leaking pipes.
5. Reducing the load, or tying the structure with beams.
6. Pile or micropile supporting under existing footing.
7. Removal of trees, or root barriers.
8. Repair of cracks after settlement is stabilised.

Causes of foundation settlement

1. Consolidation of soft clay or silt (water squeezed out slowly under load).
2. Loose, weak or filled soil and poor compaction.
3. Non-uniform soil layers and unequal loads.
4. Fall in water table, or seasonal shrinkage and swelling.
5. Overloading, extra floors, or poor foundation size.
6. Nearby excavations, vibration and dewatering.
7. Tree roots, leaking pipes, erosion and underground voids.
8. Earthquake densification or liquefaction.

Effects of unequal settlement on the structure

• Cracks in walls, beams, slabs and plaster, usually diagonal and wider at the top or bottom.
• Tilting of the building or of walls and columns.
• Distortion of frames and extra bending moments and shear in members.
• Jamming of doors and windows, and uneven floors.
• Damage to services: broken pipes, drains, and electrical lines.
• Loss of waterproofing, dampness, and leakage.
• In severe cases, partial or complete failure or collapse.

Why black cotton soil is not preferred

Black cotton soil is a clayey soil rich in montmorillonite, found in parts of Nepal's Terai and India.

1. High swelling and shrinkage: It expands in the rainy season and shrinks in the dry season, a movement that can be 10 cm or more.
2. Low bearing capacity (about 50 to 100 kN/m²) and low shear strength when wet.
3. Large cracks (up to 1 m deep) in dry weather and extreme softness when wet.
4. Differential movement under the building leads to cracks and uplift.
5. Poor drainage and high compressibility.
6. Damage to foundations and floors because it exerts uplift pressure.

Hence, the foundation should go below the active zone, or the soil should be replaced or treated (under-reamed piles, sand cushion, raft).

Causes of foundation settlement

1. Consolidation of compressible clay or silt under load.
2. Weak, loose, or filled soil, poorly compacted.
3. Non-uniform soil or unequal loading.
4. Change of water table; shrinkage and swelling of clays.
5. Overloading or addition of floors.
6. Adjacent excavation, vibrations, or construction.
7. Tree roots, erosion, leakage and underground cavities.
8. Earthquake and liquefaction.
9. Faulty design: footing too small or shallow.

Underpinning

Underpinning is the process of strengthening and deepening an existing foundation by extending it down to a stronger stratum, or enlarging its base, so as to support the structure safely. It is used for settlement, to add floors, to lower basements, or when nearby deep excavation is done.

Methods of underpinning

1. Pit (needle) method: The wall is divided into short sections (1 to 1.5 m). Pits are dug under alternate sections to the firm layer, one at a time, and filled with concrete or masonry up to the base of the old footing, packed tight with dry mortar and wedges. Then the adjacent sections are done. This is the simplest, for shallow depths.
2. Pile underpinning: Piles are driven or bored on both sides of the wall (or through it) and a needle beam or cap transfers the load; used when the firm layer is deep.
3. Pretest (Pynford) method: Jacks pre-load the new underpinning before it takes over.
4. Jet grouting / chemical grouting: Grout injected to strengthen the soil.
5. Miniature (micro) piles: Small diameter piles drilled and grouted through the existing footing.
6. Beam and needle method: Needle beams under the wall pass loads to piers on either side.

 wall
 |==== old footing ====|
   ||pit 1||    ||pit 2|| 
   (concrete filled pit, alternate sections)

During underpinning, the structure is shored, and cracks are monitored.

Problems of foundations on black cotton soils

Black cotton soil is clay with montmorillonite, with high plasticity.

1. Swelling in the wet season and shrinkage in the dry season, causing vertical movement that lifts and drops the foundation.
2. Deep shrinkage cracks (to 1 m or more) in summer, letting water in.
3. Differential movement, producing cracks in walls, floors and plinth.
4. Low bearing capacity and softness when wet; poor drainage.
5. Uplift pressure (swelling pressure) on footings, floors and pavements.
6. High compressibility and lateral movement.

Suggested foundation

An under-reamed pile foundation (as per IS 2911 Part 3) is best, with plinth beams (RCC tie beams) connecting the piles at the top.

Justification

1. The piles pass below the active zone of seasonal moisture change (3.5 m or more) and are anchored in the stable soil.
2. The bulbs (under-reams) give additional bearing and a grip to resist uplift from swelling.
3. The plinth beam is kept clear of the ground with a gap (void) or compressible filler, so swelling soil cannot lift it.
4. Isolated settlement is controlled, and it is economical for light buildings.

Alternatives: Raft foundation with a sand cushion (about 0.9 m) under a rigid RCC raft; removal of black cotton soil and replacement with a non-swelling sand/murrum cushion; and lime or cement stabilisation. Shallow strip footings are not recommended.

   plinth beam  ===========  
  (gap)  |      |      |
  under- |      |      |
  ream  (O)    (O)    (O)  <- bulbs in stable soil

A shallow foundation is one whose depth is less than or equal to its width (generally under 3 m), and which transfers the load to the soil near the surface.

Types

1. Wall (strip) footing: Continuous footing under a load-bearing wall. Wider than the wall, with an offset, made of masonry or concrete.
2. Isolated (spread) footing: Square, rectangular or circular footing under a single column; the commonest type for RCC frames. May be flat, stepped, or sloped.
3. Combined footing: One footing carrying two or more columns; used when columns are close, or one is near the property line.
4. Strap (cantilever) footing: Two isolated footings joined by a strap beam; an edge column's eccentric load is balanced.
5. Raft (mat) foundation: A thick slab under the whole building. It is used for soil of low bearing capacity or where isolated footings cover more than about half the area.
6. Grillage foundation: Steel beams in layers (crossing) embedded in concrete, to spread heavy column loads over low-capacity soil, with shallow depth.
7. Stepped foundation on sloping ground for walls.
8. Inverted arch footing for soft soil between piers.

 Wall footing         Isolated footing       Raft
   |wall|               | col |            ___________
  [_____]              [_______]          |___________|

Foundation

A foundation is the lowest part of a structure, usually below ground, which transmits the load of the structure to the soil or rock.

Basic requirements of a foundation

1. Adequate depth, below the zone of frost, shrinkage, scour and the top loose soil.
2. Safe against shear failure: Soil pressure within the safe bearing capacity.
3. Settlement within limits, and uniform (no harmful differential settlement).
4. Safe against sliding, overturning and uplift.
5. Durable: Resistance to chemicals, sulphates and moisture.
6. Resists earthquake and other special loads.
7. Economical and practical to construct.
8. Level and stable base: Bottom level on firm soil, with good drainage.

Types of shallow foundation

1. Strip / wall footing: Continuous below a load-bearing wall; it spreads the load by projecting offsets (masonry, with the width $\approx$ twice wall width).
2. Isolated (spread) footing: Separate pad under each column. Stepped or sloped; of reinforced concrete.
3. Combined footing: One footing for two or more columns, rectangular or trapezoidal, used when columns are near or near the boundary.
4. Strap footing: Two footings joined by a strap beam to take an eccentric column load.
5. Raft (mat) foundation: A large slab covering the whole plan, for weak soil and heavy loads.
6. Grillage footing: Layers of steel beams in concrete, for heavy loads.

  Strip:  |wall|      Isolated: |col|      Raft: ___________
          [____]                [___]            |_________|
  Combined: |c1|   |c2|          Strap: [F1]==beam==[F2]
            [_________]

Types of deep foundation

1. Pile foundation: slender columns driven or bored into the ground; end-bearing, friction, or both; materials: timber, concrete (precast, cast-in-situ), steel.
2. Pier foundation: Large-diameter shaft of masonry or concrete carrying heavy load to firm strata.
3. Well (caisson) foundation: Hollow cylinder or box sunk to the depth, used for bridges.
4. Drilled shaft / caisson and under-reamed piles.

Use of various shallow foundations

 Combined:  [col1]  [col2]       Raft: |c|c|c|
            [___________]              ____________
 Strap:  [F1]=====strap=====[F2]

Shallow foundations are cheap and quick; when soil near the surface is weak or when depth is needed for scour or uplift, deep foundations are chosen.

Foundation is the part of a structure, usually below ground level, which transfers the loads of the superstructure to the soil safely without shear failure or harmful settlement.

Types of foundation

A. Shallow foundations (depth $\le$ width)

1. Strip / wall footing: Continuous strip under walls.
2. Isolated (spread) footing: Under a single column; flat, stepped or sloped.
3. Combined footing: Carries two or more columns.
4. Strap (cantilever) footing: Two footings joined with a strap beam.
5. Raft (mat) foundation: A slab under the whole building for weak soil.
6. Grillage footing: Steel beams in concrete layers.

B. Deep foundations

1. Pile foundation: End-bearing or friction piles of timber, concrete or steel.
2. Pier foundation: Large shaft of masonry or concrete.
3. Well (caisson) foundation: Hollow sunk wells for bridges.

 Strip      Isolated     Raft         Pile
 |wall|      | col |   |c| |c| |c|   | col |
 [____]      [_____]   [__________]  [cap ]
                                      | | |

Choice depends on soil bearing capacity, load, water table, settlement, and cost. Shallow footing is used for firm soil; raft for low capacity; piles for deep firm layers.

Functions of foundation

1. Transfers the building load safely to the soil, within the bearing capacity.
2. Spreads the load over a large area to reduce the pressure.
3. Keeps settlement uniform and within limits.
4. Gives a firm, level base for the structure.
5. Resists lateral loads, sliding and overturning (wind, earthquake, earth pressure).
6. Gives stability against uplift (tall structures) and soil movement, such as shrinkage or swelling.
7. Protects the building from the effects of ground moisture, frost and chemicals.

Types of deep foundation

1. Pile foundation: Long slender members driven or bored into the ground.
   • End-bearing piles transmit load to a hard stratum.
   • Friction piles transfer load by skin friction.
   • By material: timber, RCC (precast or cast-in-situ), steel; by installation: driven or bored.
   • A pile cap joins the pile group under a column.
2. Pier foundation: Large diameter vertical cylinder or column of masonry or concrete, used for heavy loads on firm strata at moderate depth.
3. Well (caisson) foundation: A hollow cylinder or box (brick, concrete) sunk through soil by dredging inside, then plugged with concrete; used for bridge piers under water.
4. Under-reamed piles: Piles with bulbs at the lower end, for expansive soil.
5. Drilled shafts (bored piles): Large concrete shaft drilled in the ground.

 pile cap [=======]      pier    well
   | | |                 | |     |  |
   | | |                 | |     |__|
 =hard layer=         =hard=   sunk

Factors to be kept in mind when excavating trenches for pipes:

1. Alignment and gradient: Correct line and slope as in the drawing, set with sight rails and boning rods, so that the pipe drains properly (sewers need a fixed gradient).
2. Depth: Enough cover over the pipe to protect it from loads (about 1 m for water pipes; below frost and traffic effect), and adequate depth for the gradient.
3. Width of trench: Pipe diameter plus 300 to 600 mm working space, as narrow as practical to reduce load on the pipe and the cost.
4. Nature of soil: Soft soil needs side slopes or shoring; rock needs blasting or chiselling.
5. Ground water: Dewatering by pumping, sumps or well points.
6. Side support (shoring): Timber or steel sheeting for deep trenches in loose soil, for the safety of workers.
7. Safety and traffic: Barricades, signs, lights; spoil kept at least 0.6 to 1 m from the edge.
8. Existing underground services (cables, pipes) should be located and protected.
9. Bed preparation: Level trench bottom with a bed of sand or concrete, and joint holes at sockets.
10. Backfilling: Layer by layer after testing, and compaction.
11. Disposal of excavated materials and restoration of the road surface.

Factors affecting selection of foundation

1. Bearing capacity and type of soil.
2. Magnitude and nature of the loads.
3. Depth of the firm stratum and the water table.
4. Allowable settlement, particularly differential.
5. Type and use of structure (flexible or rigid).
6. Seismic zone and liquefaction potential.
7. Presence of expansive or filled soils; aggressive chemicals.
8. Adjacent structures; site topography, access and equipment.
9. Cost, time and materials.
10. Codes and bye-laws.

Methods of excavation in wet areas

1. Sump and ditch (open pumping).
2. Well-point system.
3. Deep wells.
4. Cofferdams or sheet piling.
5. Caissons.
6. Freezing and chemical grouting.
7. Electro-osmosis.

Three methods explained

Sump and ditch method: A drain is cut around the bottom of the excavation, leading to a sump pit deeper than the foundation level. Water collected in the sump is pumped out with a centrifugal pump. Suited to shallow excavation and low inflow, in coarse soils.

Well-point system: Many small perforated pipes are jetted into the ground at 1 to 1.5 m spacing around the excavation and connected by a header to a vacuum pump. This lowers the water table below the excavation floor, and keeps the soil dry and stable. Good for sand and silt, depth up to about 5 m for a single stage.

Sheet piling / cofferdam: Interlocking steel or timber sheet piles are driven around the area to cut off the water and hold back the soil; the water inside is pumped out. Used for deep excavations and near rivers.

 Sump:  \_______/ <- foundation level   Well point: |||  |||
         [sump]--pump                     header + pump

Method commonly used in Nepal

The sump and ditch method (open pumping) is the most common in Nepal because it is simple and cheap, and needs little equipment. The well-point method is used for larger works.

Excavation in soft soil

• Manual: Using spade, pick-axe, and shovel for shallow excavation of earth, clay, sand and murrum; labour is cheap in Nepal.
• Mechanical: Backhoe excavators, bulldozers and loaders for large areas.
• Sides are sloped or supported by timber shoring (planking and struts) for depth over 1.5 m, to avoid collapse.
• Spoil is placed at least 1 m away from the edge; the bottom is levelled and compacted.

Excavation in hard rock

1. Manual: Crowbars, wedges, chisel, pick axes, sledge hammers for small works. Wedging with feathers and plugs splits blocks.
2. Pneumatic breakers and drilling with compressed air, jackhammers.
3. Blasting: Holes are drilled, charged with explosive (gelatin, ANFO), stemmed and fired with detonators and fuse. Needs licensed personnel, warning, and safety distance. Controlled blasting limits damage.
4. Rippers and hydraulic hammers on excavators.
5. Non-explosive demolition agents (expansive cement) where blasting is not allowed near buildings. The rock bottom is cleaned and made level or benched.

Excavation in wet soil

Needs removal of water so that work is in dry conditions.

1. Sump and ditch method: Drains lead water to a sump from which it is pumped.
2. Well-point system: Pipes jetted around the trench with a vacuum pump lowering the water table.
3. Deep wells and sheet piling / cofferdam for deep works near water.
4. Shoring with close timbering for loose, saturated soils.
5. Scheduling in the dry season.

  Soft: sloped sides      Wet:  sump + pump
   \       /               \____/ 
    \_____/                  [sump]

Factors to be considered in the construction stage of trench excavation:

1. Setting out and alignment: Mark the trench lines with pegs, string and lime from the approved drawing; check dimensions and levels (profile boards, sight rails).
2. Depth and width: Excavate to the required foundation level; width is the footing width plus working space of 150 to 300 mm for formwork.
3. Type of soil: Soft soil needs slopes or shoring; hard soil or rock needs picking, drilling or blasting.
4. Side stability and shoring: Timber planking and struts for trenches deeper than 1.5 m in loose soil to prevent collapse and for the safety of workers.
5. Ground water: Dewatering by sumps, ditches, or well-point system; the bottom is kept dry.
6. Safety: Barricades, warning signs and lights, ladders for exit, spoil heaped at a safe distance from the edge (at least 1 m), no heavy loads near the trench, and inspection after rain.
7. Disposal of spoil: Store for backfilling and remove surplus; do not block drainage.
8. Existing services and adjacent structures: Locate underground pipes and cables; support nearby foundations.
9. Trench bottom: Level, clean, and compact; remove loose soil; do not over-excavate (fill with PCC if so).
10. Weather: Avoid the rainy season; protect the trench from rain run-off by diversion bunds.
11. Method and equipment: Manual or machine; cost and time.
12. Inspection and approval of the trench bed by the engineer before concreting.

Setting out is the process of transferring the dimensions and positions of the building from the drawing to the ground, so that excavation and construction follow the plan exactly.

Procedure (for a building with walls)

1. Site clearing and base line: Clear the site. Fix a base line parallel to the road or a reference line from the plan (a permanent line, set back as per bye-laws). Fix a benchmark for levels.
2. Marking the first corner: Fix a peg at one corner of the building from the base line, at the given distance.
3. Right angle: Set a right angle with the 3-4-5 method (a triangle of sides 3 m, 4 m and 5 m), or with a theodolite or a box square, to mark the adjacent wall line.
4. Other corners: Measure the length and breadth with a tape, fix pegs at all corners, and check the diagonals for equal length (they should be equal for a rectangle).
5. Marking the wall lines: Join the corner pegs with string and mark the outline of the centre lines and the trench width with lime powder.
6. Profile boards (batter boards): Fix boards on wooden posts at 1.5 to 2 m outside each corner, outside the area of excavation. Wire or nails on the boards mark the centre lines and the widths of the trench and of the footing. These stay in place during excavation.
7. Levels: Mark the depth of excavation with a level and a boning rod or a cut line at the required depth.
8. Checking: Check lengths, diagonals, and levels before the excavation starts.

    P1----------------P2     P = profile boards
     |    +--------+  |      
     |    | trench |  |      Diagonals AC = BD
     |    +--------+  |
    P4----------------P3
         3-4-5 triangle for right angle

For a framed building, the column centres are fixed by a grid of centre lines laid out by tape or total station, and marked on the profile boards.