Ground improvement

• Engineering properties
  
• Drainage
  
• Pre-consolidation
  
• Compaction
  
• Grouting
  
• Geo-textiles
  

Where poor ground conditions make traditional forms of construction expensive, it may be economically viable to attempt to improve the engineering properties of the ground before building on it. This can be done by reducing the pore water pressure, by reducing the volume of voids in the soil, or by adding stronger materials.
 
 

 

Engineering properties

• Compression
  
• Consolidation
  
• Shear strength
  
• Permeability
  

The properties of soil which most affect the cost of construction are strengthand compressibility. Both can be improved by reducing the volume of the voids in the soil mass. Water must be displaced from saturated soils in order to reduce the volume of the voids. This may take months if the permeability of the soil is low.
 
 

 

Compression

Soil which is highly compressible is prone to volume change when a load is applied. This leads to settlement. Fine-grained soils which have been compressed and then allowed to swell, experience a smaller volume change when re-compressed. Loosely-compacted coarse-grained soils may exhibit little change in volume under static loads, but become unstable and exhibit large volume changes when either vibrated or flooded and then drained.
 
 

 

Consolidation

The sudden application of a load to a saturated soil produces an immediate increase in porewater pressure. Over time, the excess porewater pressure will dissipate, the effective stress in the soil will increase and settlement will increase. Since shear strength is related to effective stress, it may be necessary to control the rate of construction to avoid a shear failure. This was the case, for example, when approach embankments were constructed on soft alluvium, for the bridge which carried the M180 motorway over the River Trent near Scunthorpe. The rate at which the excess water pressure dissipates, and settlement occurs, depends on the permeability of the soil, the amount of water to be expelled and the distance the water must travel.

   
 

 

 
 

Shear strength Collapse will occur if the shear stress along a potential failure surface exceeds the shear strength of the soil. Shear strength depends on the effective normal stress, which depends on the porewater pressure. Undrained loading causes an increase in porewater pressure equal to the change in the total normal stress so that there is no increase in strength to match the change in the shear stress. The shear strength can be increased either by decreasing the water pressure or reducing the void ratio of the soil to produce a peak strength which exceeds the critical shear stress.

       
 

 

 

Permeability

Fine-grained soils have a lower permeability than coarse-grained soils, thus excess porewater pressures take longer to dissipate. Consolidation reduces the void ratio of the soil and further decreases the permeability. Real soils are not hydraulically isotropic: the natural orientation of particles in soils which have been consolidated vertically tends to produce a horizontal permeability which is greater than the vertical permeability.

Thin horizontal layers of coarse-grained soil in a mass of fine-grained soil may dramatically increase the horizontal permeability while having little effect on the vertical permeability. It is possible to increase the drainage rate without changing the permeability of the bulk of the soil by introducing layer drains (sandwicks) or fracturing the soil. The most effective way to reduce seepage into an excavation, through or under a dam, or away from contaminated ground is to create a low permeability zone perpendicular to the direction of flow.
 
 

 

Drainage

Pumping water out of the ground will cause a local lowering of the ground water level and a decrease in water pressure. Both will return to their natural state when pumping stops. The rate of drawdown and the radius of influence depend on the permeability of the soil: Low permeability implies slow drawdown and large radius. Decreasing the water pressure increases the effective stress, which increases the shear strength and causes settlement.

The introduction of a grid of vertical drains, connected by layer of highly permeable soil, reduces the distance water has to travel through the natural soil and facilitates horizontal flow. This limits the excess water pressure generated during and after construction and increases the rate of settlement.

 
 

 

 

Pre-consolidation

Settlement due to an applied pressure occurs over a period of time. A proportion of the final settlement can be achieved prior to construction by pre-loading the soil. The larger the pre-load, the less time it will take to achieve the final settlement. Pre-consolidating the ground in this way tends to be an expensive solution compared with the use of piles to support localised loads such as columns. Pre-consolidation may be a cost-effective way of reducing the settlement due to lightly distributed loads from roads or warehouse or supermarket floors provided that material is readily available to provide the pre-loading. Pre-consolidation is normally designed to take 6 - 9 months.

   
 

 

Compaction

• Compaction of fill
  
• Dynamic compaction
  
• Vibro-compaction/replacement
  

Compaction is a dynamic process, reducing the volume of soil by expelling air. The moisture content is not altered significantly under normal circumstances. (Water may migrate a short distance from the point of application but is forced to return when compaction is applied to the adjacent soil). Compaction is most effective when applied to a thin layer because the energy dissipates with distance. Vibration is the most effective method of compacting loose coarse-grained soils.
 
 

 

Compaction of fill

Fills are normally compacted in layers between 300mm and 600mm thick. For granular soils, a motor on the back of the roller is used to rotate an eccentric mass causing the roller to vibrate. For fine-grained soils, the roller may be fitted with blunt spikes known as sheep's feet. Sheep's foot rollers produce a kneeding action which changes the shape of clods of soil and displaces air from the spaces between the clods.
 
 

 

Dynamic compaction

Dynamic compaction involves lifting and dropping a heavy weight several times in one place. The process is repeated on a grid pattern across the site. Trials in the UK indicate that the masses in the range 5 to 10 tonnes and drops in the range 5 to 10m are effective for compacting loose sand but not clay. Masses up to 190 tonnes and drops of 25m are used by TLM (Technique Louis Ménard) in France. Such heavy compaction causes fractures through which water can flow. This, according to the proponents of the system, enables fine-grained soils to be compacted. Heavy compaction tends to annoy the neighbours, which limits its use in built-up areas.

compactive energy per blow = m.g.h
where m = mass, g = gravitational constant, h = drop.

estimated depth of compaction = n.(m.h)
where n is an empirical constant between 0.3 and 1 depending on the grain size distribution and degree of saturation (0.5-1 for sands, 0.3-0.5 for silts and clayey soils).

 
 

 

Vibro-compaction/replacement

Both vibro-compaction and vibro-replacement use a vibrating poker to make a hole in the ground. Soil is displaced sideways, not removed from the ground.

vibro-compaction

in coarse-grained soils the poker may be removed slowly while still vibrating. This causes the sides of the hole to collapse and results in a depression in the ground surface.

vibro-replacement

in fine-grained soils it is usual to fill the hole with coarse aggregate (up to 50mm). The poker may be used to compact the stone column in layers. A typical column might be 5m deep and 500mm diameter. A line of columns at say 3m centres can be used to support a reinforced concrete ground beam effectively producing a piled foundation.
 
 

 

 

Grouting

Injecting cementitious material into a soil mass tends to reduce permeability, cause swelling and may increase strength.

Grout injection into fractured rock which forms the foundation of a dam is possibly the oldest and best known application. Grout injection has been used successfully to strengthen and reduce permeability of soil around a basement excavation below the water table. It has also been used to control the settlement of structures adjacent to tunnel excavations in London: predicted settlements of 60mm, which would have caused extensive damage to old buildings, were limited to 10mm.

Silty soils with high water contents are unsuitable for embankment construction in their natural state because they are difficult to compact. They can be improved by mixing hydrated lime with the soil.
 
 

 

Geo-textiles

Geo-textiles can be used for:

segregation of layers

Rock-fill laid on soft ground to form a road or embankment base can be prevented from punching into the soil below using a geotextile underlay.

tensile strength

Horizontal membranes can be used to provide tensile re-inforcement and reduce settlement. There are two primary difficulties:

(i) aligning the mebrane in the direction of the principal tensile stress, which is probably not horizontal, and

(ii) the fact that geotextiles have a low modulus of elasticity and are plastic and therefore tend to creep.

a drainage layer

Either as a water-conductor or as a filter to reduce the migration of fine particles into a granular soil drain.

an impermeable barrier

To prevent or control the flow of contaminated groundwater from or in land-fill sites.

Civil Engineering Facts

Radial Stresses and Curvature Factor

The radial stress induced by a bending moment in a member of constant cross section may be computed from
fr=3M/2Rbd
where M= bending moment, in lb (N m)
R = radius of curvature at centerline of member, in (mm)
b =width of cross section, in (mm)
d =depth of cross section, in (mm)
When M is in the direction tending to decrease curvature(increase the radius), tensile stresses occur across the grain. For this condition, the allowable tensile stress across the grain is limited to one-third the allowable unit stress in hori- zontal shear for southern pine for all load conditions and for Douglas fir and larch for wind or earthquake loadings. The limit is 15 lb/in2 (0.103 MPa) for Douglas fir and larch for other types of loading. These values are subject to modification for duration of load. If these values are exceeded, mechanical reinforcement sufficient to resist all radial tensile stresses is required.
When M is in the direction tending to increase curvature(decrease the radius), the stress is compressive across the grain. For this condition, the design value is limited to that for compression perpendicular to grain for all species.For the curved portion of members, the design value for wood in bending should be modified by multiplication by the following curvature factor:
Cc=1-2000(t/R)2
wheret is the thickness of lamination, in (mm), and R is the radius of curvature of lamination, in (mm). Note that t/R should not exceed 1/ 100 for hardwoods and southern pine or1/125 for softwoods other than southern pine. The curvature factor should not be applied to stress in the straight portion of an assembly, regardless of curvature elsewhere.

Ground improvement

Engineering properties
Drainage
Pre-consolidation
Compaction
Grouting

Geo-textiles

Where poor ground conditions make traditional forms of construction expensive, it may be economically viable to attempt to improve the engineering properties of the ground before building on it. This can be done by reducing the pore water pressure, by reducing the volume of voids in the soil, or by adding stronger materials.

Engineering properties

Compression
Consolidation
Shear strength
Permeability

The properties of soil which most affect the cost of construction are strengthand compressibility. Both can be improved by reducing the volume of the voids in the soil mass. Water must be displaced from saturated soils in order to reduce the volume of the voids. This may take months if the permeability of the soil is low.

Compression

Soil which is highly compressible is prone to volume change when a load is applied. This leads to settlement. Fine-grained soils which have been compressed and then allowed to swell, experience a smaller volume change when re-compressed. Loosely-compacted coarse-grained soils may exhibit little change in volume under static loads, but become unstable and exhibit large volume changes when either vibrated or flooded and then drained.

Consolidation

The sudden application of a load to a saturated soil produces an immediate increase in porewater pressure. Over time, the excess porewater pressure will dissipate, the effective stress in the soil will increase and settlement will increase. Since shear strength is related to effective stress, it may be necessary to control the rate of construction to avoid a shear failure. This was the case, for example, when approach embankments were constructed on soft alluvium, for the bridge which carried the M180 motorway over the River Trent near Scunthorpe. The rate at which the excess water pressure dissipates, and settlement occurs, depends on the permeability of the soil, the amount of water to be expelled and the distance the water must travel.

Shear strength

Collapse will occur if the shear stress along a potential failure surface exceeds the shear strength of the soil. Shear strength depends on the effective normal stress, which depends on the porewater pressure. Undrained loading causes an increase in porewater pressure equal to the change in the total normal stress so that there is no increase in strength to match the change in the shear stress. The shear strength can be increased either by decreasing the water pressure or reducing the void ratio of the soil to produce a peak strength which exceeds the critical shear stress.

Permeability

Fine-grained soils have a lower permeability than coarse-grained soils, thus excess porewater pressures take longer to dissipate. Consolidation reduces the void ratio of the soil and further decreases the permeability. Real soils are not hydraulically isotropic: the natural orientation of particles in soils which have been consolidated vertically tends to produce a horizontal permeability which is greater than the vertical permeability. Thin horizontal layers of coarse-grained soil in a mass of fine-grained soil may dramatically increase the horizontal permeability while having little effect on the vertical permeability. It is possible to increase the drainage rate without changing the permeability of the bulk of the soil by introducing layer drains (sandwicks) or fracturing the soil. The most effective way to reduce seepage into an excavation, through or under a dam, or away from contaminated ground is to create a low permeability zone perpendicular to the direction of flow.

Types of foundation

Shallow foundations
Deep foundations
Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. Deep foundations include piles, pile walls, diaphragm walls and caissons.



Shallow foundations
Pad foundations
Strip foundations
Raft foundations
Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is 'shallow'. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.








Pad foundations
Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or reactangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.


Strip foundations
Strip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate.


Raft foundations
Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact.
A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation.
Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.








1. Deep foundations
Piles
Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.
Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.
Piers are foundations for carrying a heavy structural load which is constructed insitu in a deep excavation.
Caissons are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.
Compensated foundations are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement.



Piles
Types of pile
Types of construction
Factors influencing choice
Pile groups
Piled foundations can be classified according to the type of pile (different structures to be supported, and different ground conditions, require different types of resistance) and the type of construction (different materials, structures and processes can be used).
(Source By: Books & net)

Foundations

Load-settlement behaviour
Types of foundation
Bearing capacity
Settlement
Foundation design
Ground improvement
The foundation of a structure is in direct contact with the ground and transmits the load of the structure to the ground. Foundations may be characterised as shallow (pad, strip or raft) or deep (piles, piers or caissons). When designing foundations, two principal criteria must be satisfied:
Bearing capacityThere must be an adequate factor of safety against collapse (plastic yielding in the soil and catastrophic settlement or rotation of the structure).
SettlementSettlements at working loads must not cause damage, nor adversely affect the serviceability of the structure.
There are other considerations that may be relevant to specific soils, foundation types and surface conditions.
(Source By: Books & net)




1. Foundation design
Design is an iterative process. Designers use their experience to estimate the dimensions, then check whether the design is safe. If it is not safe, or the check indicated that it may be possible to make economies, then they modify the dimensions and repeat the calculations. For example:
Use presumed bearing values to obtain a first estimate of the size.
Calculate the ultimate bearing capacity (qf) at which collapse will occur.
Obtain the allowable bearing pressure from
Divide the design load by this allowable pressure to obtain a required area.
Select appropriate dimensions.
Calculate the likely settlement for this size of foundation.
Check that the predicted settlement due to this allowable bearing pressure is likely to be acceptable.

(Source By: Books & net)

Civil Engineering

Engineering is a term applied to the profession in which a knowledge of the mathematical and natural sciences, gained by study, experience, and practice, is applied to the efficient use of the materials and forces of nature. Engineers are the ones who have received professional training in pure and applied science.Before the middle of the 18th century, large-scale construction work was usually placed in the hands of military engineers. Military engineering involved such work as the preparation of topographical maps, the location, design, and construction of roads and bridges; and the building of forts and docks; see Military Engineering below. In the 18th century, however, the term civil engineering came into use to describe engineering work that was performed by civilians for nonmilitary purposes.
Civil engineering is the broadest of the engineering fields. Civil engineering focuses on the infrastructure of the world which include Water works, Sewers, Dams, Power Plants, Transmission Towers/Lines, Railroads, Highways, Bridges, Tunnels, Irrigation Canals, River Navigation, Shipping Canals, Traffic Control, Mass Transit, Airport Runways, Terminals, Industrial Plant Buildings, Skyscrapers, etc. Among the important subdivisions of the field are construction engineering, irrigation engineering, transportation engineering, soils and foundation engineering, geodetic engineering, hydraulic engineering, and coastal and ocean engineering.Civil engineers build the world’s infrastructure. In doing so, they quietly shape the history of nations around the world. Most people can not imagine life without the many contributions of civil engineers to the public’s health, safety and standard of living. Only by exploring civil engineering’s influence in shaping the world we know today, can we creatively envision the progress of our tomorrows