Building Construction 4

Dr Nabil El-Sawalhi

Associate professor

Engineering Projects Management

Site preparation

• 1. Fencing the site from adjacent public or private property

• Locating and marking existing underground utility lines

• Demolishing unneeded existing structures and utility

• Lines Marking trees to be saved, and removing unneeded trees, shrubs, topsoil, and extraneous landfill, if present.

EXCAVATION

• Is the process of removing soil or rock from its original location, for constructing foundations, basements, and underground utility lines and for grading of the ground surface.

• Excavated material required for backfill or grading fill is stockpiled on the site for subsequent use.

• Unneeded material is removed from the site for appropriate disposal.

• Excavations are generally classified as:

• Open excavations

• Trenches

• Pits

• Open excavations refer to large (and often deep) excavations, such as for a basement.

• Trenches generally refer to long, narrow excavations, such as for footings under a wall or utility pipes.

• Pits are excavations for the footing of an individual column, elevator shaft, and so on.

GRADING

• Grading involves moving earth from one location to another and changing the existing land surface to the desired finished surface configuration.

• Grading is separated into rough grading and finish grading.

• Rough grading is done along with excavations for foundations, basements, and utility trenches.

• Finish grading is generally done toward the end of the project as per the landscape design.

SUPPORTS FOR OPEN EXCAVATIONS

• Excavations in the soil generally require some type of support to prevent cave-ins while the foundation system or basement walls are constructed.

• The simplest excavation support system consists of providing adequate slope in the excavated (cut) face so that it is able to support itself, Figure 11.9 .

• All su-bsoils have different abilities in remaining stable during excavation works. Most will assume a natural angle of repose or rest unless given temporary support.

• The presence of ground water apart from creating difficult working conditions can have an adverse effect on the subsoil's natural angle of repose.

• The maximum natural slope at which a soil will support itself must be determined from soil investigations.

• Excavation in coarse-grained soils requires a shallower slope than excavation in fine-grained soils.

• A sloped excavation may either be uniformly sloped or stair-stepped, called a benched excavation, Figure 11.10

• Self-supporting sloped excavations cannot be provided where the site area is restricted or adjoining structures are present.

• The excavation must consist of vertical cuts. • In cohesive soils, shallow vertical cuts (generally 5 ft or less

in depth) may be possible without any support system. • Deeper vertical cuts must be provided with a support

system. • These support systems may be temporary or permanent. • Some of the commonly used methods of supporting deep

vertical cuts in the soil are:

• Sheet piles

• Cantilevered soldier piles

• Anchored soldier piles

• Contiguous bored concrete piles

• Bentonite slurry walls

EXCAVATION SUPPORT USING SHEET PILES

• For depths of up to about 15 ft, vertical sheets of steel, referred to as sheet piles, can be driven into the ground before commencing excavations.

• Sheet piles consist of individual steel sections that interlock with each other on both sides.

• The interlocks form a continuous barrier to retain the earth.

• Structurally, sheet piles function as vertical cantilevers and, therefore, must be buried in the soil for sufficient depth below the bottom of the excavation.

• Sheet piles are available in many cross-sectional profiles.

• The most commonly used profile is a Z-section,

• The sections are driven into the ground one by one using either hydraulic hammers or vibrators

• For deeper excavations (generally greater than 15 ft), sheet piles are braced with horizontal or inclined braces or anchored with tiebacks,

• Sheet piles are removed after they are no longer required or can be left in place if needed.

EXCAVATION SUPPORT USING CONTIGUOUS BORED CONCRETE PILES

• In situations where the (deep) excavation is close to an adjacent building or the property line, tiebacks cannot be used.

• In this situation, closely spaced reinforced concrete piles, called contiguous bored piles (CBPs), are often used.

• Each pile is made by screwing an auger into the ground.

• High-slump concrete is pumped down the hollow stem of the auger to the bottom of the bore.

• Once the pumping starts, the auger is progressively withdrawn.

• The withdrawing auger brings the soil from the bore to the surface, where it is removed.

• Thus, the sides of the bore are supported at all times by the soil-filled auger or concrete.

• Immediately after the entire bore has been concreted, a reinforcement cage is lowered in the concrete-filled bore.

• CBPs up to 100 ft deep have been constructed.

• They are generally 18 to 36 in. in diameter, depending on the depth of the excavation.

EXCAVATION SUPPORT USING BENTONITE SLURRY AS TRENCH SUPPORT

• Used where the underground water table is relatively high, is a reinforced concrete wall.

• Construction of walls is done by excavating 10-ft- to 15-ft-long discontinuous trench sections down to bedrock, called primary panels .

• The width of the trench sections is the required thickness of the concrete wall.

• So that the soil does not collapse, the trench is continuously kept filled with bentonite slurry as the excavation proceeds. (Bentonite slurry is a mixture of water and bentonite clay, which pressurizes the walls of the trench sufficiently to prevent their collapse during excavation.)

KEEPING EXCAVATIONS DRY

• It is important to keep excavations free from groundwater.

• Groundwater control in an excavation consists of two parts:.

• (a) preventing surface water from entering the excavation through runoff and

• (b) draining (dewatering) the soil around the excavations so that the groundwater level falls below the elevation of proposed excavation.

• Two commonly used methods of dewatering the ground are sump pumps and well points .

DEWATERING THROUGH SUMPS

• Sump dewatering consists of constructing pits (called sumps ) within the enclosure of the excavation.

• The bottom of sumps must be located below the final elevation of the excavation.

• As the groundwater from surrounding soil percolates into the sump, it is lifted by automatic pumps and discharged away from the building site.

• The number of required sumps is a function of the excavation area. The method of discharge of pumped water must meet with the approval of local authorities.

Cut face with

Pump

DEWATERING THROUGH WELL POINTS

• Sump dewatering works well in cohesive soils, where the percolation rate is slow and where the water table is not much higher than the final elevation of the base of the excavation.

• A more effective dewatering method uses forced suction to extract groundwater.

• This is done by sinking a number of vertical pipes with a screened end at the bottom (called well points ) around the perimeter of the excavation. The well points reach below the floor of the excavation and are connected to large-diameter horizontal header pipes at the surface.

• The header pipe is connected to a vacuum-assisted centrifugal pump that sucks water from the ground for discharge to an appropriate point.

• For a very deep excavation, two rings of well points may be required. The well points in the ring farther away from the excavation terminate at a higher level than those close to the excavation.

Groundwork and Foundations

• Functional requirements

• The primary functional requirement of a foundation is strength and stability.

• Strength and stability

• The combined, dead, imposed and wind loads on a building must be transmitted to the ground safely, without causing deflection or deformation of the building or movement of the ground that would impair the stability of the building and/or neighbouring structures.

• Foundations should also be designed and constructed to resist any movements of the subsoil.

• Foundations should be designed so that any settlement is both limited and uniform under the whole of the building.

• This settlement should be limited to avoid damage to service pipes and drains connected to the building

Topsoil

• The surface layer of most of the low-lying land that is most suited to building, consists of a mixture of loosely compacted particles of sand, clay and an accumulation of decaying vegetation.

• This layer of topsoil, which is about 100 to 300mm deep, is sometimes referred to as vegetable topsoil.

• It is loosely compacted, supports growing plant life and is unsatisfactory as a foundation because of its poor bearing capacity.

• It should be stripped from the site and retained for landscaping around the site.

Ground movement

• The building shall be constructed so that ground movement caused by swelling, shrinkage or freezing of the subsoil, or land-slip or subsidence, which can be reasonably foreseen, will not impair the stability of the building.

• The foundations of the building must be selected and designed so that they overcome the problems of ground movement.

Volume change

• Firm, compact shrinkable clays suffer appreciable vertical and horizontal shrinkage on drying and expansion on wetting due to seasonal changes.

• The greater the seasonable variation, the greater the volume change.

• The more vigorous the growth of shrubs and trees in firm clay soils, the greater the depth below surface the volume change will occur.

• It is recommended that buildings on shallow foundations should not be closer to single trees than the height of the tree at maturity, and one-and-a-half times the height at maturity of groups of trees, to reduce the risk of damage to buildings by seasonal volume changes in clay subsoils.

• When shrubs and trees are removed to clear a site for building on firm clay subsoils there will, for some years after the clearance, be ground recovery as the clay gradually recovers moisture previously taken by the shrubs and trees.

• The design and depth of foundations of buildings must allow for this gradual expansion to limit damage by differential settlement.

• Similarly, if vigorous shrub or tree growth is stopped by removal, or started by planting, near to a building on firm clay subsoil with foundations at a shallow depth, it is most likely that gradual expansion or contraction of the soil will cause damage to the building by differential movement.

Made up ground

• Areas of made up ground are often used for buildings as the demand for new buildings increases.

• Because of the varied nature of the materials tipped to fill and raise ground levels and the uncertainty of the bearing capacity of the fill, conventional foundations may be unsatisfactory and investigation is required to establish the most suitable foundation design.

• The bearing ground may be some distance below the surface level of the made up ground and to excavate for conventional strip foundations would be grossly uneconomic.

• A solution is the use of piers (piles) on isolated pad foundations supporting reinforced concrete ground beams on which walls are raised, as illustrated in Figure 3.1C.

• Causes of differential settlement

• Differing building loads

• Dead loads – building structure loads not properly accommodated by the foundations

• Unexpected live loads – services and equipment installed within the building where the vibration or excessive load exceeds the foundation and soil design strength

• ❏ Different ground conditions under the building • ❏ Water courses that destabilise the ground • ❏ Weak compressible strata • ❏ Adjacent loads causing overloading under existing

foundations • ❏ Changes in adjacent vegetation that result in

different water content of clay soil, causing it to shrink or swell

• ❏ Freezing of ground below, or adjacent to, foundations. As the water in the ground freezes, it expands, which may cause shallow foundations to lift as the ground below it freezes.

• The expansion and consequent heaving of the soil occur at the surface and for a depth of some 600mm most foundations are excavated to a minimum distance of 750 mm to avoid volume changes due to seasonal movement.

• Ground instability

• Land instability may be broadly grouped under the headings:

• ❏ Landslip

• ❏ Surface flooding and soil erosion

• ❏ Natural caves and fissures

• ❏ Mining and quarrying

• ❏ Landfill

Differential settlement (relative settlement)

• Parts of the foundation of a building may suffer different magnitudes of settlement due to variations in load on the foundations or variations in the subsoil, and different rates of settlement due to variations in the subsoil.

• A common settlement problem occurs in modern buildings where a tower or slab block is linked to a smaller building or low podium. Plainly there will tend to be a more pronounced settlement of the foundations of the tower or slab block than experienced in the smaller structure

Reusing existing foundations

• The common approach is to construct new foundations (e.g. piled foundations) between the existing foundations and/or to remove existing foundations to make room for the new development. Removing existing foundations is expensive, disturbs the ground and has a high environmental cost. One way of reducing the environmental impact (carbon footprint) of building is to reuse the existing foundations.