basement excavation, underpinning

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CHAPTER ONE

1.0.Introduction

1.1.Summary of the stages involved in Building Construction

1.1.1. Site acquisition

1.1.2. Preparation of drawings e.g. Architectural, Structural, M&E, etc.

1.1.3. Site Organization

1.1.4. Setting out

1.1.5. Excavation works

1.1.6. Substructure Construction (from foundation up to over site

concrete).

1.1.7. Superstructure construction and finishes

1.1.8. Building services e.g. electricity and water (not to be covered)

1.1.9. External works

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CHAPTER TWO

2.0.Substructure Construction

This course will be limited to retaining walls, basement construction and underpinning.

2.1.Retaining Walls

A retaining wall is a wall designed to retain soil or water on one side.

2.1.2. Design of a retaining wall

Retaining walls have a primary function of retaining soils or water at an angle in excess

of the soil’s nature angle of repose.

Walls within the design height range are designed to provide the necessary resistance by

either their own mass or by the principles of leverage.

2.1.3. Design consideration:

1. Overturning of the wall does not occur.

2. Forward sliding does not occur.

3. Materials used are suitable.

4. Nature and characteristics of the subsoil.

5. Height of water table – the presence of water can create hydrostatic pressure,

affect bearing capacity of the subsoil together with its shear strength, reduce the

frictional resistance at the underside of the foundation.

6. Type of wall.

2.1.4. Types of walls

Mass retaining walls

Cantilever walls

Counterfort retaining walls

Precast concrete retaining walls

Precast concrete crib-retaining walls

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Mass (Gravity) Retaining Wall

1. Sometimes called gravity walls and rely upon their own mass together with the

friction on the underside of the base to overcome the tendency to slide or overturn

2. Generally only economic up to 1.8 m height.

3. Mass walls can be constructed of burnt clay bricks bedded in a 1:3 cement mortar

or of mass concrete.

4. Natural stone is suitable for small walls up to 1m high but generally it is used as a

facing material for walls over 1 m.

Typical example of mass retaining walls

Concrete

Brickwall

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Stone retaining wall Brick retaining wall

Mass concrete retaining wall with stone facings

Cantilever walls

1. Usually of reinforced concrete and work on the principle of leverage where the

stem is designed as a cantilever fixed at the base and the base is designed as a

cantilever fixed at the stem

2. Economic height range of 1.2 m to 6 m using pre-stressing techniques.

3. Any durable facing material can be applied to the surface to improve the

appearance of the wall.

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Two basic forms:-

A base with a large heel

A cantilever with a large toe

Cantilever T Cantilever L

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Reinforced concrete cantilever retaining wall (section)

Reinforced concrete cantilever retaining wall construction

Counterfort retaining walls

1. Can be constructed of reinforced or prestressed concrete.

2. Suitable for over 4.5 m.

3. Triangular beams placed at suitable centers behind the stem and above the base to

enable the stem and base to act as slab spanning horizontally over or under the

counterforts.

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Precast concrete retaining wall

1. Manufactured from high-grade pre cast concrete on the cantilever principle.

2. Can be erected on a foundation as permanent retaining wall or be free standing to

act as dividing wall between heaped materials which it can increase three times

the storage volume for any given area.

3. Other advantages- reduction in time by eliminating curing period, cost of

formwork, time to erect and dismantle the temporary forms

4. Lifting holes are provided which can be utilized for fixing if required.

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Application

Crib retaining walls

Wood crib retaining walls

Crib walls are one of the oldest gravity wall systems, comprising of a series of stacked

members creating hollow cells filled with soil or rock. This image shows a eucalyptus

crib wall built during the Second World War by the California Division of Highways near

Sonoma, California.

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Precast concrete crib retaining wall

These later replaced timber and are made of precast concrete elements joined together to

form a wall where hollow cells are filled with soil or rock.

Precast concrete crib retaining wall

Crib wall systems can be aesthetically pleasing because they can accommodate complex

curvature as shown below and be planted with climbing vines, giving them a much softer

appearance than more conventional support systems. Dying the concrete in an brown or

tan earthen color will also tend to enhance the final appearance.

Elevation of wood crib retaining wall Section through a wood crib retaining wall

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Curved precast concrete crib wall

One of the common failure modes for crib walls is inadequate toe embedment, on both

uphill and downhill walls, as shown below.

Failure modes for crib walls

Failure of a precast concrete crib wall without base support

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Placing aggregates or a pavement at the base of the wall will help to buttress the toe of

the walls.

Steel crib walls can also be used.

2.2. Basement Construction

A foundation is the first section of a building to be constructed and creates a base for the

rest of the building components. A foundation carries and transfers building loads onto

the ground and is usually located below the ground surface level. The bearing capacity of

the ground is the amount of load that can be supported by the ground on which the

building rests.

The following are the different loads that the foundation encounters:

1. Dead loads- the constant weight of the building arising from the walls, floors, roof

etc.

2. Live loads (imposed loads) - this is the weight of items like furniture, goods and

people and they are movable. These also include weight resulting from rain and

snow.

3. Wind loads- these are the loads caused by the wind. These may be positive of

negative.

The foundation is supposed to support the building and safeguard it from lateral and

vertical geological actions. High rise buildings require a firmer foundation because of a

high center of gravity.

When a structure is constructed on the ground, it exerts pressure on the top soil. Due

to the flexibility of the top soil, settlement occurs. The extent of settlement depends of

among other factors:

- The thickness of the subsoil layer.

- The distribution of the loads on the foundation.

- The depth of the foundation.

- The pressure at the contact points between the foundation and the

ground.

- The flexibility of the building structure.

In order to minimize settlement and make the building firm, upper layers of the

soil are removed so that the foundation of the building rests on the firmer bedrock. This

may require many meters’ depth of soil to be excavated. In most cases, the basement is

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constructed by digging out large quantities of earth and then structuring the space created

with concrete and steel in order to create room which can be occupied. In most cases,

this room is used for car parking or storage purposes. According to Newton’s third law of

motion, the downward force must be equal to the upward force for the system to be in

equilibrium. If the downward force arising from the weight of the building exceeds the

upward force, then the building will sink and the reverse of this creates a heave on the

building.

New technologies from time to time come up in order to reduce the cost of

construction for the benefit of both the contractor and the building owner of the building.

One of the most popular types of foundation used in construction of high rise buildings in

the basement foundation.

2.2.1. Advantages of basements

A basement has an advantage of being deep enough so that it rests on the solid

bedrock and hence making the building more stable and firm. It safeguards the

building from both vertical and lateral geological movements.

A basement creates room which can be used for many purposes such as car parks,

stores, saunas, disco halls, hotel rooms, etc. The space created when the ground is

excavated would be very expensive to fill with concrete and therefore

constructing a basement is actually and economical advantage.

The fact that basements are usually deep and rest on firm soils makes the

settlement of the building almost negligible and very low probability of tilting of

the building.

They are suitable in areas where there are restrictions with respect to height e.g.

near Airports,

They are also suitable in steep soils.

During the excavation, care has to be taken in order to avoid the collapsing of the soil by

supporting it (timbering using an appropriate method).

Basements are mainly constructed out of reinforced concrete in Uganda with 12mm

diameter bars tied in form of a mesh. Their construction is similar to retaining walls.

Basements are constructed below the ground and because concrete is porous it should be

waterproofed to increase on the service life of the building. This process is referred to as

tanking. A lining or liquid (slurry) asphalt compound is applied to the outside walls. The

liquid may be applied cold, though it is more effective if sprayed hot onto the outside

walls of the basement. The hot liquid is more expensive, but the results are better.

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After construction, the soil is backfilled at a slopping angle away from the building in

order to help with draining the water away. Care must be taken to avoid throwing in large

boulders as they might cause cracks in the walls of the foundation.

2.2.2. Tanking

Tanking refers to waterproofing of basements (floors below the ground surface). Below

the ground surface there is subsoil water so there is need for water proofing.

When constructing a new basement, it is possible to apply the waterproofing system to

the internal (negative) or external (positive) side of the basement wall. In many cases,

combined systems are specified to provide enhanced protection.

There are two types of tanking:-

External tanking

Internal tanking

External tanking

This refers to water proofing externally.

External tanking is usually (mostly) done on new buildings. When waterproofing

basements externally, we would normally suggest the use of a "dual-layer" system. This

consists of a primary waterproofing system protected by a secondary drainage layer

(provided by a cavity drainage membrane). This secondary layer protects the primary

waterproofing system from damage during backfilling. It also lessens the stress put on the

primary waterproofing system, reducing water ingress caused by possible imperfections

in the application of the primary system and/or structural movement.

Merits of external tanking

o It keeps the water out of the structure.

o Not damaged by internal services.

o The structural element is always kept dry.

Demerits of external tanking

It is very difficult to carry out repairs after the soil has been backfilled.

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Internal tanking

This is popular when waterproofing old basements, but can also be used to waterproof

new-build basements in situations where access to the external side of the basement is

problematic e.g. when building close to other properties, etc.

Merits of internal tanking

o It can be done after construction.

Demerits of internal tanking

o It does not protect the structure from subsoil water.

o It requires additional walls and floors to protect and hold the waterproof

membrane from exposure.

o For framed structures, columns also require waterproofing.

o When waterproofing, you have to tank to the highest level.

External tanking

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Problems associated with basement tanking

Although basement tanking will, if installed correctly, stop all water ingress into a

basement there are still a number of problems. Cracks can develop in the lining

due to the property settling or other vibrations.

Even a small crack can allow a significant volume of water into your basement. If your

property is near a rail line or underground and you are interested in a basement

conversion then take the time to look through all the basement waterproofing options.

Another major problem with basement tanking is that when the water is being

held back it builds up behind the lining and over time this pressure increases. As

the pressure reaches a certain level, as the wall is not flexible it will start to crack

and therefore let water into your basement.

Therefore, it's important when buying a house with a basement to check for signs of

dampness in the basement. It's better to find out before you buy than afterwards,

especially since you can't rely on seller disclosures. Why can't you rely on seller

disclosures? Because the sellers might not know about it. If the sellers did have

knowledge, however, and failed to disclose the water problems in a basement, it could be

very expensive to dry out the basement.

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This is why you should always, without fail, get an independent home inspection by a

qualified and accredited professional before acquiring such buildings.

Signs of a Damp or Wet Basement

Water stains along walls or floor. This could be caused by something simple such

as an overflowing laundry tub or it could be a result of water seeping in through

basement windows, the walls or the floor.

Musty odor or damp smell. Excess moisture in a basement can cause an

unmistakable smell.

Mold. It could be colored black, brown, yellow or green, and you won't know for

certain if it's mold without testing it. Often the northwest corner of a house is

known as a "cold corner" and susceptible to developing mold.

Efflorescence. This condition produces a white or sometimes grayish ash on the

walls. Sometimes it sparkles. Efflorescence is caused by salt deposits left behind

by evaporating water.

Spalling. When water gets inside the surface of concrete, brick or stone, salt

deposits from the water cause the surface to flake away or peel off.

Alternatives to basement tanking

An alternative to basement tanking includes using either an interior or external basement

waterproofing system. Exterior basement waterproofing is a common method of

basement waterproofing which involves putting a drainage system around the outside of

the basement. Exterior basement waterproofing tends to be the system that is used when

the property is constructed. Replacing this system is costly and would require that you

excavate the entire perimeter of the property.

One can construct a hidden drainage channel around the edge of the basement floor that

channels all the water to a sump pump. A membrane is installed on the walls and roof to

direct the water to the drainage channel keeping the basement warm and dry.

Video on basement waterproofing alternative.

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2.3. Underpinning

Underpinning is a method used to increase foundation depth or repairing faulty

foundations. This might be the case if you plan to add stories to an existing structure or

when the foundation has been damaged. One visible sign that your building needs

underpinning are cracks appearance. When a building needs a foundation repair some

cracks, especially wider than 10 mm appear visible, meaning that underpinning needs to

be done. Foundation failures could also be considered as heaved foundations, cracked or

buckled walls and cracked concrete floors.

Underpinning is accomplished by extending buildings foundation both horizontally and

vertically.

2.3.1. Signs of foundation failure

There are several signs that the homeowner can look for to determine if the foundation in

your home has been compromised.

Interior signs

When inspecting the interior, look for cracks around the windows and doors. When

Windows and doors that at one-time work easily and are now very difficult to open or

close. This could indicate the foundation is not stable and has shifted.

Exterior signs

Finding cracks in your buildings foundation that are larger than 10 mm is reason for

concern.

If your building has a basement check your foundation near basement windows and

vents these are the first signs that the foundation has settled. Other obvious signs are

cracked concrete floors in the basement and garage and buckling basement walls.

Reasons for underpinning

To strengthen the shallow foundation of an existing building when a building with

a deep foundation is to be constructed adjoining to it.

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To strengthen an existing foundation which has settled and caused cracks in the

wall.

When one wants to construct a basement below an existing building.

Transferring the foundation loads from a poor strata to a deeper strata of high

bearing capacity.

The means and methods of supporting a structure foundation depend on many

factors including:

- Foundation loads: static and dynamic; permanent and temporary.

- State of existing foundations.

- Type and magnitude of allowable structural movement i.e. deformations.

- Subsurface soil conditions.

- Subsurface groundwater conditions.

- Condition of the structure.

- Access and mobility to the foundations.

- Potential for environmental hazards.

Types of underpinning

Pit method

Screw pilings or helical piling method

Pit Method

The conventional pit method underpinning has been used for centuries. This method

consists of enlarging and/ or deepening existing foundations by removing soil from

beneath the foundations and replacing it with concrete, reinforcement. In some cases the

structure is temporarily shored to prevent settlement.

In this method, the entire length of the foundation to be underpinned is divided into

sections ranging from 1.2m to 1.5m and one section is taken up at a time. For each

section, a hole is made into the wall, above the plinth wall level and the needle is inserted

into the hole. These needles may be either of stout timber or steel section. Bearing plates

are placed above the needle to support the masonry above it. The needle is supported on

either side by crib supports (wooden blocks) and screw jacks. The foundation is then

excavated up to the desired level and anew foundation is laid. When one section is

finished, i.e. alternate sections are underpinned in the first round and then the remaining

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sections are taken up in the second round. If the wall to be underpinned is weak, raking

shores may be used. Similarly floors maybe supported if required.

This underpinning method is generally applied when the existing foundation is at a

shallow depth, however, the method still works very well even at 15m deep. The method

has not changed since its inception with its use of utilitarian tools such as shovels and

post hole diggers. Heavy machinery is not called for in this method due to the tight nature

of the boxes being dug. There are several advantages to using this method of

underpinning including the simplicity of the engineering, the low cost of labor to produce

the result, and the continuity of the structure's uses during construction.

The pit method often results in moderate deformation of the structure and un safe

working conditions. That is why during the last 20 years, several less disturbing methods

developed to underpin structures that result in much less deformation and more faster,

less expensive and safer operation.

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2. Screw pilings or helical piling method

Another foundation underpinning method is the use of screw pilings also known as

helical piles they are screwed beneath the building’s foundation and are installed at

various depths until stable soil is reached. These galvanize steel screw pilings are then

attached to the foundation. Helical piles have many advantages over traditional pilings.

Using the proper equipment they can quickly be installed with minimal of vibration and

noise. This method is most suitable for settling foundations.

2 video clips on helical piles

3. Pile and Beam method

This method consists on installing mini-piles on either side of the affected wall, a pocket

of brickwork is then removed below ground level then a prefabricated steel cage is

installed to span over the two piles and then concreted to form an in situ reinforced

concrete needle beam. Reducing the distance between needle beams can accommodate

very high loads.

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Installation Procedure

Install piles in pairs, driven, or bored at longitudinal centers approximately 1.0m - 1.5m;

lateral centers 1.0m - 1.2m; excavate, break out for and construct reinforced concrete

needle beams.

The bearing capacity of the underlying strata will determine the number, diameter, depth

and spacing of piles used. Very high foundation loads can be accommodated by reducing

the span between needle beams:

The advantages of underpinning with pile and beams are:

Suitable for restricted access.

Faster than traditional underpinning.

High load capability

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Less disruption, less spoil (excavated soli) generated and completed quickly.

Because large excavations are not required, disruption and mess are kept to a

minimum.

System becomes cost effective when the depth of traditional underpinning

exceeds 1.5m.

Piled Raft Method

Underpinning with piled raft, is used when the whole structure need to be underpinned. It

is recommended when foundations are too deep for other underpinning methods (bearing

strata is at depths in excess of 1.5m) or in areas where the soil is so hard that small

equipment could not excavate up to require depth.

The bearing capacity of the underlying strata will determine the number, diameter, depth

and spacing of piles used.

Pile sizes may generally be considered as follows:

Installation Procedure

Install piles, internally at centers determined by loadings. Break out and construct

reinforced concrete needle beams at 1.0m - 1.2m centers to bear the wall’s load. A

reinforced concrete ring beam is the built to link all the needle beams, then a reinforced

concrete floor slab.

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Advantages

a) Economic at depths greater than 1.5m.

b) Provides lateral and transverse ties throughout the structure with piled foundations for

internal walls and floors

c) Provides fully suspended replacement ground floor slab.

d) Quicker when compared with dig-out systems.

e) Especially suitable where access externally is restricted and lateral and longitudinal

restraint is required.

f) Minimizes disruption to existing external services (i.e. drains and service pipes) and

consequential reinstatement.

g) Recommended for total structure isolation from underlying strata, particularly in clay

heave/shrinkage situations.

There are other underpinning methods but the above are the most commonly used.

Underpinning Tips

Underpinning in foundation should be addressed and supervised by an engineer.

The underpinning process must be started from the corners and the working

inwards.

Underpinning must be made only on load bearing walls.

Do not underpin below non-load bearing walls.

Start underpinning under a strip of footing. It is recommended to start with at least

3 feet long, two feet wide and two feet depth.

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After the excavation has been completed add concrete to the cavity. Concrete

should be mixed using one part cement, three parts sand and six parts aggregates.

Remember to use formwork on the edges.

Allowed concrete placed to set for at least two days.

Use a rod bar ensuring that the cavity under the existing foundation is filled up.

Ensure that the concrete is cured thoroughly before loading it.

Once the concrete has gained sufficient strength, break off the projecting footing.

Cut the concrete with the mass of concrete surface.

Back fill and compact. If you are having problems achieving required

consolidation, use a hose to add water to the soil.