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WHAT IS BASEMENT…

A basement is a story or several stories of a building that are either completely or partially below the ground floor.

Not all buildings have basements. For buildings with basements, the furnace, water

heater, car park and air-conditioning system of a house or building are typically located in the basement; so also are amenities such as the electrical distribution system, and cable television distribution point.

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Basement Grades (BS 8102)Grade Possible Use Conditions Required Moisture Exclusion

Grade 1:

Basic Utility

Car parking,

Mechanical plant rooms

>65% relative humidity

15-32oC temperature

Minor wet seepage and visible damp patches may be acceptable.

Grade 2:

Better Utility

Retail Storage,

Electrical plant rooms

35-50% relative humidity

Temperature depends on use:<15oC for storage,

up to 42oC for plant rooms

Wet seepage unacceptable. No visible moisture patches

Grade 3:

Habitable

Offices, Residential use, Kitchens,

Restaurants etc

40-60% relative humidity

Temperature range 18-29oC, depending on use

Seepage and wet patches unacceptable. Possible active control of internal environment required to control temperature and humidity.

Grade 4:

Special

Archive storage of books, documents, art etc

35-50% relative humidity

Temperature range typically 13-22oC

Environment tightly controlled by active measures. Seepage and visible dampness unacceptable.

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Selection of types of basement

Level of ground water on site Possible contamination of ground water Natural drainage Soil type Access to site

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Requirement of wall below ground

Structural stability Durability Moisture exclusion Buildability

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Basement Type

Type A - Tanked Use impervious material internally or externally to

exclude moisture The selected waterproofing element must be

capable to withstand hydrostatic pressure from ground water & other loadings

Create a tanking effect

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Type A - Tanked

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Type B – Waterproof Utilise waterproof materials within the structure of

walls – structural integrated protection. Introduce crystallising agent inside concrete +

usage of proper construction joints

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Type B – Waterproof

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Type C – Drained Allow the ground water to pass through the wall

by providing mechanism to drain it away.

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Type C – Drained

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TYPES OF BASEMENT CONSTRUCTION

OPEN-CUT CONSTRUCTIONCUT AND COVER

CONSTRUCTIONBOTTOM UPTOP DOWN CONSTRUCTION

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OPEN-CUT EXCAVATION

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For all excavations, operators must know: the line; the final depth of the excavation; the approximate width of the excavation at

the top; and the location of any services or other

hazards.

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If there is sufficient right of way, open cut trenches can be used in almost any soil condition.

Generally a sloped open cut excavation is the most cost and schedule effective method of trenching.

When the trench is very deep and/or expensive backfill materials are required, then a vertical cut at the toe of the slope supported by shoring may be effective.

Ground water and weak lower layers may force partial shoring or flattening of the excavation slopes.

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OPEN-CUT CONSTRUCTION

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Open cut advantages

Allows continuous excavation, laying and backfilling operations. Minor breakdowns usually do not cause delays to all activities. The open trench needs only the design of the cut bank slope.

OSHA guidelines can be used, although cost saving usually will result if the bank slope is checked and designed by a registered engineer for a steeper slope. Sloping the excavation is the simplest method to design and use.

Because there are no additional support operations and equipment, it is the economical choice.

The open trench provides easy access to the work because equipment and construction materials are minimized.

The open cut method is suitable for most ground conditions, except for oozing mud and running sands.

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Open cut disadvantages The slope of the bank requires more excavation

and backfill volume than the other options. The only bank support is the strength of the soil. If

drying, flooding, or change of soil properties weakens the soil, then sloughing and collapse can happen with little or no warning.

The sloped banks require a wider work area. The bank slopes may force the use of larger

equipment because the distance to reach into the trench is increased and a greater volume of soil must be excavated and backfilled.

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Construction Method for Shallow Basement

Open excavation with sloping sides (if constraints of space on the site permits)

Angle of slope nature of soil + moisture content

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Construction Method for Deep Basement Open excavation is not suitable:

Extent of excavation required around the building would be impractical

Risk of affecting ground support for adjacent building High cost for large scale of excavation Existence of high ground water level

Use: excavation with temporary support Excavation supported by permanent retaining walls

embedded in ground

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CUT AND COVER CONSTRUCTION

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The cut and cover construction technique has been used for many years as a means for building underground transportation facilities.

This method involves the installation of temporary walls to support the sides of the excavation, a bracing system, control of ground water, and underpinning of adjacent structures where necessary.

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The main disadvantages of a cut and cover tunnel are its disruptive effects in congested urban environment.

Cost of cut and cover construction increases sharply with increased depth. Tunnel driving costs are usually higher per meter of tunnel than the average shallow cut and cover tunnel.

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Cut and cover tunnel Ramp

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Soldier piles and lagging

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Steel sheet piles

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BOTTOM UP CONSTRUCTION

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1. Form perimeter walls to enclose the area to be excavated

2. Excavation install props excavation

3. Construct foundation+column slab (suspended slab)

4. Proceed upper works: extend column, continue upper slabs towards ground level

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TOP DOWN CONSTRUCTION

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Preferred method: ↑speed, ↓cost, ↑ground stability

Make use of permanent elements to provide rigidity and bracing.

Not required temporary props

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Overall construction method: typically top down, bottom upwards or semi top-down.

Wall method. (temporary, permanent or both) king post walls steel sheet piling soil mix columns r.c. contiguous piles r.c. secant piles (hard/hard, hard/soft) diaphragm walls

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Wall selection

King post walls. Timber laggings allowed to slide downwards between king posts. Poor tolerances, loss of ground.

Sheet piling. poor tolerances, Split clutches due to obstructions.

Contiguous piles, secant piles. wasting of pile shafts. Overbreak.

Diaphragm walls. Slurry inclusions, poor joints, poor cover to steel,bleed, poor tolerances, inclusions at toe of walls.

IZATUL LAILI JABARContiguous Piled Wall - 750mm diameter at 900mm Centres

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Plunge Columns - Sequence

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IZATUL LAILI JABARrig boring wall piles Secant Walls and Columns

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Diaphragm Wall Excavation

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Reinforcing Cage Installation

IZATUL LAILI JABARGuide wall Construction for piling

IZATUL LAILI JABARSchematic of Construction Sequence

for hard secant piled wall

The hard / hard secant piled wall, consisted of 304 piles, 1180mm diameter at 1950mm spacing up to 31m long. The secant piling formed the lower part of the wall, with a new in-situ concrete retaining wall up to 1m thick built over the top. A steel beam was cast into every third pile designed to extend 7m above the pile cut off level into the existing basement. Short flat jack props were placed horizontally in the narrow gap between the beams and the existing wall, to aid the eventual demolition of the old wall. During the development of the site, six existing piles from the previous structure to occupy the site were incorporated into the wall. Water tightness was ensured between these and the new piles by injection grouting.

IZATUL LAILI JABARCompleted Hard / Hard Secant Piled Wall

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1. Formation of permanent retaining walls to the perimeter of basement area.

2. Form internal foundation and column support by using plunge method.

3. Create the uppermost floor slab (with temporary base)

4. Excavate downwards tru openings on slab or service shafts

5. Remove temp. base, excavate until the depth of next floor level.

6. Cast the next floor level, repeat the process.

7. Can use non-suspended slab for the basement slab.

Top-Down Construction

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Typical sequence of top down construction

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2) Install bearing piles with plunge columns

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3) Cast ground floor slab

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4) Excavate and cast upperbasement floor slab.

5: Extend columns and castfirst floor slab

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6: Excavate and cast middlebasement slab

7: Extend columns and castsecond floor slab (notshown)

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8: Excavate and cast lowerbasement slab.

9: Extend columns and castupper floor slabs (notshown).

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Top-down excavation for basementlevels 1 and 2

The existing massive piled raft at newbasement 2 level was broken up with explosive

charges at night and cleared during the day

Excavation at basement 3 level finallyincluded breaking up existing large-diameter piles that were part of

the observational method contingency arrangements