Underpinning 2m6n1s

UNDERPINNING

DEFINITION 

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Process of excavating under an existing foundation and building up a new ing structure, from a lower level to the underside of the existing foundation. Reason for underpinning works: To transfer the load carried by an existing foundation from its present bearing level to a new level at a lower depth. A very expensive and high risk maintenance works.

Reasons for Underpinning Works:

1. Foundation settlement happened due to: i. Uneven loading ii. Unequal resistance of the subsoil iii. Action of tree roots iv. Action of subsoil water v. Cohesive soil settlement vi. Deterioration of foundation concrete 2. To increase the load bearing capacity of a foundation, which may be required to enable an extra storey to be added to the existing structure or if a change of use would increase the imposed loading. 3. As a preliminary operation to lowering the adjacent ground level when constructing a basement at a lower level than the existing foundations of the ading structure when laying deep services near to or below the existing foundations.   

To enable excavation works to the adjacent buildings; which deeper than level of the buildings.

 

Site survey and preliminary works before underpinning works: 1.

Notice to the ading owners. Explain in detail the intention to do the underpinning works and giving full details of any proposed temporary s such as shoring.

  2. Survey of the building to be underpinned. Record any defects, cracks, supplemented by photographs and agreed with the building owner where possible. 3. Take photo of building to be underpinned as a proof.

4. Record all the buildings movement with theodolite/levels etc.

Site survey and preliminary works before underpinning works: (cont’d) 5. Survey on required temporary while doing underpinning works such as shoring works. 6. Survey on soil . Make sure that the new load impose is on the suitable strength and to determined the suitable types of underpinning work to be done. 7. Take notes on safety control for public, workers and public’s properties.

Site survey and preliminary works before underpinning works: (cont’d) 8. Bring down as much as imposed load to the building and provide temporary where necessary. 9. Determine servicing pipe to the building. Make sure it will not be interrupted or contractor have to provide alternative way for the servicing pipe. 10. Get a permission and give notice to Local Authority on proposed underpinning works.

BUILDING BY LAW 1984 ( Clause 90 Parts IV) If underpinning works is required, owner or the agents must: 1. Get a written sanctions from Local Authority before start the underpinning works. 2. Comply with all the clause stated in the Building By Law. 3. Give a written notice to Local Authority inform how and what underpinning works will be done.

Types of Underpinning:-

1. Continuous / Traditional Underpinning

2. Pre-test Method Underpinning 3. Miga @ jack Underpinning 4. Bore Pile Underpinning 5. Needle and pile Underpinning 6. Radial Piling Underpinning 7. Pynford Stooling Underpinning

CONTINOUS /TRADITIONAL UNDERPINNING

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Continuous/ Traditional underpinning can be use to replace strip foundation only. Either the width of foundation is ½ B or 1B.

CONTINOUS/TRADITIONAL UNDERPINNING

Sequence works of Continuous Underpinning:

1. Divide wall to 5 bays. E.g.: 7.50 meter/5 part = 1.50 meter each part. 2. Number each bays start from the left side. (1,2,3,4,5) 3. Excavation works will be done stage by stage to the sides and under existing foundation. 4. Timbering must be done at excavation area as a safety precaution especially to the workers. 5. Excavation will start with parts No.3. 6. Excavate till to the new level required.

Sequence works of Continuous Underpinning: 6. Construct new foundation with the length of 1.50 meter in excavation/working space area No.3. 7. Followed with construct new wall with the length of 1.50 meter on the new foundation. Make sure that the wall is close enough to existing foundation. 8. Fill the gap between existing foundation and new wall with ‘final pinning’ - mortar with ratio 1:3. 9. Repeat this method to working space No.5, followed by No.1, No.4 and lastly No.2. 10.End of new wall must be construct with ‘tooth’ to bond the wall with another new wall .

Sequence Works of Continuous Underpinning:

11. End of new foundation must have dowel bars and toothed to produce strength and neat connection between new foundation. 12. DPM can be install in underpinning works if required. 13. All excavation works must be filling back in stages. 14. It is recommended that parts of excavation works/ working area to be underpinned must not more than 6 part. 15. All part/ working area with the same No. can be excavate in one time.

Continuous Underpinning

Continuous Underpinning

Factors on determine the numbers and length of the working space: 1. Length of wall to be . 2. Width of existing foundation. 3. Condition of existing foundation. 4. Load imposed to the existing foundation. 5. Estimated ability length for existing foundation. 6. Soil condition under existing foundation.

Attention/ Precaution on construction of Continuous Underpinning: 1. Maximum length of foundation or wall floating in one time is only 1.20 meter – 1.50 meter. 2. Total length of floating foundation must be not more than 1.40 meter from total length of foundation or wall to be underpinned. 3. Floating area supposedly not more than 1/5-1/6 from total length in one time if existing underpinning is too weak or broken or load density is too big. 4. Buildings with low density load e.g.: double storey house or buildings with good condition wall, floating area permitted is 1/3 from total length of the wall.

PRE-TEST METHOD UNDERPINNING 

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This method is can be used for strip and pad foundation. It’s suitable for building which increase the load bearing capacity to the building. E.g. increased storey from 5 to 10 storey. Subsoil in new excavation level will be compact and compress to gave a predetermined loads to the soil before underpinning works done.

PRE-TEST METHOD UNDERPINNING

Sequence works for Pre-test Method Underpinning: 1. Underpinning for strip foundation will be done stage by stage as in Continuous Underpinning. 2. Construct reinforced concrete strip foundation with appropriate thickness to allow tension and compaction in foundation. 3. For pad foundation, excavation will be done under existing pad foundation. 4. New pad foundation will be construct in pit.

Sequence works for Pre-test Method Underpinning: 5. When new foundation has matured, jack hydraulic will be put on the top of the new foundation. 6. Then, precast concrete beam will be arrange on top of the jack. 7. Fast hardening concrete is laid between the precast concrete beam . 8. Before it has finally hardened, set the jack. 9. Precast concrete beam and half hardened concrete will be jack up to the existing foundation. All pores between concrete will be compacted and fill with half hardened concrete

Sequence works for Pre-test Method Underpinning: 10. When concrete have matured, jack beam and concrete one more time to the new level. It is extended to give a predetermined load on the new foundation, thus pretesting the soil beneath. 11.Underpinning material such as engineering bricks, concrete blocks and steel beam will be put in the hollow space as a to precast concrete beam before remove jack. 12. Remove jack, and fill the hollow space with underpinning material such as bricks or concretes.

MIGA @ JACK UNDERPINNING 1. This

method is suitable for strip and pad foundation.

2. Suitable for subsoil with deep bearing capacity which is not practical to use traditional wall underpinning. 3. Using jack in underpinning works can avoid deep excavation. 4. No vibration and noise in using jack which will not disturb nearest building or environment.

MIGA @ JACK UNDERPINNING

5.Can be use in as short as 1.80m depth working space 6.Existing foundation supposed to be in good condition to receive piles. At the end it will act as a pile cap to the piles. 7.The condition and ability of the existing strip foundation will determine the spacing of the pile.

MIGA @ JACK UNDERPINNING

Sequence works for Miga @ Jack Underpinning:

1. For strip foundation, excavation works is done as per continuous underpinning. Excavate bay by bay with 1.80m depth working space. 2. For pad foundation, excavation is done under existing foundation. 3. Hydraulic jack will be use to jacked precast concrete pile into the suitable subsoil. 4. Short Precast concrete pile with pointed steel toe , size of 300x300x600mm will be jack as a first section (initial pile).

Sequence works for Miga @ Jack Underpinning:

5. Followed with extension piles with the size 300x300x750mm . 6. The pile is jacked until the pile has reached the required depth. 7. Space between the top of the pile and underside of the existing foundation is fill with a ‘final pinning’ concrete, with a ratio 1:3. 

 

BORE PILE UNDERPINNING

BORE PILE UNDERPINNING

BORE PILE UNDERPINNING 1 Suitable method to replace existing foundation with piling. 2. Simple method of construction which include excavation, steel casing, reinforcement and concrete works. 3. Bore pile is most suitable piling work for underpinning because piling rig can be use in small and limited space especially under existing foundation. 4. It can be used in space as low as 1.80 meter. 5. Quite and not produce a lot of vibration. 6. The best diameter pile for the underpinning work is 300mm-950mm. 7. It will be done stage by stage. .

Sequence works for Bore Pile Underpinning:

1. For strip foundation, excavation works is done as per continuous underpinning. Excavation is done bay by bay. 2. For pad foundation, excavation is done under existing foundation. 3. Allow 1.80 meter depth for working space. 4. Locate ‘ Rig pile’ in working space where as it functioning as pit excavator. 5. Use steel casing to prevent soil from fall and fill the excavation pit.

Sequence works for Bore Pile Underpinning: 5. Excavate to suitable depth and arrange reinforcement in the excavation pit. 6. lastly, pour concrete to the pit and compact the concrete with air. 7. Steel casing will be bring out, one by one. 8. The best bored pile is the pile with toothing around . It helps pile to bond strongly and neatly with soil. 9. It is better to make the lower part of the bore pile bigger than usual to the pile.

NEEDLE & PILE UNDERPINNING

NEEDLE AND PILE UNDERPINNING 

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Needle and pile is use if the wall to be underpinned has a weak foundation that is considered unsuitable for spanning over the heads of jack piles. This method uses pairs of jacks or usually bored piles in conjunction with an in-situ reinforced concrete beam or needle place above the existing foundation.

NEEDLE AND PILE UNDERPINNING 

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The system works on the same principles as a dead shoring arrangement relying on the arching effect of bonded brickwork. If water is encountered when using bored piles a pressure pile can be used as an alternative. There are two ways of arrangements to enable the work to be carried out. Either from both sides of the wall or from the external face only.

Sequence works for Needle and Pile Underpinning:

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The needle and pile underpinning is referring to the arrangement of pile in underpinning works. If using jack, the sequence works is same as miga @ jack underpinning. If insitu pile, the sequence works is same as done in bored piles.

NEEDLE AND PILE

RADIAL/ ANGLE PILED UNDERPINNING

RADIAL/ ANGLE PILED UNDERPINNING 

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Low cost modern alternative to traditional underpinning for stabilizing existing sub structural walls and foundations. The pile is installed by using air flushed rotary percussion drill from inside and outside of the building. Spacing and depth of boring will depend on site conditions such as the occurrence of solid bearing strata and extent of structural damage. Although disruption to the building’s interior is most likely, the relatively short timescale and minimal excavation make for an economic process.

RADIAL/ ANGLE PILED UNDERPINNING 

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Although disruption to the building’s interior is most likely, the relatively short timescale and minimal excavation make for an economic process. The suitable pile diameter to be used in this underpinning works is between 120mm – 280mm.

PYNFORD STOOLING

PYNFORD STOOLING 

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This method is only suitable for strip foundation. It’s enable walls to be underpinned in continuous runs without the use of needles or raking shoring. The procedure is to cut away portions of brickwork, above the existing foundation to enable precast concrete stool to be inserted and pinned.

Sequence works of Pynford Stooling Underpinning: 1. Cut holes in brickwork in the upper part of existing foundation. This work will be done alternately . 2. Stools or precast concrete beam positioned and pinned in holes cut. 3. Remove brickworks between stools. 4. Fabricated and positioned reinforcement around stools till it be ring beam reinforcement. 5. Erect formwork to cast concrete beam along the underpinning works.

Sequence works of Pynford Stooling Underpinning 5. Removed formwork after concrete matured. 6. Do final pinning with mortar ratio 1:3 where necessary.   *** Steel stool – Heavy loads PCC stool –Light loads