Underpinning Methods, Procedure, Use in Foundation Strengthening and Repair

Underpinning is a sensitive construction technique for strengthening an existing foundation or placing a new foundation below the old foundation to a deeper depth.

 Underpinning is a sensitive repair project so, we have to choose the correct method for it. For the correct method, we have to understand and judge the soil strata, the current situation, and problems concerning the entire foundation, the required depth, and extent of the new foundation.

 In other words, underpinning is the foundation repair technique that can is Done for transfer of foundation existing bearing label roll to a new level a  lower depth a week vulnerable   Foundation can be also replaced by underpinning technique. 

Purpose of Underpinning

  Underpinning is done for the following purposes:

• For the purpose of leading an old shallow foundation to the deeper depth when the adjoining building is constructed with a deep foundation.
•   Underpinning is done for building a basement in the existing building.
•   Underpinning is done to deepen the existing foundation (resting on poor strata) and make it rest on deeper soil strata of higher bearing capacity.
•  To strengthen a settled foundation that may be caused by cracks in the wall. 

 Preparation before underpinning:

• It is necessary to notify the adjoining or not about the proposed works with details typical action about the use underpinning sorry for tying.
• bus site and its building or restricted areas should your mind and carry out a survey.   at that time and the effects of cracks are recorded and notified to the adjoining owner(s).
•  before starting any under the reason for settlement and its solutions are determined, it is done in the case where the reason for underpinning is the settlement.
• we have Meri on to reduce the load of the structure by removing imposed loads from the floors,  reducing the unwanted dead loads, and is saved by requiring props and/or shoring only after that the underpinning works should start.
• If there is any for a portion of the area which is vulnerable by proposed underpinning works should be supported and protected by identifying, tracking, and marking or exposing.

Necessary of underpinning

1. Uneven settlement is caused by the unsymmetrical loading of the building, unequal bearing capacity of all the soil beneath the foundation, By the action of tree roots, or primary or secondary settlements consolidation cohesive soil.
2.  Increase in loading: the loading process of building man changed due to the addition of more story or change in the imposed loading due to change in service loading.
3. Lowering of adjacent ground: Nearby Foundation you stop to work available then it is necessary to lower the foundation of the building.

Methods of Underpinning

Underpinning can be carried but by the following methods:

1. Pit method                
2.  Pile methods
3. Underpinning to walls
4. Jack pile underpinning
5. Needle and pile underpinning
6. ‘Pynford’ Stool method of underpinning
7. Root pile or angle PilingUnderpinning columns

Pit method

In this method, the entire length of the foundation to be underpinned is divided into sections of 1.2 to 1.5 m lengths. One section is taken up at a time.

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First of all, a hole is made in the wall for all divided sections above the plinth level, and needle is inserted in the hole. The needle may be made of a material of stout, timber or steel section.

Bearing plates are placed above the needle to support the masonry above it. The supporting arrangement of the needle is made by crib supports (wooden blocks) on both sides of the wall and screw jacks.

Watch this Video for detail about Screw jacks in Underpinning:

After that, a foundation pit is excavated up to the required level of new foundation. Then the new foundation is laid in the pit. When the work of one section is over, work on next to next section is taken up, i.e., alternate sections are underpinned in the first round, and then the remaining sections are taken up. Fig2. above shows the section.

Some important precautions should be taken during the work, such as raking shores are provided in week wall, floors are also supported.

In this process cantilever needle beams may be used when a strong interior column is there, or if there is a required foundation increasing in only one side, as shown in Fig. 3.

The following points are to be considered in the pit method:

1. Alternate sections are taken up in the first round.  The remaining intermediate sections are then taken up. Only one section should be taken at a time.

• It is better to start the work from the middle in the case of longwall extended in both directions.

• If the new foundation is deeper, proper timbering of the foundation trench may be done.

• When the foundation has gained full strength then only all the arrangements like needle beams etc. should be removed slowly.
• The needle holes etc. should be closed in masonry using cement mortar.

Pile method

In the Pile method of underpinning, as the name suggests Piles are installed by proper driving technique along both sides of the wall to be strengthened.  The piling technique normally used are borehole pile on under-reamed piles are used.

After that, concrete or steel needles are penetrated through the wall and are connected to the Pile. These needles function is as beams and is act as pile caps also.

Pile method is suitable for clayey soils, waterlogged areas, and also in weak bearing strata. 

Besides from above, followings are the Underpinning Types and Techniques used for different structures:

  Underpinning to walls:

 For underpinning work in walls, the wall should be divided into legs for bays and is bay is treated individually that prevents fracture, damage, or settlement of the walls. 

Following are the factors that affect the length of the legs and bays:

1.  The overall length of the wall.
2.  The dead and live loads on walls.
3.  The bearing capacity and types of soil under the existing Foundation.
4.  The strength and stability of the walls and foundation of the wall which is going to be underpinned.
5. The estimated differential settlement and spinning ability of the existing wall footing.

For mass concrete strip foundations supporting walls of traditional construction, the suitable bay is 1 to 1.5 m, and for moderate loading walls supported by reinforced concrete strip foundations, the bay length is 1.5 To 3 m.

(Note: In all cases, the sum of the unsupported length of wall should not exceed 25% of the total length of the wall)

A typical diagram of Underpinning schedule is shown in the figure4. above: 

Jack pile underpinning

Jack pile underpinning is done where the traditional underpinning is uneconomical because of the depth of suitable bearing capacity of subsoil. The main advantage of Jack Pile and defining is it is vibration-free and flexible because the pile depth can be adjusted to suitable subsoil conditions encountered. In this system, the existing Foundation is span over the heads of the pipe caps which are cast in onto the Jack pile heads after the hydraulic jacks had been removed that Makes the Foundation in good condition. 

The figure below shows the typical details of the Jack pile underpinning.

Needle and pile underpinning

Where the traditional or Jack pile underpinning techniques are unsuitable for the existing Foundation condition, then the needle and pile underpinning method can be used for the best result. As shown in the figure below the big work in this method above the existing Foundation should we in a sound condition. And the piles used are generally in small diameter bored piles. 

‘Pynford’ Stool method of underpinning

Pynford underpinning is used, when the existing foundation’s soil is poor bearing capacity this method of underpinning is suitable and this method makes the needle run continuously to the walls. 

Following are the detailed of this method of underpinning:

Stage1. Holes formed in the wall to receive steel or precast concrete stools.

Stage II. Stools inserted and pinned to the soffit of brickwork over the opening.

Stage II. Brickwork between pined tools removed to leave wall supported on pined stools.

Stage IV. Reinforcement fabricated and placed around pinned stools.

Stage V. Formwork erected and beam cast.

Stage VI.  Formwork removed, beam allowed to cure before being pinned to the underside of the wall.

Root pile or angle Piling

In the root pile underpinning method, there is an application of modern concrete drilling equipment to achieve fabricate concrete that is economical through time-saving. Due to the above reason, this is a simple alternative to traditional underpinning techniques. There is no need for a large volume of excavation, show this message is not a disruptive bulk work comparatively. Lined reinforced concrete piles installed in pairs at opposite angles make the wall stable in where the sound building starter is located not more than 1 to 2 m. In this process the existing floor, wall information is pre-drilled with air flushed percussion auger. Fig below details.

Through this drill hole, steel lining is driving to the low grade/clay subsoil until it impacts with firm strata. In many conditions, it is very difficult to apply angle piling to both sides of a wall. As for the subsoil condition sometimes it is down with the remedial treatment from one side only. For more stability piles are placed relativity with close space. 

Underpinning columns

In the column underpinning first of all the loadings are relieved then it can be underpinned in the same manner as walls using traditional or Jack pile methods. Dead shores are used to transfer the beam load from the column and the column is transferred by means of a pair of beams. 

Fig. Above shows the details about Underpinning Columns.

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