Week 3 - Underpinning

What is Underpinning?

Definition and Process of Underpinning

• Underpinning involves installing support to an existing foundation to enhance its depth or bearing capacity. This process includes excavating beneath the current foundation and constructing a new supporting structure.

• The goal of underpinning is to interrupt the existing foundation while integrating a new support system, ensuring stability during construction activities.

Reasons for Underpinning

• The primary objective of underpinning is to transfer loads from an existing foundation to a deeper level, which can be crucial in cases where the original foundation cannot bear additional weight. This method can also replace costly maintenance works.

• Several conditions may necessitate underpinning:

• Settlement: Uneven loading, changes in water table, or deterioration of concrete foundations can lead to structural settlement requiring underpinning for support.

• Adjacent Construction: New projects with deeper foundations adjacent to existing buildings may require underpinning to ensure stability as ground levels are altered.

• Change in Use: Modifications that increase load-bearing requirements (e.g., adding stories) necessitate underpinning for adequate support.

How Does Soil Affect Underpinning?

Soil Characteristics and Planning

• Understanding soil properties is critical during planning; factors like cohesiveness and compaction affect the need for underpinning. Poorly compacted backfill or unstable soil can lead to settlement issues that undermine structural integrity.

Monitoring Structural Integrity

• To determine if underpinning is necessary, professionals should monitor signs of distress or settlement over time through regular assessments and readings, documenting any changes meticulously for further analysis.

Underpinning: Key Concepts and Procedures

Importance of Monitoring Before Underpinning

• Daily, weekly, or monthly monitoring is essential before excavation for new structures to assess the need for underpinning.

• A professional examination of nearby structures is advisable prior to construction to determine if underpinning is necessary.

Classes of Underpinning

• There are three main classes of underpinning:

• Conversion: Alteration and improvement due to preservation needs.

• Protection: Safeguarding existing structures during new construction.

• Remedial: Addressing sudden structural defects in modern buildings that require immediate attention.

Preliminary Works Required for Underpinning

• Prior to underpinning, scientific and preliminary works must be conducted:

• Notify adjoining owners about the intention and details of underpinning work.

• Record any existing cracks with photographs as a precautionary measure.

Structural Considerations During Underpinning

• The loading on the structure should be minimized; reducing imposed loads is crucial during underpinning operations.

• Ensure servicing pipes are not interrupted; alternative routes may need to be provided for utilities like electricity during the process.

Legal and Regulatory Compliance

• Obtain permission from local authorities before commencing underpinning work, as this step is critical due to potential disruptions caused by construction activities.

Types of Underpinning Methods

• Various methods include:

• Continuous or traditional underpinning.

• Pre-test method underpinning.

• Mega or jack underpinning.

• Ball pile underpinning.

• Needle and pile underpinning.

• Radial piling underpinning.

Detailed Process of Continuous Underpinning

• Continuous or traditional underpinning can only replace straight foundations; it’s ideal when foundation width is appropriate (e.g., half brick wall).

• The process involves ensuring even distribution of load across the ground while maintaining space for workers during operations.

Sequence of Work in Continuous Underpinning

• A comprehensive sequence from steps one through fifteen outlines how continuous underpinning should be executed effectively, ensuring students grasp foundational knowledge on the subject.

Continuous Underpinning Techniques

Importance of Support and Working Space

• The wall requires adequate support, necessitating a large working space to ensure the load-bearing wall can carry the entire load effectively. Precautions must be taken during continuous underpinning construction.

Guidelines for Continuous Underpinning

• The maximum length of foundation or wall that can be unsupported at one time is between 0.9 meters to 1.5 meters, requiring additional shoring for stability. A total of 75% of the wall length should not be unsupported simultaneously.

Floating Area Considerations

• For continuous underpinning, the floating area should not exceed one-sixth of the total length of the wall if existing underpinning is weak or broken. In buildings with low-density loads, such as double-story houses, a floating area up to one-third may be permitted during construction.

Pre-Test Methods in Underpinning

• Typical methods for continuous underpinning foundations are suitable for structures needing increased load-bearing capacity (e.g., increasing stories from five to ten). New excavation levels must be compacted before underpinning begins to ensure soil stability under load conditions.

Construction Sequence and Techniques

• The sequence for constructing underpinning involves several steps that students are encouraged to read and visualize on-site processes. Understanding these sequences is crucial for effective implementation in real-world scenarios.

Jack Underpinning Method

Suitability and Advantages

• Jack underpinning is appropriate for street impact foundations where traditional methods may not suffice due to deep bearing capacities; it minimizes excavation needs while reducing vibrations and noise that could disturb nearby structures or environments.

Installation Process Insights

• Existing foundations must be in good condition to accommodate piles acting as pile caps; proper spacing between installed piles is essential based on existing foundation specifications and anticipated resistance levels during installation.

Precast Pile Installation

Steps in Jack Underpinning

• The process includes installing precast piles beneath existing foundations, ensuring minimal disruption while maintaining structural integrity throughout the operation by following specific sequences tailored for jack underpinning techniques.

Bored Pile Methodology

Characteristics and Benefits

• Bored piling serves as an effective method for replacing existing foundations due to its simple construction process involving excavation, steel casting reinforcement, and concrete work; it operates well within limited spaces (as small as 1.8 meters) without generating significant vibrations during installation.

Sequence Overview

• The sequence of bored piling consists of nine distinct steps that need careful execution; understanding these steps ensures successful implementation while adhering to measurement standards relevant in construction practices today. Students are reminded that grasping basic principles remains vital even if they forget specific sequences over time.

Understanding Needle Empire Underpinning

Overview of Needle Empire Philosophy

• The philosophy of the needle empire is crucial for understanding how it can be applied to underpin existing foundations, particularly when dealing with weak structures.

• It is unsuitable for spanning over areas with significant structural weaknesses, emphasizing the need for careful assessment before application.

Methodology and Components

• The method involves using paired objects or Bopal in conjunction with a reinforced concrete beam (needle) placed above the existing foundation.

• Two arrangements are possible: working from both sides of the wall or solely from the external face, allowing flexibility in execution.

Sequence of Works

• The sequence of works in needle and pine underpinning consists of three main methods that students should visualize to grasp construction technology effectively.

• Synchronizing knowledge delivery with practical calculations on-site is essential for comprehensive understanding.

Radial and Angle Pile Underpinning

Cost-effective Alternatives

• Radial and angle pile underpinning offers a modern, low-cost alternative to traditional methods for stabilizing substructure walls and foundations.

• Installation utilizes air flush rotary percussion drilling from outside the building, minimizing disruption while addressing structural damage.

Economic Considerations

• Although there may be some destruction to interior spaces, this method's short time scale and minimal excavation make it economically viable.

Continuous Foundation Underpinning Techniques

Suitable Applications

• This technique is specifically designed for straight foundations, enabling continuous underpinning without needing needles.

Procedure Details

• The process involves cutting away portions of brickwork above the existing foundation to insert precast concrete stools as integral parts of new beams.

Final Steps in Underpinning Process

Execution Steps

• The procedure includes alternately cutting holes in brickwork above the existing foundation and positioning precast stools securely within these cuts.

Reinforcement Integration

• After positioning stools, reinforcement bars are fabricated around them until they form a complete ring beam structure. Understanding these steps is vital for mastering underpinning techniques.