Underpinning refers to the strengthening and repair of foundations. This article will discuss the various methods and procedures for underpinning foundations. Sometimes, a foundation or footing failure can occur unexpectedly after completion of the entire structure (sub and superstructure). In such situations, it is necessary to suggest a remedy to restore structural stability.

Underpinning contractor in los angeles is a method that strengthens the foundation of an existing structure or other infrastructure. These are temporary or permanent support that is installed to increase the foundation’s bearing capacity and depth.

A Selection of Underpinning Techniques

The type of work involved and the age of the structure are factors that influence which underpinning method is chosen.

Structure categories according to its age:

  • Antiquities: Exceeding 150 Years
  • Recent Structures: Age Between 50 and 150 Years
  • Modern Structure: Less than 50 Years

Selection of underpinning methods for different types of works:

Conversion Works

To convert the structure to another purpose, it will need a stronger foundation than the one that is already in place.

Protection Works

These are the problems that a building must be protected from:

  • The foundation that is currently in place is weak and not stable.
  • The soil supporting existing footing would be affected by excavations nearby.
  • Stabilization of foundation soil to resist natural calamities
  • Basement required below an existing structure

Remedial Works

  • Subsidence of the structure was caused by mistakes in foundation design
  • Instead of building a brand new structure, you can work on the existing structure.

Structural Conditions that Require Underpinning

Engineers may recommend an underpinning method to stabilize the substructure for many reasons.

  • Settlement could be caused by the degrading of timber piles that were used as foundations for normal buildings. Water table fluctuations are responsible for the destruction of structures.
  • The soil’s bearing capacity can be affected by changes in the water table, which can lead to the soil’s ability to absorb more water and cause the structure to settle.
  • Settlement could result from structures built on soil that is not designed for its bearing capacity.

Underpinning

Based on observations, the decision to underpin can be made. It is important to take vertical readings at the offset level and to determine if structures are showing signs of settlement or distress. The severity of the settlement will determine the time frame. Before digging for any new project, professionals must examine the soil and determine its resistance to the structure being built over it. The report will determine if underpinning is necessary. Sometimes, such tests could be used to avoid the need for underpinning after the entire structure has been constructed.

Methods for Underpinning

These are the various foundation strengthening methods that can be used underpinning:

  • Mass concrete underpinning method (pit method)
  • Cantilever needle beam underpinning
  • Method of underpinning beams and piers
  • Mini-piled underpinning
  • Pile method for underpinning
  • Method of pre-testing underpinning

No matter what type of underpinning method is used to strengthen the foundation, they all follow the same idea: extend the foundation lengthwise or widthwise and then lay it over a stronger soil layer. This allows for a more even distribution of load. In the following sections, we will briefly discuss different underpinning techniques. The ground conditions and foundation depth will determine the method that is best for you.

  • Mass Concrete Underpinning Method (Pit Method)

The traditional method of concrete underpinning is mass concrete underpinning. It has been used for centuries. This involves the extension of an old foundation until it reaches a stable layer. Through pins or stages, the soil beneath the foundation is excavated. Once strata are reached, the excavation can be filled with concrete. This is done before the next excavation begins. A dry sand-cement packet is used to transfer the load to the new foundation. This method is suitable for shallow foundations as it is low-cost.

Other superior options are available for more complex foundation problems.

  • Cantilever Needle Beam Method for Underpinning

Figure 2 shows the arrangement of cantilever-pit method of underpinning. This is an extension to pit method. This method can be used to underpin if the foundation is only extended to one side or if the plan has a stronger interior column.

Advantages to Cantilever Needle Beam:

  • It is faster than the traditional method
  • Only one side access
  • Capacity to carry heavy loads

Disadvantages:

  • When the foundation is already deep, digging can be uneconomical.
  • Access restrictions limit needle beam use

Method of Pier and Beam Underpinning

This method is also known as the base and beam method, which was developed after the second world War. Because the mass concrete method was not able to work for deep foundations, this method has been successful. It works for almost all ground conditions. Reinforced concrete beams are used to transfer the load from reinforced concrete beams to mass concrete bases and piers, as shown in figure 2. Based on ground conditions and loads, the beams will be sized and deepened accordingly. It is economical to dig below 6m.

Mini Piled Underpinning

This can be used when the foundation’s loads have to be transferred to strata at a distance of more than 5m. This method can be used for soil with variable characteristics, but it is not suitable for access that is restricted and may cause environmental pollution problems. Driven piles with a diameter of 150-300mm are available. They can be augured or driven steel-cased.

Pile Method of underpinning

This method involves driving piles on the walls that support the weak foundation. As shown in figure-3, a needle or pin is used to penetrate the wall. These needles act like pile caps. This method can be used to treat soil settlement due to clayey or water clogging.

Method of underpinning that has been pre-tested

It can be used for pad or strip foundation. It can be used to build buildings with 5-10 stories. This is where the subsoil can be compacted and compressed in an excavation that provides predetermined loads to soil. This is done prior to underpinning. This will result in less noise and disruption. This method is not suitable for raft foundation.