What Is Foundation Retrofit?

Foundation retrofit (strengthening) is the improvement or increase of the load-carrying capacity and stability of the foundations of a completed building, without demolishing it. It is usually carried out when the existing foundations have problems — insufficient capacity or settlement — or when greater resistance to actions such as earthquake forces is required.

Unlike new construction that starts from scratch, foundation retrofit works with an existing structure, so it is constrained by limited working space, the loads the building is already carrying and disruption to building use. This kind of work therefore requires especially careful planning and engineering design.

The key point to understand is that foundation retrofit cannot be assessed by eye or done to a standard template, because every building has a different cause of the problem, different soil conditions and a different structural condition. Diagnosing the true cause before taking action is the heart of this work.

Related: If your building sits on soft soil and you are concerned about earthquakes, read more in Earthquake-resistant foundations on soft soil, and learn about liquefaction in Earthquakes, liquefaction and their effects on foundations

When Should Foundation Retrofit Be Considered?

Not every crack or every building after an earthquake needs foundation strengthening — the decision must come from an engineering inspection. However, the following signs are good reasons to have an engineer carry out an assessment:

  • Differential settlement: sloping floors, doors and windows that no longer close properly, or diagonal cracks at the corners of openings may indicate a foundation problem
  • Structural cracks after an earthquake: particularly cracks in columns, beams or structural joints, which differ from ordinary plaster cracks
  • Need for increased loading: such as adding storeys, changing the building's use or installing more machinery, which may exceed the capacity of the existing foundations
  • Older buildings on soft soil in earthquake-prone areas: which may have been built before seismic design requirements existed, so their risk should be assessed

A word of caution: cracks and settlement have many causes, and some are not related to the foundations at all. Whether foundation strengthening is needed must always be decided after inspection and diagnosis by an engineer.

Related: Learn how to spot foundation problems in 5 signs of foundation problems — check before it's too late

What Foundation Retrofit Methods Are There?

There are many foundation retrofit techniques. The choice depends on the cause of the problem, the soil conditions, the structural condition and site constraints. In principle, the common methods are:

  • Adding supplementary piles (micropiles / small-diameter bored piles): small piles are installed to transfer load down to a stronger soil layer. Well suited to confined spaces and retrofitting existing buildings, because small equipment can get in to do the work
  • Enlarging or deepening existing footings (footing enlargement / underpinning): increasing the size or depth of the footing to spread the load into more soil
  • Improving the soil beneath the foundation (ground improvement): for example by grout injection to strengthen the soil and reduce further settlement
  • Strengthening the superstructure as well: in some cases the problem is not only in the foundations, and columns, beams or joints may need to be strengthened at the same time

The details of each method — size, number, depth and location — must be designed and calculated by an engineer from actual building and soil data. This article explains the principles only and cannot replace project-specific design.

Cross-section of underpinning an existing foundation with micropiles, with hydraulic jacks to transfer the load The left image is a section beneath a building. A building column carries load down to the existing footing, which sits in soft clay or fill. A working pit has been dug around the footing. Micropiles are drilled close to both sides of the footing, through the soft layer down to stiff soil or dense sand. Steel brackets are fixed to the sides of the existing footing, and hydraulic jacks are placed between the pile heads and the brackets to transfer the building load from the existing footing to the new piles. Arrows show the load path down to the pile tips. The right panel shows that a small drilling rig can work in confined spaces with low headroom inside the building, along with a three-step work sequence. Underpinning an existing footing with micropiles Not to scale Building load Fill Soft soil Low capacity Settles Stiff soil / dense sand Old column Old footing Hydraulic jack Load to piles Micropile Drilled beside footing Bracket Working pit Works in tight spaces Ceiling Low Small drill rig fits inside buildings 1 Drill micropiles at footing 2 Fit bracket + jack 3 Transfer load to piles Micropile size, number and depth designed by an engineer from site data
Principle of foundation strengthening with micropiles: small piles are drilled beside the existing footing through the soft soil layer down to stiff soil, then hydraulic jacks transfer the building load from the existing footing to the new piles. Suitable for confined spaces because small equipment is used. Size and number must be designed by an engineer.
Caution: Never specify the type, size or number of supplementary piles yourself without an engineer. Incorrect strengthening can worsen differential settlement or create new problems that are harder to fix than the original.

What Must Be Inspected or Tested Before Foundation Retrofit?

The heart of foundation retrofit is finding the true cause before taking action. Inspection and testing let the engineer see what cannot be seen, such as the piles and soil layers underground. The data typically needed includes:

  • Structural condition and crack survey: assess the pattern, location and severity of cracks, track whether they are still widening (Crack Monitoring), and measure building settlement and tilt with Settlement Points and Tiltmeters
  • Testing the length and integrity of existing piles: if the pile heads are still accessible, use the Seismic Integrity Test. In most existing buildings, however, footings or structures already cover the pile heads, so Parallel Seismic is usually used: a borehole is drilled beside the pile, the structure above is struck, and the waves are measured in the borehole to find the length and any defects of the existing pile (see a real project in Case Study: Parallel Seismic at a factory in Samut Prakan)
  • Non-destructive structural evaluation (NDE): assess concrete quality, reinforcement and internal damage without breaking anything. See the service at Non-destructive structural evaluation
  • Soil data: soil investigation provides the soil parameters needed to correctly design supplementary piles or ground improvement

Data from these inspections lets the engineer design the strengthening precisely, fixing the real cause rather than the symptoms, which reduces the risk of the problem returning.

The 3D view below brings the four main inspections together in a single building and summarizes which question each data set answers before the strengthening method is chosen.

3D model of the inspections and tests to carry out before designing foundation retrofit for an existing building A conceptual 3D view of an existing single-storey column-and-beam building, with the front cut away to show the existing footings and piles underground. The front wall has a diagonal crack from the corner of a window, fitted with a crack-measuring gauge. On the right, a level on a tripod sights onto a settlement marker on a column. A non-destructive testing instrument is placed against the left column. Beside the right pile is a borehole for the Parallel Seismic test, with receivers arranged in a line deeper than the pile tip, and the base of the column is being struck. At the back right is a soil boring rig. The right panel summarizes what each data set answers: is the problem still developing; are the concrete and reinforcement still in good condition; how long is the existing pile and is it intact; and how much load can the soil carry — before diagnosing the cause and then designing the strengthening method. 3D model: what to inspect before foundation retrofit Not to scale 1 Cracks + settlement/tilt Crack gauge · level 4 Soil investigation New soil layers & data 2 NDE columns–beams Concrete + rebar 3 Parallel Seismic Existing pile length/integrity What each data set answers 1 Still developing? Track cracks + levels 2 Concrete/steel OK? UPV · GPR · Rebound 3 Pile length/intact? Parallel Seismic works under caps 4 Soil capacity? Boring results + SPT Diagnose cause first then the engineer designs a fix that targets the root cause Conceptual view, not to scale
3D model — ① crack gauges and a level track whether the problem is still developing ② NDE checks the concrete and reinforcement of columns and beams ③ Parallel Seismic: a borehole beside the existing pile measures waves from striking the column base, and works even when a footing covers the pile head ④ soil boring determines the soil layers and new parameters. The right panel summarizes the question each data set answers. Conceptual view (Image: SPN Soil Engineering)

How Do You Start, and How Can SPN Help?

If your building shows signs of foundation problems or has been affected by an earthquake, the recommended first steps are:

  1. Have an engineer inspect the actual condition: assess cracks, settlement and the overall structural condition to diagnose what is causing the problem
  2. Test and collect the necessary data: such as pile integrity testing, structural inspection or soil investigation, as the engineer considers appropriate
  3. Design a case-specific strengthening method: the engineer designs and prepares calculations from actual data, with certification
  4. Carry out construction with inspection during the works: with quality control to match the design, such as load testing the supplementary piles as specified by the engineer (read Do micropiles need testing?), and monitoring settlement after strengthening

SPN Soil Engineering provides full geotechnical engineering services, including soil investigation and testing, pile testing, non-destructive structural evaluation and geotechnical engineering design, by a team of licensed engineers — helping from data collection and diagnosis of the cause through to designing the strengthening approach that suits your building.

For consultation or a quotation, call +66 64 746 9601 or LINE @spnth. Free quotation within 24 hours (fees depend on the scope of work and exclude travel and equipment mobilization costs, which are charged extra depending on the distance to site).

📌 Disclaimer: This article provides general information and is not design advice for any specific project. The method, size and details of foundation strengthening must be inspected, designed and certified by the responsible engineer based on actual site data.