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Route to Successful Underpinning

Contents:

It is the Association of Specialist Underpinning Contractor's (ASUC) objective to introduce quality standards, standards in health and safety and codes of conduct for the industry. This article is intended as a practical guide for supervising officers and for contractors alike.

The effects of a dramatic rise in the underpinning market in 1989/90 were particularly emphasised by the recession and seized upon as a lifeboat by professionals and contractors alike. Some tried hard to get it right, others realised they were in the market for a quick killing and understandably standards dropped to an all time low. A similar lowering of standards took place in the down-turn of 1983, where again, professionals and contractors normally engaged in other activities turned to underpinning as a source of revenue.

This inflow of non-experts resulted in many projects being either highly over-designed and therefore, significantly more expensive or under-designed leading to failures. There are even examples of schemes which have been completely over-designed without acknowledging the cause of the original problem producing the combination of a very expensive job and subsequent failure.

Following this, we have seen the long awaited publication on the subject "Has your House got Cracks" from the Institution of Civil Engineers and the "Subsidence of Low Rise Buildings" from the Institution of Structural Engineers. Both publications are extremely welcome. However, neither recognises that underpinning is possibly the most difficult construction technique to manage and monitor the quality of workmanship. Nor do they recognise that the underpinning techniques which have been developed and practised by specialist underpinning contractors and consulting engineers are highly advanced and as such, the most cost effective scheme would undoubtedly be designed and constructed by an underpinning specialist.

The progression of activities involved in carrying out an underpinning project may be listed as follows:

Desk Studies

This has been well covered in the publications described above, however, the industry is still suffering from the activities of some contractors and even professionals who are not "expert" in the field. No doubt, this trend will be gradually reversed.

Preliminary Investigation

A proper site investigation should allow the expert to predict the potential for further subsidence. Long term monitoring, the continual repairing of cracks and redecoration work is sometimes a needless expense.

The expert should be in a position to instruct on the necessity for underpinning and repair/decoration work or repair and redecoration work only. Whilst it is acknowledged that monitoring is appropriate in some circumstances, there is a tendency for the "expert" to absolve himself/herself of responsibility by monitoring the property for a period of years. It is ASUC's view that monitoring is generally required only when the site investigation indicates that movement is unlikely to occur.

Another common problem with the investigation phase of the work is the method of borehole construction and the limit in number of boreholes and their depth. For example, the ground conditions themselves would determine the depth of a hand augured hole, i.e. unstable ground will collapse and small obstructions will prevent progress. The majority of underpinning schemes which run over budget do so unnecessarily and often due to the lack of site investigation information. This point is made abundantly clear in the Institution of Structural Engineers publication. The interpretation of information is very much the job of the "expert", however, it is often the case that there is not sufficient information to interpret rather than the information misinterpreted.

Design

Here skills of the specialist underpinning engineers and contractors should be sought. Such specialists are designing various types of underpinning schemes on a daily basis. They are aware of the limitations of available equipment and will inherently produce the most cost effective scheme.

Contract Award

Notwithstanding the above, there are still too many contracts awarded purely on price without taking into consideration the potential for additional costs, other associated problems such as noise, vibrations, and - most importantly - the likelihood that the scheme will work. The attitude that the contractor is "guaranteeing it anyway", is all too often used by the "expert" as a means of absolving his/her own responsibility.

Workmanship

Together with other forms of groundwork, underpinning works are hidden from view almost immediately after their completion, and unlike all other forms of construction are not available for inspection for any significant period of time. Underpinning, however, unlike other forms of groundwork tends to be carried out in much small sections and those sections tend to be backfilled and covered over immediately. There is simply not sufficient money available for the supervising officer to remain on site permanently, or indeed inspect on a daily basis. The expert must therefore rely on his contractor to carry out his own supervision work and to employ workmen, equipment and systems which inherently generate good workmanship.

A BS5750 accreditation is, in part, a move in the right direction, however, it does not necessarily guarantee a workable scheme.

ASUC has set out some areas which the supervising officer should be particularly aware of. Many problems stem from failing to realise the original cause of the subsidence problems. This often occurs more with limited traditional schemes as they are looked upon as "low technology, cannot go wrong" and often more complete high technology schemes are given more thought.

The three most important planks of a successful underpinning project are firstly to establish the cause of subsidence, secondly, to confirm that cause while carrying out work on site and thirdly, to confirm that the work being carried out is addressing the cause of the problem. These three stages are rarely consciously acknowledged.

On site, the more dramatic failures are normally as a result of taking out too much support. Support removal and substitution should be clearly identified in the contractors' proposals, and reliance should not be placed on the initiative of the site foreman.

When involved in underpinning work the following should be considered in addition to normal constructional factors:

Traditional Excavated Underpinning (no beams)

  1. Failure to identify the cause of subsidence and placing bases to a specified (arbitrary) depth.

  2. Base sizes inadequate therefore cause over-stressing and settlement.

  3. Base left exposed before concreting (only a matter of hours can cause severe softening in highly sensitive clays) can cause settlement problems.

  4. Underside of foundations not cleaned, leaving a soft layer between underpinning and underside of existing foundation.

  5. Dry packing is very critical, rarely supervised and often the cause of subsequent failure.

  6. Failure to identify potential heave problems and therefore the provision of anti-heave precautions. Also inadequate fixing of heave precautions.

  7. Pinning up to loose, damaged, or decayed footings.

Traditional Underpinning with Beams

The above, but include for associated quality problems with concrete. Site concrete materials have been known to come into contact with waste materials. Proper storage of materials and careful mixing is essential.

  1. Cover to reinforcement (steel is not easy to fix or inspect).
  2. Arbitrary moving of bases to overcome local problems leading to excessive spans and the over-stressing of bases and beams.
  3. Collapse of void formers below beam.
  4. Failure to isolate support stools in anti-heave scheme
  5. Use of "standard" beam designs can lead to inadequate design.
  6. Proper consideration of realistic applied loads is essential to ensure a properly designed scheme both in terms of providing adequate load-bearing capacity and preventing over-design.

Piled Schemes

Normally piling works on main contracting sites are subject to routine inspections by a resident engineer, clerk of works, building inspector and other supervising officers. On underpinning sites, with the exception of the building inspector, there is no enforced supervision regime except for that which the contractor imposes on himself/herself. Mini piling rigs can now be hired easily and literally anybody can call themselves a piling specialist. It does not need much imagination to realise the potential of problems that can be caused as a result of this ignorance particularly where the supervising "expert" is not a piling specialist.

Bored Pile Scheme

  1. Inadequate design or more usually inadequate design revisions on site.

  2. Inability to identify cause of subsidence and therefore inadequate design.

  3. Failure to confirm ground conditions, cause of subsidence and ability of ground to support pile scheme proposed.

  4. Pile integrity (particularly small diameter piles), will not be noticed by inexperienced crew or supervising officer.

  5. Weakened concrete with water inflow (rarely noticed by inexperienced contractors).

  6. Shallow pile depth a common problem if crew are financially incentified and supervision is poor.

  7. Inadequate provision of reinforcement or slip membranes in heaving ground.

Driven piles

  1. Reliance on the fact that "a set" is "a set" is "a set" is "a set", regardless of ground conditions. Ground conditions must be taken into consideration when designing a driven pile - often they are ignored!

  2. Driven piles being used in heaving ground. Unless proper precautions are taken the will fail.

  3. Steel casings, finishing within concrete footing, thereby leaving unbonded section.

  4. Failure to carry out check sets particularly where pore water pressure is a significant factor. When using pneumatic hammers a small drop-off in air pressure can lead to dramatic reductions capacity. Air pressures and flows are rarely checked.

  5. The belief, by some contractors, that the old Cornfield formula is conservative and therefore the use of a "lighter drop weight will do", the supervising officer is given the impression that the weight is heavier than it actually is. Drop weight should be checked and the mass calculated.

  6. Small diameter mini piles may very easily set on an obstruction or within consolidating made-up ground. This fact is rarely checked.

  7. Damage to the building itself or adjacent buildings through vibration related problems.

Piled Rafts

  1. Over use of standard details and therefore overlooking more highly loaded areas.

  2. Collapsing of any void forming materials.

  3. Failure to properly isolate support jacks.

  4. Failure to identify locations of support jacks and to subsequently remove them.

  5. The usual steel/concrete quality problems associated in all RC work.

  6. With a nibbed scheme the size of the nibs can often create over-stressing in the concrete

  7. Punching shear is very often overlooked and can become critical.

  8. All associated piling problems.

Common to all underpinning systems

  1. Taking away too much support - leading to severe structural damage and collapse.

  2. Failure to confirm the assumed cause of subsidence and to redesign if necessary.

  3. Failure to employ responsible operatives - regardless of supervision, fraud can occur on any site.

© 2008 I ASUCplus, Tournai Hall, Evelyn Woods Road, Aldershot, Hampshire GU11 2LL I Tel: 01252 357833 I Fax: 01252 357831