Spread Underpinning.

Occasionally, due to site constraints, underpinning is achieved by spreading the foundation load over a greater area of ground, rather than transferring to a bearing strata at a lower level.

An example undertaken by the authors’ practice was in the restricted cellars of a series of large Victorian properties being redeveloped as office accommodation. The load-bearing walls sat on stepped brick footings, just beneath a cellar floor of compacted earth. In this case it was possible to cut pockets out of these footings, run reinforcement through the holes, and cast the whole of the cellar floor area as a reinforced concrete raft slab. This proved a very cost-effective and practical way of enhancing the load-bearing
capacity of the premises and providing a basement slab at the same time.

Discontinuous Underpinning.

Where the existing foundation has reasonable spanning capability it is sometimes possible to excavate and install piers in mass concrete or concrete and brick at a spacing  to suit the spanning capability of the original foundation.

The area of the base of this underpinning needs to be  capable of distributing the ground pressure from vertical and horizontal loading into the sub-strata without allowable limits being exceeded (see Fig. 15.13 for typical details).

Fig. 15.13 Typical discontinuous underpinning.

In other situations where good ground exists but the foundation is not capable of spanning, a pier and underpinned beam can be used, the beam being inserted in sections in a similar manner to that of the mass concrete underpinning.

This operation tends to be more tedious and more time  consuming, but where excavations are deep it can prove economic (see Fig. 15.14). As with continuous underpinning, the engineer must carefully consider the risk to site personnel before specifying these techniques.

Fig. 15.14 Typical pier and beam underpinning.

It is particularly useful for foundation jacking where subsidence or settlement requires re-levelling, the jacks being inserted between the soffit of the beam and the top of the piers. In some cases, particularly where the building to be underpinned forms part of the new construction, piles can be inserted on either side of the structure to support needle beams inserted through the existing structure to bear onto the piles. This is particularly useful where a basement extension is to be added to an existing building; the piles form the basement columns and the beams the framework for the ground floor structure. Typical pile beam under-pinning is shown in Fig. 15.15.

Fig. 15.15 Typical pile and beam underpinning.

Temporary lowering of the water-table by sump-pumping for underpinning operations requires careful consideration relative to the effect on new and existing foundations. As previously discussed, there is a danger that soils such as fine sands may suffer from loss of fines and may cause settlement of adjoining structures. There is also the possibility that in certain soils when the dewatering process stops, running sand or clay softening may occur. It is therefore important under these circumstances that the effects of  the temporary works and methods of construction are considered at design stage.

There are numerous ingenious piling systems available which minimize disruption of the existing structure, while maximizing economy and practicality of construction and a reputable specialist contractor should be approached at an early stage where appropriate.

Continuous Underpinning.

In this part the authors have considered only underpinning of existing buildings adjacent to new developments
and not underpinning required due to structural settlement or subsidence, which is a separate subject beyond the scope of this book.

All foundation types may require underpinning when development takes place alongside or under an existing structure. The possible combinations of ground conditions, foundation details and levels is endless and complex. The basic methods and principles are quite simple. Where a new foundation or structure is to be constructed with  its foundation soffit below that of an adjoining foundation, underpinning is usually necessary. The exception to this is where the adjoining building is built upon a substantial ground strata such as hard rock.

The ‘traditional’ method of underpinning described below has been used extensively and effectively in the past and  is included here for completeness. However, the risks to site personnel involved in working beneath a temporarily supported section of superstructure of sometimes dubious integrity must be weighed against other underpinning options such as the use of mini-piles and needle beams  (see Fig. 15.15), which are now easily available and equally effective.

Fig. 15.15 Typical pile and beam underpinning.

Traditional underpinning is generally carried out in sequenced construction and in short lengths (commonly  1.0 to 1.2 m). The sequence is arranged to allow limited undermining of the structure at any one time. The limit of this undermining is dependent upon the structure’s capability of spanning over the undermined section and the  stability of the short section of unrestrained earth. In some cases beam underpinning may be provided to help the structure to span over greater distances. Typical underpinning is shown in Fig. 15.11.

plan
Fig. 15.11 Typical continuous underpinning.

The simplest and most common form of underpinning is  to remove a series of short lengths of sub-soils from below the adjoining building in a sequenced operation. As each section is excavated it is replaced immediately with mass concrete, which is allowed adequate time for curing prior  to the construction of the adjoining section. The top of the concrete is either cast with a pressure head so that it rises  to the underside of the foundation, or is cast low to allow wedging with dry pack or slate. Figure 15.12 gives a typical example of mass concrete underpinning.

Fig. 15.12 Construction methods for mass concrete underpinning.

In the authors’ opinion the preferred method of construction is to cast whenever possible with a pressure head. Concrete shrinks, and so theoretically this method encourages some slight settlement as the building above follows this shrinkage downwards. However, in the authors’ experience such settlement is usually negligible and is offset by the following advantages of the pressure head method:

(1) The underpinning is completed in one operation, rather than waiting up to seven days before dry packing. Also since concrete continues to shrink for weeks, even months, the logic of dry packing is inconsistent.
(2) The workmanship of the dry packing process is often of poor quality due to the difficulty of the technique. This requires increased supervision, and slows the whole sequence down even further.

It is rarely necessary to mechanically key mass concrete underpinning across the joints and the majority of mass concrete underpinning will perform successfully without  a key. The need for keying depends upon the requirement for vertical shear and/or tensile strength across the face, neither of which is usually necessary.

Slip Circle Example.

Slip circles have been mentioned previously and the following example, showing the calculation process for
deriving the factor of safety for a single slip circle with an arbitrarily selected radius, is included here for com-
pleteness. In order to find the most critical condition, i.e. the minimum factor of safety, a number of slip circle  calculations should be carried out using different radii. The engineer experienced in this field is able to identify the most likely mode of failure based on a knowledge of soil parameters and boundary conditions and hence reduce the computational effort required. This iterative calculation process is time-consuming and can be more cost-effectively carried out by one of the commercial software packages for slip circle analysis that are available.

A detached house, 9 m × 9 m on plan, is to be constructed on a sloping site; a section through the proposal is as shown  in Fig. 15.10.

Fig. 15.10 Slip circle design example.

Assuming an average value of  cu = 50 kN/m2, consider  a one metre wide strip for the case where Øu  = 0, i.e. the undrained condition immediately following construction

Next set up a circular arc using compasses, to pass through the edge of the excavation for the basement of slab/footing and close to the bottom of the new embankment. Measure the radius, and compute the arc length, r = 12.5 m. The angle subtended by the arc =α= 90°. Therefore

Deduct a length (arbitrary) disturbed by excavation and subsequent filling, i.e. 19.6 − 3.6 = 16 m.

Weight of ground is F1, assuming the small area of fill above the chord line equals the area omitted within the house.

Using 16 kN/m3 for existing ground and compacted fill
weight, F1 = 45 × 16 = 720 kN/m

The weight of a detached house of two storeys, including external and internal load-bearing walls, when averaged per metre run, equates to 170 kN/m. Therefore

By simple geometry, the centroids of the areas are located, and scaling their lever arms

Combining these gives

    2304 + 680 − 194 = 2790 kNm

Therefore

Since 3.6 is greater than 2, the factor of safety commonly adopted for slope failures involving buildings, then there  is an adequate factor of safety against slip circle failure.

However, as mentioned above, other circles should be checked in order to find the critical case.