1 Shoring

Lateral support for the sides of an excavation is usually provided by shoring. Shoring is the means of providing temporary support to structures that are in an unsafe condition till such time as they have been made stable, or to structures which might become unstable by reason of work being carried out on or near them, such as underpinning of foundations.

Shoring is also used as temporary support to sides of excavations until soil can be stabilised or otherwise supported.

Types of Shore

Raking shore are generally used where external support is necessary. The angle of the shores is generally 60o to 75o. If the feet of raking shores obstruct construction operations then flying shores can be used provided that there is conveniently placed wall or other structure to strut against. For practical reasons the length of flying shores is generally limited to about 10m.

Support to the vertical load of a wall, where it is required in conjunction with flying shores or horizontal ties, can be given by means of dead shores. Dead shores are vertical struts bearing on the ground at the required distance away from the wall to be clear of underpinning operations and surmounted by a horizontal beam or needle spanning between a pair of shores.

2 Underpinning

Underpinning is required :

(a) to support a structure which is sinking or tilting due to ground subsidence or instability of the superstructure;

(b) as a safeguard against possible settlement of a structure when excavating close to and below its foundation level;

(c) to support a structure while making alterations to its foundation or main supporting members;

(d) to enable the foundations to be deepened for structural reasons, e.g. to construct a basement beneath a building;

(e) to increase the width of a foundation to permit heavier loads to be carried, e.g. when increase the storey height of a building;

(f) to enable a building to be moved bodily to a new site.

[(a), (b) and (c) apply to shoring as well.]

If underpinning is necessary to arrest settlement, it is essential that the underpined foundations should be taken down to relatively unyielding ground below the zone of subsidence.

E.g., if the bearing pressures of existing foundations are such that excessive settlement is occurring due to consolidation of a compressible clay soil it is quite useless simply to widen the foundations by shallow underpinning. This will merely transmit the pressures to the same compressible soil at a lower level (Figure 12.6(a)), and the cycle of settlement will start all over again.

The underpinning must be taken down to a deeper and relatively incompressible stratum, if necessary by piers or piles (Figure 12.6(b)).

If the structure to be underpined is close to the excavation, it is often convenient to combine the underpinning with the supports to the excavation. E.g. the supports can consist of steel universal beam soldiers inserted in pre-bored holes with horizontal sheeting members to the tops of the piles (Figure 12.7(a)). Or, a system of close-space bored piles can be used (Figure 12.7(b)).