Malcolm’s underpinning systems allow excavation adjacent to and directly below sensitive structures and facilities. Low vibration and sequenced construction techniques are employed to provide vertical load capacity while maintaining ground support. After new support systems are installed, the load is transferred either at the base of existing foundations or directly applied to columns or walls higher in the structure through “column picks.” Underpinning is highly specialized and requires case-by-case design and implementation to minimize risk during each step of the construction process. Systems are designed with consideration for vertical loads imposed from the supported structure, lateral loads from the retained ground, and any temporary induced loads. Malcolm draws from a toolbox of techniques to safely and securely perform underpinning, with careful consideration for the existing structure, ground conditions, and the proposed site improvements to optimize solutions for each case. Some key techniques are described below:
HAND-DUG PIERS mine a small vertical pit below the existing foundation. Reinforcing steel is placed within the pit, and then excavation is backfilled with concrete. A hydraulic jack pre-loads the underpinning pier, then stiff cement mortar is packed to complete the connection. Hand-dug piers are installed in a phased sequence. They are suitable for relatively stable soils above groundwater or when water can be readily drawn down by wells without the risk of subsidence to existing structures.
SLANT DRILLED SOLDIER PILES are placed directly below the existing structure by drilling an inclined oversized hole, and then pushing the steel pile into a vertical position below the foundation. A shallow hand-dug pier is used to create clearance for the pile head directly below the existing structure. As excavation proceeds, lagging is placed between slant drilled piles. Alternatively, jet grouting can construct a continuous soil cement wall spanning between the soldier piles before the start of adjacent excavation.
JET GROUTING injects grout slurry through horizontal nozzles at high pressure and flow rate, hydraulically eroding and mixing soil in situ. Small diameter tooling (2 to 4 inches) is advanced, then slowly rotated and withdrawn while jetting to form columns of cemented ground, with a diameter typically from 2 to 4 feet. Underpinning jet grout columns are installed on an incline below adjacent structures. The system is applied in erodible soils, such as sands, silts, and clays, and is particularly useful below the water table. Jet grouting creates an improved soil block that acts as a gravity mass providing direct vertical support as well as lateral earth retention. Jetting can be combined with other structural underpinning solutions such as micropiles or slant-drilled piles.
MICROPILES are small diameter steel-reinforced drilled and grouted piles. They are constructed using specialized drilling equipment that can fit through conventional doorways and can operate with headroom clearance of less than 9 feet. Micropiles are installed in almost all ground conditions, including caving soils below water, and into rock. For underpinning applications, the load can be transferred by drilling through and bonding into existing footings or via transfer frames.