Robust waterproofing of underground structures is one of the most cost effective ways to enhance safety and function as well as increase design life of new and existing structures.
Meyco has conceived a method for supporting ground by integrating a state-of-the-art fully bonded cementitious polymer-based spray-applied membrane (Masterseal 345) into concrete linings thereby establishing a thinner monolithic waterproof structure that relies on the primary lining as a permanent solution and prevents water migration behind the membrane. This unique system consists of a first layer of sprayed concrete, a fully bonded intermediate layer of spray-applied impermeable membrane and a second layer of sprayed or cast in place concrete. It provides both a rapid and economical alternative to the more traditional sprayed concrete support systems in a number of situations and particularly where the traditional system faces difficulties such as complex geometries or space limitations.
The membrane consists of a polymer-rich cement base imperative in preventing de-bonding and ensuring the long-term stability of the system. The strong bond is given by crystals produced during the hydration of the cement. These crystals are embedded on the surface of cured concrete and grow into fresh applied Masterseal 345 paste. The crystals are also embedded in the open exposed surface of the cured Masterseal 345 membrane and grow into fresh applied concrete, providing the chemical and mechanical bond.
The membrane bonds equally to the primary and secondary concrete layers with significant adhesive and shear bond strength, giving the system unique mechanical properties and waterproofing features. The bond is unaltered by the concrete placement, be it sprayed or cast insitu, or the presence of fibres, and allows the lining to act as a composite structure.
The adhesive (normal) bond strength to concrete is significant and easily reaches 1.2MPa ± 0.2MPa when dry but reduces to 0.5MPa locally when water saturated. Even in this eventuality watertightness is not compromised because of the bond to the inner lining. (The bond strength to metals ranges from 0.5 to 1.2MPa.) The tensile strength of the membrane is greater than 1.5MPa (at +20°C) with elasticity values between 80% and 140% (between -20°C and +20°C). Thanks to this, the membrane provides crack bridging capability some hours after application. Since hardening of the membrane reduces the elasticity, the minimum crack bridging ability is achieved once fully cured. For a 3mm thick membrane layer this ability is 3mm. Due to the tensile strength of the entire structure, the membrane does not delaminate.
When a shearing load is slowly applied to the structure, the membrane, thanks to its particular chemistry, has sufficient time to adapt to the imposed deformation through the rearrangement of its molecules. In this case the membrane behaves in a viscous manner, i.e. like a fluid, and is not comprised (maximum load at maximum rate of load application need to be established). However, when load deformations are applied at a higher rate, the composite lining resists with shear strength parameters in the following ranges: 20 = T = 45 and 0.5MPa = T = 1.05MPa. These shear strength parameters are taken from direct shear tests carried out under zero normal displacement conditions to a 2mm thick membrane applied to a smooth substrate and a 5mm thick membrane applied to a rough substrate. These two specimen conditions represent two ends of a spectrum. The minimum average flexural strength is 3MPa (tested in accordance with EN 12390: Part 1 and 5) when the membrane is installed centrally with respect to the composite system. Analyses conducted by Mott MacDonald, have shown that these outstanding properties of the Meyco system, lead to loads being transferred from outer/primary lining to inner/secondary lining so that the full lining acts compositely.
Typically designers consider the primary lining to carry the ground loads and the secondary lining to carry the hydrostatic loads. This new system allows the full thickness of the composite lining, primary and secondary layers together, to take the combined applied loads. In a real life situation this is carried out either as a true “single shell” or by acting monolithically through the deformation of the membrane, depending on the magnitude and direction of the applied loads (radial and tangential) and subsequent relative deformation of each lining layer. The two layers of concrete are both to be considered as permanent and durable elements that will fulfill the structural requirements both during construction and throughout the designed life of the structure.
Advances in sprayed concrete technology have made it possible to rely on sprayed concrete as a permanent solution. Durability issues derived from poor mix design, involving inappropriate admixtures, and poor application have been overcome thanks to better understanding of sprayed concrete, advances in chemical formulations and robotic applications and approved guidelines and certification for hand applicators.
The membrane itself should have a nominal thickness of 3mm (localized minimum of 2mm can be accepted and maximum of 10mm) in order to be watertight. Laboratory tests show that the composite system has excellent resistance to water ingress and can sustain up to 20bar pressure over 1 year.
Thanks to the fully bonded nature of Meyco’s combined ground support and waterproofing system the location of any potential leakage can be easily identified, as water cannot migrate along the membrane-concrete interface. Any remedial measures will be simple, cost-effective solutions.
Hampton Pump Out Shaft
Thames Water Utilities required an upgrade to its existing system with the Hampton Pump Out Shaft. This shaft has 15m i.d. and is approximately 40m deep. The shaft pumping station is to replicate the duty of an existing shaft and associated connections to the supply network. The shaft connects to two tunnels for the supply of water and lies almost entirely in London Clay. The external water pressure can be attributed to leakage from the gravels above and/or infiltration through any London Clay fissures.
The design of the shaft follows the approach of a hybrid sprayed concrete lining (SCL)/segmental shaft – increasingly used for permanent works in London.
The top of the shaft is formed of pre-cast concrete segments with a SCL below. The advantage of this SCL method over segments is that the junction with the tunnel is simpler and quicker to form. Due to the depth of the shaft it would also be necessary to thicken the segments, to accommodate the increased ground loading, which presents its own difficulties, particularly in the transition between segments of different thicknesses. Also, forming low-level openings in the shaft lining would be time-consuming with pre-cast segments necessitating large steel sections if using jamb-and-lintel frames.
The construction of the lower section of the shaft, the low level portals and soft-eye was therefore constructed using SCL instead of pre-cast concrete segments. This was felt by Costain Ltd. (Contractor) and Mott MacDonald (Designer) to offer a simpler, safer and more durable solution in terms of ease of construction and performance in the long-term with savings in cost and programme.
For an efficient design a varied profile consisting of a thinner section at top, at the interface with the PCC lining, and a thicker section at the bottom, at the connection with the base slab, was considered. The thickness of SCL is 425mm at the segment interface and 625mm at the shaft bottom. Crack limitation and improved flexural capacity was achieved using polypropelene fibres rather than steel fibres or steel mesh as they removed concerns over durability. Strux structural polypropelene fibres from Grace were used in the sprayed concrete.
To waterproof the shaft, Masterseal 345 was sprayed between primary and secondary sprayed concrete layers. The membrane could be applied to the primary lining using the same equipment (Meyco Oruga) as for the sprayed concrete. Standard waterproofing detailing, typically consisting of re-injectable grout tubes, hydrotites and waterstops were adopted at the joint with the PCC lining and the junction with the connecting tunnel and also where differential movement could occur and in areas of geotechnical concern such as areas of silt pockets within the London clay.
A smoothing layer was applied to rougher areas of the primary lining profile arising from joints and the inherent roughness of the 10mm aggregate. Shotpatch 12 was sprayed for this “regulating” process achieving considerable cost savings on Masterseal 345. A finishing coat was applied to the secondary sprayed concrete lining in areas where access is required.
Meyco’s innovative ground support and waterproofing system proved very efficient, safe and economical for this deep shaft with multiple openings. The use of sprayed concrete as part of the permanent works has improved the cost-effectiveness of constructing underground structures.
The integration of a fully bonded membrane into the lining to establish a thinner monolithic waterproof structure ensured considerable savings both in terms of cost as well as to the programme.
A permanent composite SCL lining combined with a fully bonded waterproof membrane proved very successful and is recommended for future projects where it is deemed an appropriate solution. T&T
The lower segmental lining ring and primary sprayed concrete lining Meyco Oruga in operation during the first SCL section of the shaft Hand spraying the membrane
