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1 - Site Conditions
Pier Foundation The least expensive foundations use variable depth piers. Concrete piers are poured under each of the scale’s load-bearing points. The total capacity of the scale determines the footprint of the piers, which are then dug to undisturbed soil below the frost line. The soil must have a minimum bearing capacity of 12,200 kg/m 2 (2,500 psf). It can be helpful to include a thin washout slab poured around the piers to aid in periodic cleaning.
There are a few site condition considerations that must be accounted for: subsurface obstructions, drainage, and soil bearing pressure.
Obstructions Subsurface obstructions include human-fabricated obstacles, such as water lines, gas lines, power lines, sewers, drains, and old landfills. They must be moved or avoided. Natural obstacles that must be considered include high water tables, boulders, bedrock and sinkholes. If you have any doubt about what’s under your site, consider ordering test borings before you start excavation. Drainage Every scale located outdoors needs adequate drainage. You do not want excessive storm water or snow-melt flowing over, through, or into your scale. Open-sided scales usually allow water to flow off the foundation naturally. Pit scales, on the other hand, need sufficient drain piping and/or sump pumps. If your area experiences freezing temperatures, also consider frost heave. Damage to the scale foundation can result from the subsurface around the scale expanding and contracting. Adequate subsurface drainage reduces that risk. Soil Bearing The soil at the site needs to be strong enough to support the foundation, the scale and the loads the scale will weigh. That is determined by establishing the soil bearing pressure at the site. Many facilities will have this information on file from construction records when the site was developed. If not, and depending on your area, a civil engineering agency can assess the soil. The soil’s strength may have an impact on the design of the foundation to be used. Specific requirements will be noted on your scale company’s foundation drawings. These typically range from 7,300 kg/m 2 to 12,200 kg/m 2 (1,500 psf to 2,500 psf). If the soil bearing capaci- ty is too low, the foundation design may need to be modified to compensate. 2 - The Foundation A stable foundation is critical. Any movement or settling may throw your scale out of adjustment and neces- sitate recalibration, or worse. Continued movement would mean a continuing need for recalibration. Over time, an unstable foundation could move enough to exceed the scale’s corrective capacity, in which case you must start all over again and build a new one. The foundation must be designed and installed properly. Work closely with your scale supplier. They can advise you on acceptable foundation designs for your locale and they probably have the names of several contractors who have done good foundation work in the past – those who can get the foundation in straight, plumb and level. There are several types of foundation designs. Here, we discuss the two most common ones.
Beam Slab The beam slab foundation has extra excavations beneath the slab into which additional concrete is poured for added support. It will typically include beams running the length of the scale along each side, as well as beams running the width of the slab. In combination, those beams make a ladder-bar formation below the visible slab. Minimum bearing capacity is approximately 7,300 kg/m 2 (1,500 psf). A beam slab foundation is stronger than a pier-style foundation.
Pit Foundation Pit foundations may be designed to allow service technicians adequate space to access components be- neath the weighbridge. The recommended soil bearing capacities are similar to the beam-slab foundation.
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