Greenhouse Foundation

The foundation of Dutch greenhouses

The foundation is required to anchor the greenhouse in the ground. All the forces exerted on the greenhouse are transferred to the ground via the foundation. These forces include the weight of the greenhouse, the wind-pressure, wind-suction, snow-load, crop load and that of the installation loads. Before installing the foundation, a comprehensive soil analysis must be carried out. Depending on this analysis, the decision can be made to use drive-piles to ensure the greenhouse construction will not prolapse through the years.

Inner Foundations

The inner greenhouse foundation transfers the loads from the steel post to the ground or, if necessary, the drive-piles. Pre-fabricated concrete or steel dollies are used for the inner foundation. Concrete dollies come with a steel strip for easy fixation to the greenhouse posts. DutchGreenhouses® works exclusively with pre-fabricated steel foundation posts. This provides more options for the connection to the steel columns, in many cases this looks better and is less heavy for workers to install.

The foundation posts are placed in boreholes with a calculated diameter and filled with concrete. Boreholes are drilled with an auger mounted to a skidsteer (Bobcat), a crane or a backhoe, into untouched soil. This allows for perfect holes, with exactly the right amount of concrete.

The borehole under the cross-braced columns is always larger than that of the rest of the inner columns, because more load is transferred.

Perimeter Foundation

The foundation for the perimeter of the greenhouse differs from that of the inner foundation. The perimeter foundation is a continuous concrete beam with a minimum height of 30 cm. This outer perimeter can be pre-fabricated or can be poured with concrete on the construction site, the latter is the most common. Similar to the inner foundation, the beam is poured on site and is is connected to the foundation posts with hairpins, which are casted into a borehole that is filled with concrete. The perimeter foundation is not a structural foundation element as a tie-beam. It serves as the base for the self-supporting walls, rather than transferring loads.

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Slope in the foundation

Unlike many conventional industrial foundations, Dutch greenhouse foundations are not level in a single direction; instead, they feature a slight slope oriented towards the gutter. While this incline is subtle and hard to notice, it becomes more evident in greenhouses that stretch beyond 50 meters (or 55 yards) in length, as one end of the foundation will visibly sit higher above the soil level than the other.

Greenhouses deal with rainfall and also often have cultivation gutters suspended from the steel frame. To allow for the greenhouse shape to function as rain-water discharge and irrigation drainwater collection, it is built on a slight angle. Enough to lead water in the right direction, but not enough to jeopardise the structural properties.

The slope is applied lengthwise, in the gutter direction of the greenhouse. To ensure optimal water discharge, the foundation has to be created accordingly. The most common used slope is 0.2% of gutter length. Horizontally, the foundation has to be perfectly levelled out.

In greenhouses with larger lengths, the slope for water discharge has to be created on two sides. This means that the highest point of the greenhouse is in the centre of structure.

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FAQ

Why do you need a soil analysis before the construction of a greenhouse?

A soil analysis is essential before constructing a greenhouse because it helps determine the stability and suitability of the ground for bearing various forces. These forces range from the weight of the greenhouse itself to external factors like wind pressure, wind suction, snow loads, as well as the weight of the crops and installation components. Based on the results of the soil analysis, decisions can be made about the type of foundation needed. For instance, drive-piles may be used to provide additional stability and to ensure that the greenhouse remains structurally sound over time.

What is the foundation of a greenhouse made out of?

The foundation of a greenhouse is made of concrete and steel, depending on its location within the structure. For the inner foundation, pre-fabricated concrete or steel posts are used, DutchGreenhouses® specifically uses pre-fabricated steel foundation posts. These are placed in calculated boreholes filled with concrete, which are drilled into untouched soil. For the perimeter foundation, a continuous concrete beam with a minimum height of 30 cm is used, this can be pre-fabricated or poured on-site.

Why is a greenhouse foundation built on a slope?

A greenhouse foundation is built on a slope to facilitate proper drainage and water management. The slight incline, oriented toward the gutter, helps channel excess water away from the crops and structure. This is especially important for large greenhouses exceeding 50 meters in length, where the slope becomes more noticeable.

Why choose steel foundation posts over concrete dollies?

Opting for steel foundation posts over concrete dollies in a greenhouse construction offers several advantages. Steel foundation posts are generally lighter, making them easier and less labor-intensive for workers to install. They also provide more options for connection to the steel columns of the greenhouse structure, which can provide greater flexibility in design and construction.

And they look better.