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Sewage holding tank anchoring: when and why is it needed?

A sewage holding tank (reoveemahuti; in legal texts reovee kogumismahuti; UK readers may know a sealed tank as a cesspool or cesspit) stores wastewater, has no outlet and is pumped out by a tanker (purgimine). Vesmaco’s reinforced-fibreglass (GRP) tanks weigh only 150–800 kg, and in wet ground an empty tank can be pushed upwards. Holding tank anchoring (ankurdamine) straps the tank to a concrete base so that it cannot move. This article covers when the guides call for it, slab versus concrete blocks, why the straps must be textile, and why water goes into the tank before they are tensioned.

Why a light, empty tank has to be held down

Groundwater pushes a sealed, empty tank upwards with a force equal to the weight of the water it displaces. If groundwater stood at the top of an empty MK-5 D1400 (5 m³, 290 kg), the uplift would be about 5 000 kg (5 m³ × 1 000 kg per m³ of fresh water; shell and riser ignored). The tank’s own weight offsets under 6 % of that; soil cover and anchoring must do the rest.

The Ministry of the Environment’s 2015 guide to rural wastewater systems (official guidance, though dated) says that where soil water (pinnasevesi) is high, a concrete slab under a holding tank and anchoring are essential, so that an empty or half-empty tank cannot rise and be damaged. Its septic-tank chapter adds that a lifted tank can break its pipe connections. For a holding tank that is serious: the Water Act (Veeseadus, § 124(2)) requires it to be leak-tight (lekkekindel).

This is not a one-off risk: the tank is nearly empty again after every visit from the tanker, and with permanent living the ministry guide expects emptying about once a week, in every season. How to find out your groundwater level is covered in holding tanks and high groundwater.

When is anchoring needed?

Neither guide gives one rule for every plot. Taken together, the sources point to four situations:

Your situation What the sources say What to do
Groundwater is high, or rises to tank level in some season Ministry guide: slab and anchoring are essential Anchor; a designer sizes the base
Clay or other tight soil, where the sand-filled pit can collect water Ministry guide, septic-tank chapter: anchor where the pit may fill with rainwater Treat it as wet ground; see holding tanks in clay soil
Water level not known No guidance Find out before ordering; meanwhile ask for installation prices with and without anchoring
Dry, free-draining ground all year Ministry guide links anchoring to high soil water; Vesmaco’s guide lists the slab as step 2 without stating a condition Ask the supplier and your designer whether your site needs the slab or blocks

Check your municipality too. Holding-tank requirements sit in its rules for on-site wastewater treatment and removal (reovee kohtkäitluse ja äraveo eeskiri, Water Act § 104(7)). Saaremaa, for example, requires anchoring according to the soil and the manufacturer’s instructions, so that the tank cannot move. That is only an example: rules differ by municipality, so check your own.

Anchoring is not the reinforced-concrete slab that Vesmaco’s guide puts above the tank in a trafficked area or when it is installed deeper than 1.5 m: that one spreads loads from above. The ministry guide rules out a holding tank under a road or parking area altogether; both views are in can a holding tank go under a driveway?

Slab or concrete blocks: the two bases in Vesmaco’s guide

Vesmaco’s installation guide for its horizontal GRP underground tanks (in Estonian, 2016, written “according to EN 976-2”) offers two bases for the straps:

Reinforced-concrete slab (raudbetoonplaat) Reinforced-concrete blocks (raudbetoonplokid)
When the guide uses it Step 2 of the installation sequence “In stable soil”, instead of the slab
Thickness 20 cm At least 20 cm
Material named Frost-resistant concrete, double wire mesh 150 × 150 mm, wire Ø 8 mm Reinforced concrete, no further detail
Layout in the guide’s drawing One slab under the whole tank Two long blocks, one along each side
Size and weight Set by a certified design or construction organisation Same

The tank never sits on concrete. The slab rests on the sand bed of step 1 (at least 20 cm) and is levelled with another 20 cm of compacted stone-free sand (fraction 0–3 mm, compacted to 95 % of natural density, not less than 1500 kg/m³). The ministry guide likewise requires 200 mm of compacted stone-free sand between slab and tank and forbids setting a holding tank directly on a slab or any other hard base, which would include blocks. With a slab, the pit is about 40 cm deeper (20 cm slab + 20 cm sand).

A designer sizes the base. The guide has no size table: a certified design or construction organisation (sertifitseeritud projekteerimis- või ehitusorganisatsioon) determines the weight and dimensions of the slab or blocks. Give them the model, the highest known water level, the depth and the soil. “Stable soil” is not defined either, so leave the choice between slab and blocks to the designer. Vesmaco’s services include consultation and design; ask whether that covers the base.

From the bottom up, the slab version is: at least 20 cm of compacted sand, the 20 cm slab, 20 cm of compacted sand, then the tank, with textile straps over it fastened to the slab. The block version has two long blocks along the tank’s sides and the same straps.

Straps: textile only, and how many

Vesmaco’s guide says to fasten the tank only with textile straps and to ask the seller about strap type and quantity. The ministry guide mentions “cables or load straps”, but for a Vesmaco tank follow Vesmaco: its guide states that the product warranty does not apply if the installation instructions are violated or ignored.

The order drawings offer an “anchor strap with tensioner” (polyester, 5.5 + 0.5 m) as an option. On two 5 m³ drawings, the longer tank has more straps:

Drawing (rev. B, 15.08.2025) Tank length (model table) Straps
MK-5 D1400 about 3.6 m 4
MK-5 D1100 about 5.7 m 6

The manufacturer specifies the number of straps per model, so check the drawing for yours in the model table, and ask the supplier exactly what the anchoring option includes, for example whether fixings are part of it.

In the guide’s drawings each strap passes over the tank and is fastened with a fitting at the slab or blocks; the fittings are not specified. The ministry guide’s septic-tank chapter (a different system, facing the same uplift) passes the straps under the slab or uses anchor bolts, tensioned so that they cannot slip over the tank’s ends. Settle the method, and who supplies the fixings, with your designer and installer before the concrete is cast.

Order of work: water in first, then tension

Once the sand bed and base are ready, Vesmaco’s guide continues:

  1. Set the tank on the compacted sand and level it.
  2. Fill a quarter of the tank with water, then fit and tension the straps.
  3. With the first 30 cm of backfill sand, compact carefully under the tank and between its support legs.
  4. Backfill in compacted 30 cm layers of stone-free sand up to the inlet/outlet pipe, adding water to the tank at the same time.

The rest of the sequence is in installation step by step.

The guide gives the order, not the reasons. It also does not say whether “a quarter” is measured by volume or by height, or how tight the straps should be, so ask the supplier. Read by volume, a quarter of a 5 m³ tank is 1 250 L, about 1 250 kg of water (1 L ≈ 1 kg), roughly four times the tank’s own weight, so agree beforehand who brings it.

Before the first layer goes in, photograph the sand between base and tank (no contact with concrete anywhere), the levelled tank with its water, the tensioned straps and the fixings. Once the pit is filled, photos are the only record; Saaremaa, for example, asks for an act of covered works with photos when a holding tank is taken into use.

Frequently asked questions

Does a tank on dry, sandy ground need anchoring? That depends on the highest water level, not the day you dig: the ministry guide notes that the soil-water level usually changes with the seasons and asks for both the highest and lowest to be recorded. For what each guide says, see the table above.

Can steel cable or chain replace textile straps? Not with a Vesmaco tank. Its guide allows textile straps only and ties the warranty to following the guide.

Are the straps and the concrete base part of the delivery? The basic delivery is the tank with a discharge pipe, a plastic cover and documents. Anchoring systems are offered in addition, and straps with tensioners are an option on the order drawing. The slab or blocks and the sand are not on the drawing, so ask your installer to price them.

Add anchoring straps to the quote. In the quote form, say that the tank will be anchored, or that the groundwater level is not yet known, and name the model, so that straps with tensioners are quoted in the right number. Mention if you would like advice on the slab or blocks. Phone: +372 641 8550.

Sources

Last checked: 17 September 2026.