Keeping the excavation dry: calculating capacity and choosing the right construction pump
Quick answer: For a construction pit, choose a robust construction pump that can handle sand and sludge, preferably with a stirring head so that mud does not accumulate at the bottom. Be generous with capacity: inflow (seepage + rain) plus margin, at the head you need. If the pump needs to run for days on end, choose a jacket-cooled pump.
→ Go directly to: which construction pump is right for my job?
Step 1: Determine how much water needs to be removed
In a construction pit, you're dealing with two types of water:
- Water that is already present. Surface area × depth. A pit of 4 × 5 meters with 50 cm of water contains 10 m³ (10,000 litres).
- Water that continues to flow in: groundwater (seepage) and rain. Estimate rain by surface area × precipitation: a heavy downpour of 20 mm on 20 m² yields an additional 400 litres. Seepage varies greatly depending on soil type and groundwater level. Measure it by emptying the pit and observing how quickly the water rises again.
Example calculation: if the water rises 20 cm per hour after emptying in a 20 m² pit, then 4 m³ per hour is flowing in. In this case, choose a pump that delivers significantly more at your required head, for example 6 to 8 m³ per hour, so it can keep up even during a downpour.
Step 2: Determine the discharge head
Add up the height difference from the bottom of the pit to the discharge point, plus approximately 1 meter per 10 meters of hose. Read from the pump curve how much the pump delivers at that height. The maximum flow rate stated on the box applies at 0 meters. Detailed: calculate discharge head.
Step 3: Choose the right model
| Feature | Why important on the construction site |
|---|---|
| Stirring head | Stirs up sludge and sand so it's pumped along instead of clogging the inlet |
| Wear-resistant (cast iron, polyurethane or stainless steel) | Sand abrades the impeller and housing; a wear-resistant pump lasts much longer |
| Vortex impeller | Less susceptible to stones and dirt |
| Jacket cooling | For continuous operation: the motor stays cool, even at low water levels |
| Bottom discharge connection | Pump does not tip over in a muddy pit |
| Float switch | Switches automatically, even at night and on weekends |
| 230V or 400V | 230V runs on a construction site outlet; 400V gives heavy-duty pumps more power |
Use a coarse filter or place the pump on an elevated platform (e.g., a bucket with holes or a gravel bed). This way, it sucks in less sand and lasts longer.
Connecting on the construction site
Use a flat, rollable discharge hose with quick couplings. For larger flow rates, C-Storz is the standard: quick to connect without threads. Lay the hose without kinks and discharge the water according to agreements with the municipality or water board. For groundwater pumping, reporting or permit rules often apply.
Maintenance
After use in muddy water, flush the pump with clean water. Regularly check the impeller and stirring head for wear and the cable for damage. Wear parts on SPT pumps are easy to replace.
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Frequently asked questions
Which pump do I need for a construction pit?
A robust construction pump that can handle sand and sludge, preferably with a stirring head and vortex impeller. For continuous dewatering, choose a jacket-cooled pump.
How do I calculate the capacity for a construction pit?
Add up the water already present (surface area × depth) and the hourly inflow (seepage and rain). Choose a pump that delivers significantly more than the inflow at your required head.
What does a stirring head on a construction pump do?
A stirring head stirs up settled sludge and sand, so the pump discharges it with the water instead of the inlet clogging.
230V or 400V construction pump?
230V is convenient on a standard construction site connection. 400V gives heavy-duty pumps more power and is more economical for long-term use.
Unsure which pump is suitable?
Provide us with the dimensions of the construction pit, the soil type, and where the water needs to be discharged. We will calculate the required capacity for you free of charge.
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