Learn how to build an underground rectangular water tank using standard masonry blocks. Discover design tips, construction steps, waterproofing methods, and maintenance for a durable water storage solution.
How to Build an Underground Rectangular Water Tank Using Concrete CMU Blocks
Why Are More People Choosing Underground Masonry Water Tanks?
Imagine having a strong, long-lasting, and low-maintenance water storage system hidden beneath your property. An underground rectangular water tank by standard masonry blocks offers exactly that. Whether you need water for a home, farm, commercial building, or industrial project, this type of tank provides excellent storage capacity while protecting water from sunlight, algae growth, and temperature changes.
In this guide, you'll learn everything about underground water tanks, including their benefits, construction process, waterproofing methods, design considerations, and maintenance tips. If you're planning a reliable water storage solution, this article will help you make the right decision.
This guide is especially useful for engineers, contractors, and students working with BS EN 1992-3:2006 (Eurocode 2) and is equally valuable for construction projects in the United Kingdom, Europe, Australia, New Zealand, the Middle East, and other regions where Eurocode standards are adopted or referenced.
What Is an Underground Rectangular Water Tank?
An underground rectangular water tank is a reinforced masonry structure built below ground level using standard concrete cinder blocks (CMU) or solid concrete blocks. The tank is designed to safely store clean water while resisting soil pressure, groundwater pressure, and water pressure from inside the tank.
Because the structure is buried underground, it remains protected from weather conditions and occupies no valuable surface space. This makes it an excellent choice for residential buildings, schools, hospitals, commercial complexes, and agricultural projects.
Benefits of Using Standard Masonry Blocks
Standard masonry blocks have become one of the most popular construction materials for underground water tanks because they are affordable, easy to install, and widely available.
Some major advantages include:
- Lower construction cost compared to cast-in-place concrete tanks
- Easy availability in most regions
- Faster construction time
- Good structural strength when properly reinforced
- Easy repair and maintenance
- Excellent durability with proper waterproofing
With proper engineering design and quality workmanship, masonry water tanks can serve reliably for several decades.
Common Materials Required
The quality of construction depends heavily on the materials used. A typical underground masonry water tank requires:
- Standard concrete masonry blocks
- Cement mortar
- Reinforcement steel bars
- Reinforced concrete base slab
- Reinforced concrete roof slab
- Waterproof plaster
- Waterproof coating or membrane
- PVC water inlet and outlet pipes
- Overflow pipe
- Vent pipe
- Manhole cover
Using certified construction materials greatly improves the tank's strength and service life.
Construction Process Step by Step
Building an underground water tank requires careful planning and proper sequencing of work.
1. Site Selection and Excavation
The first step is selecting a suitable location. The soil should have adequate bearing capacity and good drainage. After marking the tank dimensions, excavation is carried out to the required depth while maintaining safe side slopes or shoring where necessary.
The bottom of the excavation should be level, compacted, and free from loose material before construction begins.
2. Base Preparation
A layer of lean concrete is normally placed over the compacted soil to create a clean working surface. After that, reinforcement steel is fixed according to the structural design, and the reinforced concrete base slab is cast.
The base slab forms the foundation of the tank and must be designed to resist both water pressure and uplift forces if groundwater is present.
3. Masonry Wall Construction
Once the concrete base gains sufficient strength, masonry block walls are constructed using cement mortar. Vertical and horizontal reinforcement bars are installed as specified by the structural engineer.
The hollow cores of reinforced masonry blocks are often filled with concrete grout to improve structural strength and increase resistance against soil pressure.
Proper alignment, level, and joint thickness are essential for a strong and watertight structure.
4. Waterproofing
Waterproofing is one of the most important stages of construction.
The internal wall surfaces are plastered with waterproof cement mortar before applying waterproof coatings or crystalline waterproofing products. External waterproof membranes may also be installed to prevent groundwater seepage into the tank.
A high-quality waterproofing system significantly increases the life of the structure and reduces maintenance costs.
5. Roof Slab Installation
After completing the walls, a reinforced concrete roof slab is constructed. The roof normally includes a manhole for inspection and maintenance, along with ventilation openings where required.
The roof slab should be capable of carrying soil loads, vehicle loads (if applicable), and any additional surcharge loads.
Important Design Considerations
A properly designed underground masonry tank must safely resist several different forces during its service life.
Important design factors include:
- Water pressure inside the tank
- Earth pressure outside the walls
- Groundwater uplift pressure
- Foundation settlement
- Crack control
- Reinforcement detailing
- Waterproofing requirements
- Seismic loads where applicable
Professional structural design is recommended, especially for large-capacity water storage tanks.
For projects designed according to BS EN 1992-3:2006 (Eurocode 2), special attention should be given to crack width control, serviceability requirements, water-tightness, reinforcement detailing, and durability. These requirements are widely followed in British, European, and many Australian engineering projects, making Eurocode one of the most recognized international structural design standards.
Free PDF: Underground Rectangular Water Tank Design Calculations
If you're looking for the complete engineering calculations behind an Underground Rectangular Water Tank by Standard Masonry Blocks, we've prepared a detailed design calculation PDF for you.
The document includes:
- Design assumptions
- Loading calculations
- Soil pressure considerations
- Water pressure calculations
- Wall stability checks
- Reinforcement design
- Base slab design
- Structural safety checks
- Material specifications
- Step-by-step engineering calculations
Whether you're a structural engineer, civil engineer, contractor, or engineering student, this PDF can serve as a valuable reference for understanding the complete design process.
The design calculations provided in this PDF are prepared according to BS EN 1992-3:2006 (Eurocode 2), making them an excellent reference for engineers working on projects in the UK, Europe, Australia, New Zealand, the Middle East, and other regions that adopt Eurocode principles for water-retaining structures.
📥 Download the Design Calculation PDF here:
[Download Underground Rectangular Water Tank Design Calculations (PDF)]
Free Sample Construction Drawing (PDF)
Understanding construction becomes much easier with detailed engineering drawings. To help you visualize every part of the structure, we've also included a complete set of Underground Rectangular Water Tank Design Drawings.
The drawing package includes:
- Plan view
- Cross sections
- Wall reinforcement details
- Base slab reinforcement
- Roof slab details
- Pipe layout
- Construction dimensions
- Reinforcement schedules
- Typical structural details
These drawings are useful for engineers, architects, site supervisors, and contractors during planning and construction.
📥 Download the Sample Design Drawing PDF here:
[Download Underground Rectangular Water Tank Design Drawing (PDF)]
Waterproofing Tips for Longer Service Life
Even a well-built tank can develop leakage if waterproofing is ignored.
For better durability:
- Use waterproof cement additives.
- Apply crystalline waterproofing systems.
- Seal all pipe penetrations carefully.
- Provide proper construction joints.
- Perform water-tightness testing before backfilling.
- Repair cracks immediately after detection.
Routine inspection helps identify small problems before they become expensive repairs.
Maintenance Requirements
Regular maintenance keeps the tank clean and extends its lifespan.
Maintenance activities include:
- Cleaning the tank every 6 to 12 months
- Inspecting waterproof coatings
- Checking for cracks or leakage
- Cleaning inlet and outlet pipes
- Inspecting overflow and vent pipes
- Testing water quality periodically
Simple preventive maintenance can help the tank remain functional for many years.
Typical Applications
Underground rectangular masonry water tanks are widely used in many sectors.
Common applications include:
- Residential houses
- Apartment buildings
- Schools
- Hospitals
- Hotels
- Commercial buildings
- Industrial facilities
- Farms
- Irrigation systems
- Rainwater harvesting projects
Their flexibility makes them suitable for both small and large storage capacities.
Relevant Standards and Guidelines
Construction should always follow recognized engineering standards and local building regulations. Helpful references include:
- American Concrete Institute (ACI)
- International Code Council (ICC)
- Bureau of Indian Standards (BIS)
- Portland Cement Association (PCA)
These organizations provide technical guidance for concrete, masonry, structural design, and construction best practices.
Frequently Asked Questions
How long does an underground masonry water tank last?
A properly designed and waterproofed masonry water tank can last 50 years or more with regular maintenance.
Which blocks are best for underground water tanks?
High-quality concrete masonry blocks with adequate compressive strength are generally preferred because they provide good durability and structural performance.
Is waterproof plaster enough?
Waterproof plaster improves resistance to leakage, but combining it with a waterproof membrane or crystalline waterproofing system offers much better long-term protection.
Can masonry tanks store drinking water?
Yes. When built using approved materials, proper waterproofing, and food-safe internal finishes, masonry tanks can safely store potable water.
Conclusion
An underground rectangular water tank by standard masonry blocks is an economical, durable, and practical solution for long-term water storage. With proper structural design, quality materials, effective waterproofing, and regular maintenance, these tanks can provide decades of reliable service for homes, commercial buildings, and industrial facilities.
Whether your project follows Eurocode 2, British Standards, Australian Standards, or another recognized structural design code, the engineering principles discussed in this guide can help you build a safe, durable, and watertight underground masonry water tank.
If you're planning to build an underground water tank, investing in good engineering and quality construction today can save significant repair costs in the future. Are you planning a residential water tank or a large commercial storage tank? Let us know your project requirements in the comments or whatsapp, and we'll help you choose the right design.