How thick must a concrete or block wall be to withstand hydrostatic pressures up to three meters' height?

Question

I'm planning a basement 3.6m depth, ~0.5m of which extended above ground surface level, in a deep clay/alluvial terrain (bedrock inaccessible). Torrential rainfall during rainy season saturates the region, causing considerable flooding in low-lying areas. The basement, while not where flooding would occur much above ground level (no more than about 20 cm./8 in.), would have no option for drainage for anything below grade. Power outages are not uncommon, especially during a strong storm, making a sump pump an unreliable option. I expect, therefore, to essentially be building an inverted pool.

To prevent water entrance, 2 or 3 mm. thick steel plates could be welded together to create a waterproof liner for the concrete walls. I had planned to use either 8" blocks, poured full, or to pour the entire wall outside the liner. But as concrete is porous, and could admit some water, I then realized the need to account for hydrostatic pressure potential, and, quite likely, would need to have the steel as exterior cladding to the wall. Sooner or later, ground penetration of the water would be inevitable, and rising water levels would create a lateral load against the basement wall. Assuming the maximum pressures (worst case scenario), what is the minimum wall strength/thickness required to maintain the wall's integrity?

Basic Considerations

• Basement cladded by steel plating for waterproofing
• Alluvial/clay soil, no bedrock
• Basement dimensions about 8m x 12m (about 26ft x 40ft) and about 3m/10ft below grade
• Heavy monsoon rains--Limited pooling of water may be expected above grade, no more than about 20 cm./8 in.
• No viable option for drainage below grade **

Further details/considerations

• Two floors are planned above the basement, with exterior walls of 8" block (very uncommon for this region, and hard to obtain--the common block size being 3").
• Would like to consider the potential for seismic activity and accompanying liquefaction.
• Steel is expensive, but concrete blocks and cement are not.
• Termites are ubiquitous and will ravage most woods in a short time; i.e. wood is not a good option.

Question

• What are the wall requirements to withstand the worst-case hydrostatic pressures?

** I took measurements of the grade today and asked a new question HERE regarding the possibility of drainage. Perhaps there's a small chance of it.

Answer 1

This is out of my head only - for basement walls cast in place integrally with concrete slabs (top and bottom), I suggest a 10" (ideally 12") reinforced concrete wall with fc' >= 4000 psi (compressive strength), and two layers of rebar meshes, each mesh consists of #6 (M25) rebars at 6" on center vertically and horizontally. The concrete cover on the outermost rebars shall be at least 2" clear. The vertical bars shall be continuous from the base slab all the way to the top of the wall. You can cut and splice the bars per the local building code requirement.

Also recommend waterproofing the exterior wall, and backfill with free-draining soil. You should also provide continuous PVC water-stop at the base slab and wall joint if it is available. If not, you should build a key way away from the base level.

For construction, place the concrete continuously in several lifts over the height; vibrate the concrete thoroughly to allow for an uniform mixture but without separation of aggregates and the water (caused by over vibration). Again, if available, vibrating the face of the formworks has been proven to be an effective method to achieve a better quality concrete product (do not allow excess bleeding over the form). After concrete placement, maintain the wall in a moist state for 7 days. Place backfill only until the concrete has gained its full strength (21 days), or with the interior formworks stay in place.

I encourage you to go over these recommendations with a "licensed" structural engineer and make necessary adjustments per material availabilities and prevalent local practices.

Note: There is not much you can do about earthquake-induced liquefaction but to consult with an experienced geotechnical engineer to determine the extent of the susceptible soil and modify the layer by removal or applying a method of subgrade strengthening. However, if you extend the base slab a distance beyond the exterior wall face (d >= wall thickness), it will greatly increase the stability of the building during such an event.

Answer 2

A retaining wall to support wet soil (saturated, maybe) has to withstand this loading plus any additional earthquake loads which are usually a factor of 1.15 to 1.25 in our city of Los Angeles.

$P=\frac{H^2}{2}(\gamma'_s + \gamma_w)1.25$

• $\gamma'_s= \text{design bouyant lateral load of the soil}$
• $\gamma_w= water density= 62.4lbs/feet^3$

The design lateral pressure of soil can be found in the ASCE 7-05 3.2 table 3.2-1 added below.

They act at H/3.

In the absence of a soil report, it is conservative to consider the full density of the wet soil and add the water hydrostatic pressure.

You have to consider the uplifting force of the water. I have seen the basements that were pushed out of the earth in the rainy season,

I strongly recommend you seek the advice of a local engineer.

Here is the ASCE 7-05 3.2 table 3.2-1

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