##### Design of Sand Drains

The typical design parameters for the sand drains along with its permissible values are given as :

(i) **Diameter **of sand drains (**ɸd**) = 450 mm to 600 mm.

(ii) **Spacing **between two adjacent sand drains (**S**) = 1.8 m to 5 m cc (centre to centre).

(iii) **Depth **of sand drains (**H**) = 3 m to 35 m.

(iv) **Thickness **of sand blanket = 0.6 m to 1 m.

During designing, the values of diameter is rounded off to higher side and spacing is rounded off to lower side.

Click here to know – What is Sand Drains

## Steps to Design Sand Drains

### Method 1

**Total Settlement = mv.∆σ.h**- Degree of consolidation,
**U = Settlement controlled * 100 / Total settlement** **Tv = Cv.t / h²**- Tv at U = 60% is 0.2827, So if Tv < 0.2827 i.e. U < 60% then
**Tv = Uv².∏/4**otherwise if Tv >0.2827 i.e. U > 60% then**Tv = 1.781 -0.933 log (100 – U%)** - After getting the Uv value from above equations, find Ur (required) value from the formula,
**(1-U) = (1-Uv)*(1-Ur)** - Assume triangular or square pattern and spacing S;
**R = 0.525 S or R = 0.564 S** **Tr = Ch.t/4R²**- Assume Diameter (ɸd) or Radius (rd) of Sand Drains; then find
**n = R/rd** - From the given graph find the value of
**Ur**(calculated) corresponding to obtained n and Tr values. - If Ur (calculated) > Ur (required) then Design of Sand drains is OK otherwise assume another values from step 6 and keep on trying safe design values.

### Method 2

**Total****Settlement = mv.∆σ.h**- Degree of consolidation,
**U = Settlement controlled * 100 / Total settlement** **Tv = Cv.t / h²**- Tv at U = 60% is 0.2827, So if Tv < 0.2827 i.e. U < 60% then
**Tv = Uv².∏/4**otherwise if Tv >0.2827 i.e. U > 60% then**Tv = 1.781 -0.933 log (100 – U%)** - After getting the Uv value from above equations, find
**Ur value**from the formula,**(1-U) = (1-Uv)*(1-Ur)** - Assume
**n value**; ranging from 5 to 40 - From the given graph find the
**value of Tr**corresponding to obtained Ur and assumed n value. **Tr = Ch.t/4R²**; get R value.- Find radius (rd) of Sand Drains,
**n = R/rd** - Obtain Diameter of Sand Drains (ɸd) = 2(rd)
- Let the pattern triangular or square pattern
- Find spacing S from formula;
**R = 0.525 S or R = 0.564 S**

Method 2 is more convenient than Method 1 as it does not require number of trails. The design obtained via Method 2 is all OK in first attempt. But if restricted values of few parameters are there then we should go for Method 1.

**Representations used are :**

**mv**= co-efficient of volume compressibility**∆σ**= change in stress**h**= Depth of clay layer (h = D/2 when two way drainage is there, h = D when one way drainage is there)**ch or cvr**= co-efficient of consolidation in horizontal / radial direction**cv or cvz**= co-efficient of consolidation in vertical direction**R**= Radius of zone of influence**Tr**= Time factor in radial direction**Tv**= Time factor in vertical direction**U**= Degree of total consolidation**Ur**= Degree of consolidation in radial direction**Uv**= Degree of consolidation in vertical direction