Guidelines for dewatering during construction

IS 9759: Key Formulas, Tables & Specifications (Foreword & Dewatering Flow)

1. Foreword Highlights

• Provides guidelines for dewatering in normal civil construction (excluding river valley projects like earth dams).
• Covers common dewatering methods for soils like gravels, sands, silts.
• Developed with help from Central Building Research Institute, Roorkee.
• Values to be rounded as per IS 2-1960.

2. Key Dewatering Flow Formulas (Clause 1.10, Tables 2 & 3)

• Discharge to a fully penetrating slot (artesian flow):

  [ Q = k D x L (H - h_e) ]

• Discharge to a fully penetrating slot (gravity flow):

  [ Q = 5.7 (H - h_e) k_x 2L ]

• Discharge to a partially penetrating slot (gravity flow):

  [ Q = (0.73 + 0.27 \frac{H - h_e}{H - h_o}) k_x H' L ]

• Discharge to a slot from two line sources (partially penetrating):

  [ Q = 2 k D x (H - h_e) L + Y D ]

  Where (Y) depends on ratio (W/D).

3. Parameters

4. Dewatering Methods Comparison (Table 1 Summary)

IS 9759: Scope - Key Formulas, Tables & Specifications

Scope:
Guidelines for dewatering during construction, excluding river valley projects and powerhouses in boulder/gravel reaches.

Key Discharge Formulae (Clause 6.2)

Well Discharge Correction (Clause 1.10)

For fully penetrating wells:
[ H - h = \frac{n + \ln(N_f)}{k D} \times \left( N_f + 2n^2 \right) \times N_e ]

For partially penetrating wells:
[ H - h = \frac{n + 0.4}{k D} ]

Where:

• (a) = well spacing
• (D) = depth of pervious stratum
• (T_w) = effective well radius
• (f_a) = uplift factor (depends on well screen penetration)

Table 1: Dewatering Systems Summary

IS 9759: Definitions, Terminology & Key Discharge Formulae

Key Definitions (Clause 2.0)

• H: Total head or water table height above a datum.
• he, he2, ho: Various piezometric heads at specific points.
• k, kx, kD: Hydraulic conductivity coefficients.
• L, D: Geometrical dimensions (length, depth).
• Q: Discharge or flow rate to a slot or well.

Discharge Formulae (Clause 6.2, Tables 2 & 3)

Notes on Variables Substitution (per IS 9759 corrections)

• Substitute (N_1 N_e') for (N_0 N_f).
• Replace (Q) in Tables 2 & 3 with corrected expressions involving (k D x), (H - h_e), and penetration depths.
• Use (h = h_e + (H - h_e) \frac{J + D}{L + D}) for head at distance.
• For flow at distance (y > 1.3 D), head varies linearly:

[ h = h_e + (H - h_e) \frac{y + D}{L + y D} ]

Summary Diagram: Flow to a Slot

flowchart LR
    A[Water Table Height H] --> B[Slot with Penetration Depth H']
    B --> C{Flow Condition}

IS 9759: General Principles of Dewatering

Key Points from Clause 7.9 & 5.2:

• Dewatering Operation (7.9):
  Dewatering methods must be selected based on soil conditions, groundwater level, and construction requirements.

• Particle Size Distribution (5.2):

  • Soil particle size distribution is critical for selecting the dewatering method.
  • Conduct sieve analysis to obtain grading curve.
  • Use Fig. 1 (soil classification chart) to match soil type with suitable dewatering technique.

Typical Dewatering Methods vs Soil Type (from Fig. 1 concept):

General Dewatering Design Formulas:

• Drawdown (s):
  [ s = H - h ]
  Where:

  • (H) = initial groundwater level
  • (h) = lowered groundwater level after dewatering

• Flow rate for wellpoint system (Q):
  [ Q = \frac{2 \pi K D s}{\ln(R/r)} ]
  Where:

  • (K) = coefficient of permeability (m/s)
  • (D) = thickness of the aquifer (m)
  • (s) = drawdown (m)
  • (R) = radius of influence (m)
  • (r) = radius of wellpoint (m)

Summary:

• Step 1: Determine soil particle size distribution by sieve analysis.
• Step 2: Refer to soil classification (Fig. 1) for suitable dewatering method.
• Step 3: Calculate drawdown and flow rate using above formulas to design the system.
• **Step

IS 9759: Subsurface Investigation and Soil Conditions

Key Points from Clauses 4.3.1 & 5.1

• Subsurface investigation is essential to determine soil type and select the appropriate dewatering system.
• Table I (Clause 5.1) guides the selection of dewatering based on soil type.
• Permeability of pervious strata must be determined by field pumping tests for accuracy.

Coefficient of Permeability (k) for Sands (Clause 4.3.1.5)

Approximate Formula for Coefficient of Permeability (if no pumping test):

[ k = C_1 \times D_{10} ]

• (k) = coefficient of permeability (cm/s)
• (C_1) = constant (100 to 150)
• (D_{10}) = effective grain size (cm)
• Applicable for uniform sands with uniformity coefficient ≤ 2 in loose state.

Notes:

• For large/deep excavations, permeability should be measured through full depth.
• Use field pumping tests for precise permeability values.

flowchart LR
    A[Subsurface Investigation] --> B[Soil Type Identification]
    B --> C[Determine Permeability]
    C --> D{Pumping Test Conducted?}
    D -- Yes --> E[Use Measured Permeability]
    D -- No --> F[Estimate k using k = C1 * D10]
    E & F --> G[Select Dewatering System]

This ensures proper design and safety of dewatering operations.

IS 9759: Selection of Dewatering Methods - Key Points

1. Soil Particle Size Distribution (Clause 5.2 & Fig.1)

• Dewatering method depends on soil type (gravel, sand, silt, clay).
• Determine soil gradation by sieve analysis.
• Plot grading curve on Fig.1 to select the suitable system.

2. Comparative Table of Dewatering Methods (Table 1)

3. Flow to a Slot from a Single Line Source (Clause 6.2.2, Table 2)

Key Formulas & Tables for Analysis and Design of Dewatering Systems (IS 9759)

1. Discharge from Wells / Wellpoints (Clause 1.10)

For fully penetrating wells: [ H - h = \frac{n + \ln\left(\frac{a}{r_w}\right)}{f_a} ]

• (H): Original groundwater level
• (h): Drawdown at well
• (a): Well spacing
• (r_w): Effective well radius
• (f_a): Uplift factor (depends on penetration)
• (n): Number of wells in the group

For partially penetrating wells, a modified formula applies (refer to IS 9759 for exact terms).

2. Flow to a Slot from a Single Line Source (Clause 6.2.2)

3. Soil Suitability & Dewatering Methods (Table 1 Summary)

IS 9759: Key Formulas & Specs for Wellpoint and Deep Well Systems

1. Deep Well Pumps (Clause 6.3.13.6)

• Pump Types: Turbine or Submersible pumps for wells ≥ 150 mm diameter.
• Selection Criteria: Based on pump capacity, not just well yield.
• Capacity Estimation: Use Table 8 (IS 9759) for approximate max capacity.
• Operation: Pumps should run at rated speeds for efficiency; higher speeds increase capacity with safety margin.

2. Wellpoint Pumps (Clause 6.3.10 & 6.2.1)

• Discharge-Drawdown Relation:
  For wellpoints closely spaced, treat the line of wells as a slot for flow calculations.

• Fundamental Formula (Dupuit-Thiem approximation for steady flow to a line sink):

  [ Q = \frac{2 \pi K (H^2 - h^2)}{\ln \frac{R}{r}} ]

  Where:

  • (Q) = discharge (m³/s)
  • (K) = hydraulic conductivity (m/s)
  • (H) = initial water table height (m)
  • (h) = drawdown height (m)
  • (R) = radius of influence (m)
  • (r) = radius of wellpoint or slot (m)

3. Design Steps (Clause 6.3)

• Determine soil permeability (K).
• Estimate drawdown (h) and radius of influence (R).
• Calculate discharge (Q) per wellpoint or deep well.
• Select pumps based on calculated discharge and rated speeds.
• Ensure spacing and number of wellpoints meet dewatering needs.

Summary Table (Example for Deep Well Pump Capacity)

Refer IS 9759 Table 8 for detailed values.

flowchart

IS 9759: Installation & Operation of Deep Well Systems - Key Points

1. Selection of Deep Well Pumps (Clause 6.3.13.6)

• Use turbine or submersible pumps for wells ≥ 150 mm diameter.
• Select pumps based on pump capacity, not just well yield.
• Refer to Table 8 (IS 9759) for approximate max capacity of deep well pumps.
• Pumps should operate at normal rated speeds; capacity increases at higher speeds (safety margin).

2. Installation Guidelines (Clause 8.1)

• Ensure proper alignment and secure anchorage of pump units.
• Prevent flooding/slushing at pump base by controlling water flow through sandy soil.
• Use wellpoints and sand drains on basin side to intercept flow and stabilize pump base (Clause 7.9.4).

3. Pump Base Protection (Clause 7.9.4)

• Avoid tilting/slushing caused by water flow through soil.
• Install sand drains and wellpoints to maintain stable soil conditions around pump base.

Example: Approximate Maximum Capacity (from Table 8, IS 9759)

Note: Actual Table 8 should be consulted for precise values.

flowchart LR
    A[Deep Well] --> B[Turbine/Submersible Pump]
    B --> C[Pump Base]
    C --> D{Water Flow through Soil?}
    D -- Yes --> E[Install Wellpoints & Sand Drains]
    D -- No --> F[Stable Pump Base]

Summary: Select pumps per Table 8 capacity at rated speeds; protect pump base from soil erosion by installing wellpoints/sand drains; follow installation steps for alignment and anchorage.

IS 9759: Key Formulas & Specifications for Sump Pumping

1. Pump Capacity Formula (Clause 10.4.6)

To estimate the required pump capacity for surface runoff:

[ ID = QR - V \times T ]

Where:

• Q = Total pump capacity (volume/time)
• QR = Average rate of runoff (volume/time)
• V = Volume of sump (volume)
• T = Duration of rainfall (time)

This helps size pumps to handle runoff without overflow.

2. Pump Types (Clause 9.2.8, Table 9)

Suitable pumps for open sumps include:

3. Surface Water Discharge (Clause 6.2)

• Use standard discharge formulae (e.g., Manning’s equation) for surface water flow estimation.

4. Wellpoint Pumps (Clause 6.3.10)

• Used for dewatering by lowering groundwater level around excavation.

flowchart TD
    A[Rainfall] --> B[Surface Runoff QR]
    B --> C[Sump Volume V]
    C --> D[Pump Capacity Q]
    D --> E[Discharge]
    E --> F[Prevent Flooding]

Summary: Use the formula (Q = QR - \frac{V}{T}) to size pumps; select pump type per Table 9; apply discharge formulas for surface water control; wellpoint pumps for groundwater.

IS 9759: Safety and Stability Considerations in Dewatering

Key Formulas for Discharge (Clause 6.2 & Table 2)

Filter Design Criteria (Clause 6.3.5)

Note: Proper filter design prevents piping under large hydraulic gradients.

Dewatering Methods & Suitability (Table 1)

• Sump Pumping: Simple, suitable for clean gravels/coarse sands; risk of instability.
• Wellpoint Systems: Sandy gravels to fine sands; economical for short durations; limited suction lift (~4.5-6 m).
• **Deep Bored

IS 9759: Electro-Osmosis Method Key Points

1. Principle (Clause 2.4 & Appendix A)

• Electro-osmosis uses an electric field to move pore water in fine-grained soils (silts, clays) where gravity drainage is ineffective.
• Useful when soil permeability < 0.5 × 10⁻⁴ cm/sec.

2. Discharge Estimation (Appendix A-1.8)

[ Q = -k_g \cdot i \cdot z / a ]

• (k_g) assumed same for sands, silts, and clays.

3. Precautions (Clause 11.2)

• To maintain groundwater levels:
  • Discharge water into absorption ditch at ground level for shallow deposits.
  • Use recharging wells to maintain head in lower pervious layers.

4. Usage Notes (Appendix B)

• Electro-osmosis is costly; apply only if soil permeability is very low.
• Effective for soils where capillary forces hinder gravity drainage.

flowchart LR
    A[Electric Field Applied] --> B[Pore Water Movement]
    B --> C[Water Collected at Wells]
    C --> D[Discharge to Absorption Ditch or Recharging Wells]

Summary: Use electro-osmosis for low permeability soils (<0.5×10⁻⁴ cm/sec). Calculate discharge using (Q = -k_g i z / a) and ensure proper water disposal to maintain groundwater equilibrium.

IS 9759: Hydraulic Calculations & Flow Formulas Summary

Key Discharge Formulas (Clause 6.2, 6.2.2)

• For fully penetrating slots, flow is twice that from Table 2 (single line source).
• At distances (y > 1.3 D) from slot, head (h) varies linearly:
  [ h = h_e + (H - h_e) \frac{y + D}{L + y D} ]

Corrections for Well Penetration (Clause 1.10)

[ H - h = (n + \ln \frac{a}{r_w}) \frac{Q}{2 \pi k D} \quad \text{(Fully penetrating wells)} ]

[ H - h = \text{(similar form with correction factor for partial penetration)} ]

• (a) = well spacing, (r_w) = effective well radius, (f_a) = uplift factor (depends on penetration).

Notations

IS 9759: Key Formulas & Specifications for Pump Selection and Performance

1. Horse Power of Pump (Clause 6.3.7)

Calculate required pump horsepower (HP) as:

[ \text{Horse Power} = \frac{\text{Total discharge (gpm)} \times \text{Total dynamic head (m)}}{3960 \times \text{Efficiency}} ]

• Total dynamic head = Operating vacuum at pump intake - Discharge friction losses
• Efficiency = Combined pump and engine efficiency (decimal)

2. Selection of Deep Well Pumps (Clause 6.3.13.6)

• Use turbines or submersible pumps for wells ≥ 150 mm diameter.
• Select pump capacity based on Table 8 (approximate max capacity).
• Pumps should operate at normal rated speeds; higher speeds increase capacity (margin of safety).

3. Wellpoint Pumps (Clause 6.3.10 & 6.3.10.1)

• Vacuum Pumps:

  • Use self-priming centrifugal pumps with vacuum pumps.
  • Typical vacuum developed: 6 to 7.5 m (design with 6 m vacuum).
  • Must handle air volume and produce high vacuum.

• Jet-eductor Pumps:

  • Suitable for lowering water table > 4.5 m with low flow (<10-15 gpm).
  • Can lower water table by 15 to 30 m.
  • Use single-stage wellpoint systems with jet-eductor pumps.

Summary Table (Example)

flowchart TD
    A[Pump Selection] --> B[Calculate HP]
    B --> C{Total Discharge x Head}
    C --> D[Divide by 3960 x Efficiency]
    A --> E[Select Pump Type]
    E --> F[Deep Well Pump (≥150 mm)]
    E --> G

Maintenance and Troubleshooting per IS 9759

1. Discharge Formulae (Clause 6.2)

• Fully penetrating wells:

[ H - h = \frac{n + \ln\left(\frac{a}{r_w}\right)}{k D} \times Q ]

• Partially penetrating wells:

[ H - h = f_a \times \frac{n + \ln\left(\frac{a}{r_w}\right)}{k D} \times Q ]

Where:

• (H) = Original groundwater level
• (h) = Drawdown at well
• (a) = Well spacing
• (D) = Depth of pervious stratum
• (r_w) = Effective well radius
• (f_a) = Uplift factor (depends on penetration)

2. Filter Design for Piping Prevention (Clause 6.3.5)

• Proper filter design prevents piping under high hydraulic gradients.

3. Maintenance & Troubleshooting Tips

• Standby Equipment: Keep standby pumps ready for emergencies (Clause 6.3.10.5).
• Regular Inspection: Check for clogging in filters and wells.
• Pump Operation: Ensure continuous operation for wellpoint systems.
• Noise & Vibration: Monitor for abnormal noise indicating pump or motor issues.
• Water Level Monitoring: Regularly measure drawdown to detect system inefficiencies.

Quick Reference: Dewatering Methods (Table 1)