Republic of the Philippines
DEPARTMENT OF PUBLIC WORKS AND HIGHWAYS
OFFICE OF THE SECRETARY
Bonifacio Drive, Port Area Manila
RAcOHG PILIP\HAS
Mar £? o 2025
DEPARTMENT ORDER SUBJECT: DPWH Standard Specification for
Item 501 -Underdrains
N.. 90
Series of 2025
To support the Department's commitment in updating its standard specifications and adopting
effective/appropriate solutions for specific project needs, the attached Standard Specification
for Item 501 - Underdrains is hereby prescribed for adoption in DPWH infrastructure
projects that require the utilization of such on the Program of Works.
The Standard Specification shall form part of the DPWH Standard Specifications for Highways,
Bridges and Airports, Volume 11, Likewise, the new Pay Item is now included in the Standard
Pay Item List and in the Project and Contract Management Application (PCMA).
This Order shall take effect immediately.
Departmen5fflofcopuji'{;ewfr:rae?anr?H!ghways
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Encl: DPWH standard specification for Item 501 -Underdrains
14.1.2 JDV/AGC
DPWH Standard Specification for
ITEM 501 -UNDERDRAINS
501.1 Description
This Item shall consist of constructing underdrains, using pipe and granular filter materials,
underdrain pipe outlets using granular material in accordance with this Specification and in
conformity with the lines and grades shown on the Plans to effectively reduce excessive
moisture content of existing subgrade for stable and long-lasting pavement.
501.2 Material Requirements
501.2.1 Underdrains
The size of the underdrain pipes to be furnished shall be based on the nominal internal
diameter of a pipe as shown in the Plans. All sizes shall be supported with hydrologic data
analysis and hydraulic design.
Underdrain materials shall conform to the applicable requirements of Item 706, Concrete, Clay,
Plastic and Fiber Pipe.
Pipe fittings shall match the pipe material type to be used.
501.2.2 Granular Backfill
Granular Backfill Filter Material - Granular backfill filter material shall be permeable and shall
meet the requirements of AASHTO M 6 - Fine Aggregates for Hydraulic Cement Concrete, except
that soundness tests will not be required and minor variation in grading and content of
deleterious substances may be approved by the Engineer.
Table 501.1 Grading Requirements for Fine Drain Fill
Fine Aggregate Size Percent Passing by Weight
9.5 mm 100
4.75 mm 95 to 100
2.36 mm 80 to 100
1.18 mm 50 to 85
600 urn 25 to 60
300 Llm 10 to 30
150 urn 2 to 10
When the location of manufacturing plants allows, the plants will be inspected periodically by
compliance with specified manufacturing methods, and material sample will be obtained for
laboratory testing for compliance with material quality requirements. This shall be the basis
for acceptance of manufacturing lots as to quality.
``AII material shall be subjected to inspection for acceptance as to condition at the latest
practicable time.
501.2.3 Filter Cloth
Filter cloth shall conform to the applicable requirements of Item 715, Geotextile.
Department Order No. 9fl, s. 2025
DPWH Standard Specification for Item 501 - Underdraihs
Page 2 of 3
501[3 Construction Requirements
501.3.1 Pipe Installation
Trenches shall be excavated to the dimensions, depths, and grades required by the Plans or
as directed by the Engineer. The excavation shall begin at the outfall and proceed toward the
upper end. A minimum of 150 mm bedding layer of granular backfill material shall be placed
and compacted at the bottom of the trench and a width equal to the outer diameter of
underdrain pipe plus 150 mm at each side and length.
Filter cloth shall be placed as shown on the Plans and the Standard Details.
Subdrainage pipe of the type and size specified shall be embedded firmly in the bedding
material.
Perforated pipe shall normally be placed with the perfo'rations down and the pipe sections
shall be joined securely with the appropriate coupling fittings or bands.
Other pipes shall be laid with the bell end upgrade and with open joints wrapped with suitable
material to permit entry of water, or unwrapped as may be specified. Upgrade end sections
of all subdrainage pipe installations shall be closed with suitable plugs to prevent entry of soil
materials.
After the pipe installation has been inspected and approved, granular backfill material shall be
placed to a height of 300 mm above the top of pipe. Care shall be taken not to displace the
pipe or the covering at open joints. The remainder of the granular backfill material shall then
be placed and compacted in 150 mm maximum layers to the required height, Any remaining
portion of trench above the granular backfill shall be filled with either granular or impervious
material, as may be specified, and thoroughly compacted.
501-.3.2 Underdrain Outlets
Trenches for underdrain outlets shall be excavated to the width and depth shown on the Plans
or as otherwise directed. Pipes shall be laid in the trench with all ends firmly joined by the
applicable methods and means. Pipe screens shall be placed in the end of the outlet pipe.
Underdrain outlet protectors shall be constructed as indicated on the plans. After inspection
and approval of the pipe installation, the trench shall be backfilled in accordance with Item
103, Structure Excavation.
501.4 Method of Measurement
Underdrains and outlets shall be measured by the linear meter or lump sum for pipe of the
type and size specified. Blind drains shall be measured by the linear meter or lump sum
including all excavation and backfill materials required.
``Granular backfill filter material, when specified in the Contract as a Pay Item shall be measured
in place by the cubic meter, completed and accepted. Cross-sectional measurements will not
exceed the net dimensioris shovyn on the Plans or as directed by the Engineer. Filter cloth
shall not be measured separately whe-n specified on the plan. Its cost shall be considered as
a subsidiary item under Underdrain.
Department Order No. qo_, s. 2025
DPWH Standard Specification for Item 501 - Underdrains
Page 3 of 3
Excavation for underdrain pipe will be measured and paid for as provided in Item 103,
Structure Excavation.
501[5 Basis of Payment
The accepted quantities determined as provided in Section 501.4, Method of Measurement,
shall be paid for at the contract unit prices for underdrains which price and payment shall be
full compensation for furnishing and installing all materials, labor, equipment, tools and
incidentals necessary to complete this Item.
Payment shall be made under:
Pay Item Unit of
Description
Number Measurement
501 '1)a Underdrajn, 100 mm dja. Linear Meter
501 (1)b Underdrain, 125 mm dia. Linear Meter
501 (1)c Underdrain, 150 mm dia. Linear Meter
501 [1)d Underdrain, 175 mm dia. Linear Meter
501 (1)e Underdraih, 200 mm dia. Linear Meter
501 (1)f Underdrain, 250 mm dia. Linear Meter
501 (1)a Underdrain, 300 mm dia. Linear Meter
501 (2) Blind drain Linear Meter
501 (3) Granular backfill filter material for Underdrains Cubic Meter
501 (4) Underdrain Lump Sum
501 (5) Blind drain Lump Sum
