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Drains and Culverts at CH50

Invert Levels, Drain Depths, Culvert Depths, Levels 

 Invert Levels of Drains & Box Culverts

Dwg No:

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There appears to be some discrepancy in the top and invert levels of the box culverts and drains. Information extracted from Drawings, please check and verify on site.

  • TOP of Drain = +4.91m
  • BOT of Drain = + 3.71m
  • DEPTH of Drain = 1.20m
  • CROWN of Road = +5.41m
  • DEPTH OF COVER = 5.41-4.91=0.5m <-- can be much less
  • DEPTH of Culvert = 1.2m, WIDTH=2m

Note that drain depth is shown to be 1.20m as against what was interpreted earlier. However, if a shallower depth of 900mm is used, then the Invert Levels and Gradient should not change, a constant depth could be maintained throughout the full length by just changing the side berm slope, not exceeding 1:1.5 or as per drawing. If side wall of drain is varied, then formwork will be more difficult to do.

Use any CAD software, such as AutoCAD to plot road cross-section at the box-culvert section to scale and any discrepancies or problems should show up clearly. Alternatively setup in Excel Spreadsheet auto calculation for drops, gradients and inverts, then use this as template for calculations of cross falls for road sections, but must check carefully for errors in formula.

Issued 05/Feb 2011, MEC Engineers, "Discrepancy, To Re-Confirm on Site Conditions".

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Cross-sections A-A at CH 50

 Cross Section AA CH 50

Dwg No:

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  • 2000 Width x 1500 Height
  • 1:500 Slope
  • Drop both ends or slope grade to fit drop
  • Earth drain to suit depth and side slope (1:1.5), invert not critical, decide on site.
  • Ends of Culvert have channelled flow.
  • Concrete Drain at 1.2m depth (if 900mm fix invert same, only top change)

Issued 05/Feb 2011, MEC Engineers, "Cross Section A-A, Box Culvert Profile & Inverts".

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The following are Additional Links.

New Bridge Span (issues 09/Feb/2011)


Where the soil foundation for the culvert is good and piling is not required, then the culvert can be simply constructed directly, based on soil bearing capacity. In this case where the culvert is long there will be areas where soft soil exist. These soft areas must be treated properly otherwise differential settlement will occur to the box culvert, leading to uneven settlement of the road itself. A good way to physically check the quality of base subgrade is to use a wooden stud probe, and manually punch on the subgrade along the length of the culvert and if you find little resistance then these are "soft areas". Generally for wet areas the bearing capacity should be halved that of dry areas.

Hard & Soft Soil Foundation Areas can be locally treated as follows :-
  • For those areas which are known to be firm, just cast the Concrete Base.
  • For those areas which appears weak or soft as denoted by the pile with 1 blow, over-excavate until you reach firm soil and backfill with coarse sand or crusher run/sand mix and compact. Concrete can be cast directly.
  • Where piles are already driven, do not extract, just cut below the base of the Culvert for about 450mm - 600mm, and backfill with crusher run/sand mix, compact and cast the Concrete.
  • For those piles already extracted, make sure the holes left are well plugged with coarse sand only, vibrate to quick sand effect (add water) and topped up with crusher run/sand mix, compact before concreting. If the holes are too big, filled with mass concrete as the worst case.

The above are best & economical practices, however for simplicity, in areas of uneven hard/soft spots, just over excavate by 450mm or deeper in softer areas, and whole length of culvert base backfilled with crusher run/sand mix, compact well. Put lean concrete and then the Base of Culvert can be constructed. If you do not want to use lean concrete, just increase the concreting depth by about 40mm, and make sure the rebars are placed higher up. An easier but more expensive approach would be to just over-excavate by about 450mm and cast mass concrete grade 15 throughout.

Note that in the absence of any reliable data, for making decision in the field on safe bearing capacity, assume 2 tons/ft2 for hard and firm soil, 1 tons/ft2 generally and halved for wet areas. In areas of fill, with some compaction, use 1 tons/ft2, without compaction, use half that value. With reliable/supervised compaction, use 2 tons/ft2.

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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng) MEC Engineers, Civil & Structural Engineer HP: 016-8672189, Email: Widget: email cloaker