October_1966 — Page 70

Ok, narrow, penetration-asphalt road at Km. 22.00 near Don Muang airfield

Recent view of same spat with dual four-lane concrete highway

A design speed of 100 kilometers per hour was chosen largely because of the level terrain.

The final horizontal alignment selected provided for safe operation at design speed. The maximum de- gree of curve permissible, for a 100 kph. design speed is a 4.6 degree (15° metric) curve. The maximum degree of curvature used on the Bangkok-Saraburi Highway is a 3.4 degree curve.

The superelevation rate was 0.06 ft ft. Controlling factors were the high intensity rainfall and the uncer- tainty of well-controlled operation of vehicles at a higher rate of supereleva- tion.

Total width of the highway from Don Muang to Saraburi was specified in the engineering contract to be 12 metres, consisting of two traffic lanes each 1.5 metres wide with 2.5 metre wide shoulders on each side. Using the as

a satisfactory standard, the four-lane portion of the highway from Bangkok to Don Muang was also de- signed with 3.5 metre wide traffic lanes. Outside shoulders on the four- lane portion were 2.5 metres wide, as were the inside shoulders which com- bined to form a 5 metre wide median.

The choice of surfacing was limited by the engineering contract, which specified Portland cement concrete

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from Bangkok to Don Muang Air- port. A cost study, conducted by the engineers, compared the estimated cost of Portland cement concrete with asphaltic concrete. The results in- dicated that cement concrete was slightly more economical than flexible type pavement to about km. 27.

metre

As it was not necessary to con- struct embankments over above the surrounding ground between Bangkok and Rangsit (km. 31.30) to keep the roadway surface above high water, it was decided to use Portland cement concrete pavement all the way from Bangkok to Rangsit, Asphalt concrete surfacing was specified be- tween km. 31.30 and Saraburi.

A bituminous prime and seal coat with crushed rock cover aggregate was included in the grading contracts for the portion which was later to be surfaced by an asphalt plant-mix sur- facing. The prime and seal coat were added to preserve the crushed rock base course from damage by traffic and weather until the plant-mix sur- facing could be placed. This measure paid dividends in reduced mainten- ance and reduced reshaping costs, as it was more than two years after the first seal coat was completed before the asphalt surfacing was begun.

On the lime-stabilized portion of the road sand was used to provide the

cushion between the stabilized layer

and the concrete pavement, to pro- tect the stabilized layer during the curing period and to provide drainage under the pavement.

Pavement Design

For reasons of economy a thicken- ed edge pavement was selected and designed In accordance with pro- cedures recommended by the Portland C'ement Association. Steel mesh was used ลา temperature reinforcement with deformed reinforcing hars as tie bars, and smooth bars as dowel.

The thickened edge slab saved ap- proximately 10 per cent on concrete and Cost insignificantly more in labour, because subgrade preparation in Thailand is almost entirely a hand- labour operation. It is as easy to prepure the grade for a thickened edge as for a flat slab.

Sawed joints were specified at 15. metre centres. Sawed joints offer a much superior riding surface and eliminate spalling of the lattice that nearly always collects at a formed joint.

For the first time in Thailand a transverse broom finish was specified rather than a float finish.

For the asphalt-surfaced road it was determined that the pavement must be approximately 90-100 cm. thick to support the design wheel load of 5,500 kg. From examination of the existing road it was considered that the existing pavement would supply only about 30 cm. of the total thickness required.

The estimated maximum flood level plus an allowance for settlement also required that the surface be about 70 cm. above the old road, which would keep the base course and surfacing above maximum high water.

The select borrow course was con- sidered as a matching and leveling course for the old pavement. The subbase course was intended to be the same as the select borrow, but up- graded in quality by crushing over- size material to $ cm. and adding other granular material as necessary to achieve the specified CBR. The hase course was dense graded for maximum stability. This was the first major project in Thailand where Thai contractors used a well-graded base course instead of one of uniform gradation.

Minor Drainage Structures

Minor drainage structures consisted of reinforced concrete culvert pipe Some were of standard strength and some of higher strength where they were to be placed under low roadway embankments with minimum cover.

or

All the culvert pipe used was round and either of the bell-and-spigot_type tongue-and-groove type, and all were cast in 1-metre lengths. Internal diameters were either 60 cm., 80 cm.. 100 cm., or 120 cm. For all sizes larger than the 60 cm. two lines of circular reinforcement were used in

Far East Architect & Builder October, 1966

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