Question 3 of 7: Intersection Sight Distance and Pavement Distress

Question 3: Intersection Sight Distance and Pavement Distress (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given.

Find. (a) the length of major-road pavement that must be visible to a driver stopped on the minor road, so that a truck can cross without a major-road vehicle having to slow; (b) the causes of seven named distresses and their classification as structural or functional.

Approach. Part (a) is a departure-sight-triangle problem: compute the distance the truck must travel to clear the major road entirely, convert that to a crossing time through an assumed acceleration, add the perception–reaction time, and multiply the total by the major-road design speed. Part (b) is descriptive, and each distress is classified by asking whether it reduces load-carrying capacity (structural) or only ride quality and safety (functional).

1. Part (a) — distance the truck must travel to clear the crossing. The front of the vehicle starts 3.0 m from the near edge, and the vehicle is clear only when its rear bumper has passed the far edge, so the travelled distance is the set-back plus the pavement width plus the vehicle length: $$D = d_s + w + \ell = 3.0 + 7.5 + 9.1 = 19.6\ \text{m}$$
2. Time to traverse that distance. Starting from rest under a constant acceleration a, $D = \tfrac{1}{2}a t_a^2$, so $$t_a = \sqrt{\frac{2D}{a}} = \sqrt{\frac{2(19.6)}{1.5}} = \sqrt{26.13} = 5.11\ \text{s}$$ using a = 1.5 m/s², a representative value for a loaded single-unit truck accelerating from a stop. The paper does not supply an acceleration, and Note 2 permits the assumption.
3. Total occupancy time of the intersection. The driver must first perceive the gap and react, then execute the crossing: $$t = t_{pr} + t_a = 2.5 + 5.11 = 7.61\ \text{s}$$
4. Sight distance along the major road. A major-road vehicle travelling at the design speed must not close on the intersection during that time, so the visible length of major road is $$d = v\,t = \frac{80}{3.6}(7.61) = 22.22(7.61)$$ $$\boxed{d = 169\ \text{m}}$$
5. Cross-check against the gap-acceptance model and adopt a design value. The alternative AASHTO and TAC procedure sets the sight distance directly from a tabulated critical gap, 8.5 s for a single-unit truck crossing a two-lane road: $$d = 0.278\,V\,t_g = 0.278(80)(8.5) = 189\ \text{m}$$ The two methods bracket the answer, and the larger governs, so a design sight distance of 190 m is adopted along the major road in each direction from the intersection. Measured from the driver's eye on the minor road, that distance defines the legs of the departure sight triangle within which no obstruction — sign, barrier, planting or parked vehicle — may stand.
6. Sensitivity to the assumed acceleration. Because the crossing time varies as $a^{-1/2}$, the result is only moderately sensitive: a = 1.2 m/s² gives 183 m and a conservative a = 0.9 m/s² gives 202 m. All three lie within about 10 per cent of the adopted 190 m, so the design value is robust against the assumption.

Part (b) — causes and classification of the named distresses. The distinction used throughout is the standard one: a structural distress reflects or causes a loss of load-carrying capacity in one or more layers, and its remedy is structural (overlay, reconstruction, subgrade improvement); a functional distress leaves capacity intact but degrades ride quality, safety or surface texture, and its remedy is a surface treatment.

Asphalt pavements. (i) Bleeding is a film of free asphalt on the surface, shining and tacky in hot weather. It arises from an excessive asphalt content or too low an air-void content in the mix, from an over-applied prime or tack coat, or from binder expanding into the remaining voids as pavement temperature rises; heavy channelised traffic accelerates it. Bleeding does not weaken the structure, but it destroys surface texture and skid resistance, so it is classified as functional.

(ii) Fatigue (alligator) cracking begins as fine parallel longitudinal cracks in the wheel path that interconnect into a many-sided pattern resembling alligator hide. It is caused by repeated tensile strain at the bottom of the asphalt layer exceeding the material's fatigue capacity — the classic symptoms of an under-designed section, of a weak or saturated subgrade, of poor drainage, or of traffic loading heavier than the design ESAL. It is the archetypal structural distress and normally calls for a structural overlay or reconstruction.

(iii) Polished aggregate is the gradual wearing smooth of the coarse aggregate exposed at the surface, so that the microtexture that generates wet-weather friction is lost. The cause is an aggregate with a low polished-stone value — typically soft carbonates such as limestone — combined with high traffic volumes. Load capacity is unaffected, so the classification is functional, but the safety consequence is serious enough that a friction course or micro-surfacing is usually applied promptly.

(iv) Rutting is a longitudinal depression in the wheel path. It has two distinct origins that must be separated before treatment: plastic flow within the asphalt layer itself, caused by an unstable mix (excess binder, rounded natural sand, insufficient VMA, inadequate compaction), and consolidation or shear failure in the base, subbase or subgrade under repeated loading. The first is a surface-mix problem, but the second is a loss of layer capacity, and because rutting is normally driven by the lower layers it is classified as structural; it also carries a functional consequence, since ruts pond water and cause hydroplaning.

Concrete pavements. (i) D-cracking appears as closely spaced crescent-shaped cracks parallel to joints, cracks and free edges, often with a dark staining. The mechanism is freeze–thaw disruption of a frost-susceptible coarse aggregate that becomes saturated where water collects beneath the slab near joints; the aggregate itself fractures and the surrounding paste disintegrates. Because the concrete progressively loses integrity, D-cracking is a durability distress with structural consequences, and it cannot be arrested once started — the affected concrete must be removed.

(ii) Pumping is the ejection of water and fine material through joints and cracks, and along the pavement edge, under the passage of heavy axles, leaving a fan of fines on the shoulder. It needs three ingredients simultaneously: free water beneath the slab, an erodible fine-grained subbase or subgrade, and frequent heavy loads with poor load transfer across the joint. Each cycle removes support, and the resulting voids lead to faulting, corner breaks and slab cracking, so pumping is structural. Prevention is by drainage, a stabilised non-erodible subbase and effective dowelled load transfer.

(iii) Map cracking (craze cracking) is a network of fine, shallow, interconnected cracks on the surface. Two causes are common: improper finishing and curing, in which the surface dries and shrinks faster than the concrete beneath, and alkali–silica reaction, in which a reactive aggregate and the pore-solution alkalis form an expansive gel. In its ordinary shrinkage form the cracking is confined to the surface and is functional, becoming a scaling problem only under de-icing salts; where alkali–silica reaction is the cause it progresses through the slab and becomes structural, so a petrographic examination is warranted before choosing a treatment.