A FIFTH DIMENSION TO ENGINEERING - Ziveli Village · calculations for the 20 years are illustrated...

October 2016 i

PREPARED FOR:

PREPARED BY:

Square Foot Develepments P.O. Box 2680 SWAKOPMUND Tel: +264 (0) 81 300 7292 Contact: Mr. D Oosthuizen [email protected]

Element Consulting Engineers 6 Otto Nitzsche Street KLEIN WINDHOEK Tel: +264 (61) 30 9416 Fax: +264 (61) 30 9412 Contact: Mr. B Cilliers [email protected]

A FIFTH DIMENSION TO ENGINEERING

mailto:[email protected]

mailto:[email protected]

Preliminary Investigation Report for the Road Capacity of DR1504 to Ziveli Lifestyle Estate

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TABLE OF CONTENTS

1 Introduction ................................................................................................................................................... 1

2 Scope of Works .............................................................................................................................................. 1

3 Available data ................................................................................................................................................ 1

4 Traffic estimation ........................................................................................................................................... 2

4.1 Traffic Counts ........................................................................................................................................ 2

4.2 Trip generation ...................................................................................................................................... 2

4.3 Design Traffic......................................................................................................................................... 3

5 Pavement Design ........................................................................................................................................... 4

5.1 Pavement Investigation ........................................................................................................................ 4

5.2 Preliminary Pavement Design ............................................................................................................... 4

6 Geometric Standards ..................................................................................................................................... 6

6.1 Cross section ......................................................................................................................................... 6

6.2 Intersection ........................................................................................................................................... 6

7 Conclusion ...................................................................................................................................................... 7


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1 INTRODUCTION Element Consulting Engineers were approached by Square Foot Developments, the developer of Ziveli Lifestyle

Estate, to conduct an investigation into the remaining life of District Road 1504 which will serve as the access

road to Ziveli Lifestyle Estate.

DR1504 is located about 12km south of Windhoek on the B1 as is indicated in the Locality Plan below. The

road currently serves as access for a number of small holdings / developments as well as a military base

located at the end of DR1504. The existing road varies from surfaced, to gravel, to surfaced again up to the

military base.

The concerned portion consists of 2.75km of surfaced road (6.8m wide) followed by 0.8km of gravel road up to

the access to Ziveli Lifestyle Estate. The surfaced section is generally in a fair condition although the riding

quality is not good. The gravel section is in a relatively good condition.

2 SCOPE OF WORKS The scope of works for this assignment consists of the following tasks:

Conduct a road condition survey

Do traffic estimation including projected generated traffic for the development

Perform a pavement investigation and analysis

Conduct a preliminary pavement design based on the projected traffic loading

Determine the remaining life of the pavement

3 AVAILABLE DATA Square Foot Developments provided the following data:

The Aris Town Planning Scheme Map indicating all plots contributing to traffic on D1504


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Proposed layout of Ziveli Lifestyle Estate

Density calculation / number of proposed living units for the planned development

Calculations on the developments / density of the area envisaged in the near to medium future,

including projects currently planned (from the town-planner active in the area)

Initial comments received from the Roads Authority

An e-mail of Stephanie van Zyl, the planning consultant, with her comments / requests in this regard

Other data collected include:

Traffic count data acquired from the Roads Authority (RA)

4 TRAFFIC ESTIMATION Design traffic was estimated based on traffic counts (from the RA) on the existing road plus projected

generated traffic for the new development.

4.1 TRAFFIC COUNTS

Two sets of traffic count data were acquired from the RA; both counts were conducted in 2015. Although the

traffic in both instances is fairly low, the two sets of data do vary considerably:

- The Total Traffic varies by 18.5% (from lowest Total Traffic)

- The Heavy Vehicle counts vary by 32% (from the lowest HV count)

These variances are deemed unacceptable, especially since the HV’s contribute the bulk of the design traffic. A

sensitivity analysis will be conducted to determine the pavement’s sensitivity to the varying traffic data. It is

proposed that manual traffic counts be done to confirm the accuracy of the data.

The two sets of traffic counts are summarised in Table 1 below.

TABLE 1 SUMMARY OF ROADS AUTHORITY TRAFFIC COUNTS

Data Set 1 Data Set 2

Light Vehicles 216 270

Heavy Vehicles (& %HV’s) 33 (13%) 25 (8.5%)

Total Traffic 249 295

4.2 TRIP GENERATION

Generated traffic is determined in accordance with COTO’s TMH 17 Trip Data Manual from the information

received regarding the development. All residential units in the development are defined as Townhouses

(Simplexes and Duplexes) according to THM 17, which relates to dwelling units typically provided in clusters or

in complexes. Units can be detached or provided within one building structure. Parking is often provided

within a communal area.

Trip generation is the amount of traffic generated by a development. A trip is defined as a single (one

directional) movement with either the destination or the origin of the trip at a development. The development

is the destination when a person visits the development and an origin when the person leaves the

development. Should a person both visit and leave a development, two trips are counted.

Daily trip generation rates and other associated traffic parameters are provided in Table 3.1 of TMH17. These

parameters are intended for use in the determination of engineering service contributions. Trip generation


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adjustment factors are provided in Table 3.2 (TMH17) which is used both for establishing engineering service

contributions and traffic assessments.

The following trip generation rates and parameters are provided in Table 3.1 for different land uses:

a) AADTD, the Annual Average Daily Trip generation rate (per size unit), the estimated total in- and

outbound traffic generated by one size unit of the development over one year divided by the number

of days in a year

b) The traffic factor FQD used to convert the AADTD to an equivalent impact hourly flow rate for the

development. It is a combined factor which accounts for factors such as the heavy vehicle generation

as well as the traffic peaking characteristics of a development. The factor is determined using

formulae provided in the South African Engineering Service Contribution Manual for Municipal Road

Infrastructure

For the planned 298 dwelling units the generated traffic amounts to:

298 𝑈𝑛𝑖𝑡𝑠 × 3.75 𝐴𝐴𝐷𝑇𝐷 = 1117.5 𝐴𝐴𝐷𝑇 (𝑏𝑜𝑡ℎ 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛𝑠) ≅ 560 𝐴𝐴𝐷𝑇 𝑝𝑒𝑟 𝑙𝑎𝑛𝑒

For the planned Lodge / Hotel the generated traffic amounts to:

(150 𝑅𝑜𝑜𝑚𝑠 × 2 𝐵𝑒𝑑𝑠 𝑝𝑒𝑟 𝑟𝑜𝑜𝑚) × 3.25 𝐴𝐴𝐷𝑇𝐷 = 490 𝐴𝐴𝐷𝑇 (𝑏𝑜𝑡ℎ 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛𝑠) = 280 𝐴𝐴𝐷𝑇 𝑝𝑒𝑟 𝑙𝑎𝑛𝑒

* Assume 150 rooms at 2 beds per room

The total generated traffic for the whole development is therefore 560 + 280 = 840 trips / lane.

4.3 DESIGN TRAFFIC

The design traffic will be calculated for a 20 year design life, starting from the commencement of construction

in 2019 (assumed). The start traffic was calculated by applying a moderate growth rate (4% growth rate

assumed (Table 17, SAPEM) since traffic data is insufficient to determine an actual growth rate) to the last

traffic counts (2015) up to the start date (2019).

The design traffic takes into account the following periods:

2019 – 2023 (5 years), which includes a 5 year construction period (assumed) with resulting additional

construction traffic

2024 – 2038 (15 years), which includes the 15 years after construction including generated traffic

The design traffic for 20 years was calculated for each traffic count data set. The two cumulative design traffic

calculations for the 20 years are illustrated graphically in Error! Reference source not found. below.


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FIGURE 1 CUMULATIVE DESIGN AXLES FOR 20 YEAR DESIGN LIFE FOR BOTH TRAFFIC COUNTS DATA SET

Figure 1 clearly shows the impact of the different traffic counts on the design traffic, resulting in an 86 396

design axles variance, or nearly 16%.

5 PAVEMENT DESIGN

5.1 PAVEMENT INVESTIGATION

Element Consulting Engineer visited the site and did a brief visual assessment of the road. The surfaced section

is in a fair condition, with only some stone loss and a few mechanical damage issues observed. The riding

quality of the surfaced section is however poor. The surfaced section used to be a gravel road before it was

sealed with a low volume seal, which basically implies that the old gravel wearing course was reworked and

then sealed. Low volume sealed roads are typically not fully engineered, with substandard alignments and can

potentially develop other issues such as poor riding quality.

The short section of gravel road up to Ziveli Lifestyle Estate is in a good condition.

Element Consulting Engineers conducted limited material investigation tests on the existing pavement to

determine the structure and characteristics of the existing pavement. Tests included 1 test pit and 5 DCP tests.

Standard laboratory tests (Grading Analysis, MDD, CBR and Atterberg Limits) were conducted on the samples

taken from the test pit to establish the material properties.

Awaiting material investigation results…

5.2 PRELIMINARY PAVEMENT DESIGN

It is anticipated that the pavement will consist of the following pavement structure:

Seal Low volume seal (to be confirmed) Base ±150mm G6 (to be confirmed) Subbase ±150mm G7 (to be confirmed) Subgrade G9 (to be confirmed)

0

100,000

200,000

300,000

400,000

500,000

600,000

2014 2016 2018 2020 2022 2024 2026 2028 2030 2032 2034 2036 2038 2040

Cu

mu

lati

ve D

esi

gn A

xle

s (E

80

's)

Year

216 LV, 33 HV 270 LV, 25 HV


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TRH4 catalogue designs do not include pavements with G6 base material therefore the above pavement

structure cannot be compared with the TRH4 designs.

Four possible scenarios of the pavement structure were therefore analysed using mechanistic design software

(Rubicon) which yielded the following results:

150mm G6 Base < 1 000 standard axles Fail within 0.25 years

150mm G7 Subbase G9 Subgrade 150mm G5 Base

1 000 standard axles Fail within 0.25 years


< 1 000 standard axles Fail within 0.25 years


3 000 standard axles Fail within 0.25 years

150mm G7 Subbase G8 Subgrade

In all instances the analysis calculated nearly no design life, with the subgrade failing in each scenario. This

clearly shows that the 300mm cover over the subgrade is insufficient and more cover is required. The pavement

(anticipated) is therefore insufficient to carry the design traffic loading and will required strengthening.

Possible strengthening measures were investigated which included the addition on a new base layer to increase

the subgrade cover and also provide a higher quality base. The calculations yielded the following results:

150mm G4 Base 97 000 standard axles Fail after 4.5 years

150mm G6 Subbase 150mm G7 Selected G9 Subgrade 150mm G4 Base

0.3 mil standard axles Fail after 11 years

150mm G6 Subbase 150mm G7 Selected G8 Subgrade 175mm G4 Base

0.7 mil standard axles Fail after 21.5 years

150mm G6 Subbase 150mm G7 Selected G8 Subgrade

Adding a new 175mm G4 base will strengthen the pavement sufficiently to carry the design traffic loading of

0.46 / 0.55 million axles.

Further options will be investigated when the actual material properties are known, but it is recommended

that further testing will be required during the detailed design stage.


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6 GEOMETRIC STANDARDS

6.1 CROSS SECTION

The RA’s RMS Division lists the surfaced section of the road as 6.8m wide, which in terms of the RA Standard

Drawings is suitable for up to 1 000 AADEVU (Average Annual Daily Equivalent Vehicle Units). The figure below

compares the projected AADT (Average Annual Daily Traffic) with the 1 000 AADEVU threshold.

FIGURE 2 AADT GROWTH OVER DESIGN PERIOD

From the above figure it is clear that initially the current cross section is adequate, but as traffic is generated

by the development, the AADT will surpass the 1 000 AADEVU threshold in 2023 / 2027. This implies that the

upgrading of the road will be required at some stage. Depending on the material quality and width of the

existing gravel shoulder this can be achieved with relatively limited work by simply widening the existing

surfacing. If however the existing road profile or structure does not allow for this, greater effort and cost will

be required.

6.2 INTERSECTION

DR1504 starts at the intersection with the B1 (TR1/5) between Windhoek and Rehoboth. Figure 3 below shows

an aerial view of the intersection. It is clear from the figure that the intersection includes a southbound left

turn lane for traffic turning into DR1504 from Windhoek.

0

200

400

600

800

1,000

1,200

1,400

2014 2016 2018 2020 2022 2024 2026 2028 2030 2032 2034 2036 2038 2040

AA

DT

Year

216 LV, 33 HV 270 LV, 25 HV Threshhold


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FIGURE 3 INTERSECTION OF DR1504 WITH B1 (TR1/5)

There are no acceleration lanes for traffic turning from DR1504 into the B1. This may present a problem during

peak hours with the already high commuter traffic travelling from / to Rehoboth, but a detailed investigation

will be required to confirm this. It is anticipated that most traffic will travel to / from Windhoek and therefore

the right turning action from DR1504 into the B1 may especially present problems. Vehicles turning right

towards Windhoek will conflict with the fast moving stream (also towards Windhoek) due to their initial slow

movement while they accelerate to normal operating speed. These conflicting speeds may increase the

potential for collisions, especially under high traffic flow. If traffic flow volumes warrant it, slow moving

vehicles should be separated from higher speed through lanes.

The following detailed investigations are proposed in this regard:

The sight distances either way of the intersection must be verified

Detailed hourly traffic counts, especially peak traffic, should be done at the intersection to determine

the need for possible upgrading of the intersection (if required, TRH17 (Table 8.4.2(a)) suggests an

acceleration lane length of 550m (including 1:50 taper))

The geometry of the existing intersection should be verified

7 CONCLUSION The concerned portion of DR1504 consists of 2.75km surfaced road and about 800m gravel road up to Ziveli

Lifestyle Estate. The surfaced section was constructed as a low volume seal which implies that the old gravel

road was merely reworked and surfaced to its existing alignment. The poor riding quality could be a symptom

of the low volume seal. Other than the poor riding quality the road is in a fair condition.

Traffic count data obtained from the Roads Authority varies considerable and needs to be verified. Along with

the traffic generated by the development the expected 20 year design traffic is about 0.5 million standard

axles.


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The pavement was investigated by test pit and 5 DCP tests, which results are still awaited. Based on an

assumed pavement structure the pavement is insufficient, yielding almost no structural capacity. Strengthening

of the pavement will be required.

The current cross section of the road, i.e. 6.8m wide, is still adequate according to RA standards, but will need

to be upgraded as traffic volumes increase; projections show that the threshold for upgrading will be reached

in 2023 / 2027.

Detailed traffic counts will have to be done to assess the safety and capacity of the intersection of DR1504

with the B1. Sight distances and the intersection geometry will also have to be verified.

A FIFTH DIMENSION TO ENGINEERING - Ziveli Village · calculations for the 20 years are illustrated...

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