Geosynthetic Basement Drainage Block 6, Government Enclave, Gaborone, Botswana

Transcription

1 GEOSYNTHETICS Ref No: DRAIN /05 Geosynthetic Basement Drainage Block 6, Government Enclave, Gaborone, Botswana Project: Basement Drainage Client: Ministry of Works, Botswana Date: 1995 Consultants: Burrow, Binnie & Partners William Lee & Associates HKS Law Gibb Inc Geotechnics Africa Product: Kaymat U14 Kaymat U24 Flownet DN3 Geopipe M100R Contractor: NV BESIX SA Quantity: 4 040m² 1 590m² 4 800m² 750m INTRODUCTION In 1993 the Ministry of Works of the Botswana Government issued a design and construct tender for two new government buildings (Blocks 6 and 8) to be built in the Government Enclave at Gaborone, Botswana. The contract was awarded to BESIX Construction of Gaborone, who had employed consulting engineers William Lee & Associates, also of Gaborone, to design the structures and prepare the bid documents. In February 1995 Kaytech was contacted by the Johannesburg branch of consulting engineers HKS Law Gibb, who were providing specialist structural design services to William Lee & Associates of Gaborone. The latter were finalising the design details for Block 6. HKS were concerned about the drainage arrangements for the five metre deep parking basement of Block 6 as there was a perched water table associated with the site. They were concerned that this water would exert pressure on the parking basement floor slabs, as well as the basement retaining wall, despite conventional drainage provisions made for this at tender stage. TENDER STAGE DRAINAGE DESIGN A no-fines concrete drainage layer had been allowed for under the floor slabs as well as against the basement retaining wall, for the purposes of the tender submission. However, there were two main concerns about this provision: There was no reliable published information on the volume of water that no-fines concrete can transmit within a drainage design. HKS had approached the Portland Cement Institute and had been told that there was no design standard which provided such information, as the production of no-fines concrete on site was subject to many variables which led to an unpredictable void size and ratio in the finished product. The nature of the groundwater in the perched water table was such that it had been proven to cause mineral deposits within the voids of no-fines concrete drainage structures of older buildings in the vicinity. This had resulted in a reduction of the permeability of these structures. JOHANNESBURG (011) PINETOWN (031) CAPE TOWN (021)

2 Case History: Geosynthetic Basement Drainage, Gaborone Page 2 of 8 The combination of these two factors had led to doubts being expressed as to the advisability of using no-fines concrete to drain Block 6, and Kaytech had been approached for advice on an alternative based on the use of geosynthetics. ALTERNATIVE DRAINAGE DESIGN After inspection of the design drawings Kaytech offered the drainage design suggestion as detailed in the sketch given below. Figure 1: General Layout DESIGN RATIONALE Underfloor Drainage: Kaymat U14, a nonwoven, continuous filament, needlepunched, polyester geotextile, is laid on hard compacted earth of a pervious nature (Decomposed Granite). Its purpose is to act as a separation and filtration material between the underlying soil (through which water will flow) and the drainage net. As it is laid on a granular, hard, smooth surface with a low PI and is overlaid by further layers of geosynthetics, high mechanical characteristics are not required of it. A HDPE drainage spacer, Flownet DN3, provides the main conduit for groundwater and conducts it to the collector drains, which are spaced at ten metre intervals. In association with the geotextiles above and below it, it has the flow characteristics as detailed in Figure 2.

3 Case History: Geosynthetic Basement Drainage, Gaborone Page 3 of 8 Figure 2: Flow Data for DN3 Flownet The Silty Sand/DN3/Silty Sand ( ) boundary condition was chosen as the condition applying to this application as, although the upper surface of the Flownet would be overlaid by concrete, this would be cast over the geomembrane and geotextile. It was thus felt that the pressure of the wet concrete would partially occlude the Flownet voids. Kaytape S120, a high-modulus, woven, polypropylene geotextile, was included to restrict penetration of the 250 micron waterproofing membrane into the Flownet voids under the pressure of the 125 mm layer of compacted cast-in-situ floor concrete (filtration characteristics were unimportant in this application). There was some doubt as to whether this measure was actually required so an on-site test was done. This test took the form of two test pads, which were constructed in the same way as the floor would be. One pad was constructed with the Kaytape and the other was not.

4 Case History: Geosynthetic Basement Drainage, Gaborone Page 4 of 8 Figure 3: Test Pad with Kaytape Figure 4: Test Pad without Kaytape The two pads were cast as shown and after 24 hours were turned over. The concrete surface that had been in contact with the 250 micron was examined for signs of intrusion into the Flownet void space. Photographs 1 and 2 below show what was observed.

5 Case History: Geosynthetic Basement Drainage, Gaborone Page 5 of 8 Photograph 1: Test Pad with Kaytape This photograph clearly shows the impression of the Kaytape geotextile in the concrete surface. However, there is little sign of concrete protrusion into the void space in the DN3 Flownet. The unevenness of the compacted substrate is evidenced by the ripple appearance of the surface. Photograph 2: Test Pad without Kaytape Photograph 2 quite clearly shows that the 250 micron geomembrane was forced into the void space in the DN3 Flownet. The lines of the upper ribs of the Flownet stand out clearly, and the raised mounds of concrete between the rib lines stood nearly as high as the void height between the upper and lower Flownet ribs.

6 Case History: Geosynthetic Basement Drainage, Gaborone Page 6 of 8 Based on these tests it was decided that Kaytape would be an indispensable part of the geosynthetic drainage system. The 250 micron geomembrane was a linear, low-density, geomembrane specifically formulated as a low-cost Damp Proof Membrane (DPM). It had originally been specified as a 500 micron membrane over the no-fines concrete, but it was felt that with the support of the Kaytape and Flownet its thickness could be reduced as there was a lower risk of installation damage. This material was installed on site with 100mm overlap joints sealed with adhesive tape to prevent leakage of concrete when it was being placed. Basement Retaining Wall Drainage: The approximate 4 metre high, circumferential, basement wall was cast on a footing as shown in Figure 1. This wall was to be waterproofed with a painted-on, bituminous, waterproofing membrane to prevent any capillary transfer of water through the concrete from the drainage layer. A Kaymat U24 geotextile layer was to be placed against the waterproofing membrane to protect it from possible damage by the ribs of the DN3 Flownet. As the Flownet would be pressed against the comparatively soft bitumen by a pressure of four metres of compacted soil backfill, and it was felt that the waterproofing layer would thin out under the ribs of the Flownet and possibly be penetrated, the geotextile was inserted to act as a protection membrane in this application. DN3 Flownet, an HDPE drainage spacer, provides the main conduit for groundwater reaching the retaining wall, and conducts it to the circumferential collector drain which runs along the bottom of the wall on top of the footing. In association with the geotextiles on either side of it, the DN3 has the flow characteristics of the Rigid Surface/DN3/Silty Sand ( u ) configuration as detailed in Figure 2. The wall collector drain is equipped with a M100R Geopipe bedded in 6mm size crushed stone wrapped in a Kaymat U24 filter jacket, as shown in Figure 1. Outlets spaced at ten metre intervals penetrate the retaining wall and run under the floor in 100mm diameter solid UPVC pipes laid in concrete, to the main collector drain situated under the basement floor. Water from these drains and the underfloor drains is pumped from a final sump equipped with a submersible pump into the municipal stormwater system. Kaymat U24 geotextile acts as the filter jacket between the drainage system and the compacted granular backfill around the retaining wall. In this application the filtration attributes and the mechanical strength of the geotextile are important parameters. The Kaymat U24 was chosen as being an economical fabric which Photograph 3: Construction of the Retaining Wall Photograph 4: A Solid Drain Pipe from the Retaining Wall

7 Case History: Geosynthetic Basement Drainage, Gaborone Page 7 of 8 displays superior filtration characteristics and adequate mechanical strength for the application. INSTALLATION The installation of the floor drainage system proved to be speedy and easy. Supervision was facilitated by it being easy to see that each layer of the multi-layer system had been placed in its correct location. Photographs 5 to 12 show the progress of the floor drainage. The many columns in the floor area did not interfere with the laying of the drainage system as all the components were easily cut with scissors to enable them to be fitted around the columns and beams. The Kaymat U14 does not cover the top of the drain as the DN3 Flownet is continuous across the top of each drain, allowing the water to flow freely into the drain. The U14 jacket overlaps the U14 laid over the compacted earth to provide a continuous filter zone. The earth floor area was levelled and compacted, and rolled, raked and broomed to ensure that an even surface without low spots was presented for the drainage system. Soft wet spots were excavated and new material brought in and compacted. Great attention was paid to ensuring a smooth, even fishing for the drainage system to lie on. This is the first documented case history of an underfloor drainage system incorporating DN3 Flownet. Prior to the design being approved Kaytech was asked for case histories where the material had been used in a similar application. No such case history could be found, either locally or overseas (contact was made with Netlon UK and USA several times), or in any geosynthetics literature or conference proceedings. However, there were several case histories where the material had been used for vertical drainage in association with retaining walls. The client s review consultants, Burrow, Binnie & Partners, were dubious about the application and Kaytech had to supply many technical papers from geosynthetics conference proceedings examining the behaviour and characteristics of drainage spacers before approval was finally given. This involved two trips to Gaborone for meetings with the contractor, his consultants and Photograph 5: General Overview of Site Showing Retaining Wall and Columns Photograph 6: Finishing of Compacted Earthworks. Great attention was paid to ensuring a smooth, even finish for the drainage system to lie on. Photograph 7: Underfloor Collector Drain with Kaymat U14 Jacket and M100R Geopipe

8 Case History: Geosynthetic Basement Drainage, Gaborone Page 8 of 8 the client s consultants. Many faxed letters and papers were involved and many telephone calls. In the end the logic of the application won the day, and BESIX were authorised to place the order for materials on Kaytech. It is significant that the cost of the geosynthetic drainage system exceeded the allowable for the no-fines system by a considerable margin, and that the contractor was recompensed for this extra expenditure It is hoped that this case history will assist specifiers to design underfloor drainage with geosynthetic materials in the future. It appears to work well. Water had been discharged by accident at one corner of the site and had freely drained through the system, under the floor, to another sector of the site. This had impressed BESIX who felt very confident about the success of the design. Photograph 8: Sewing Edges of DN3 together with nylon line to ensure uninterrupted flow path. Photograph 9: Installing the Kaymat U14 Layer and the DN3 Flownet Photograph 10: General Overview of Floor Drainage Laying Operation Photograph 11: Bringing in Geotex S120 Geotextile Photograph 12: Compacted Earth, Kaymat U14, DN3 Flownet, Kaytape S120, 250 Micron Geomembrane and 125mm concrete floor with curing sand