Replace the white boss arrangement with a tee and a standpipe. You might be able to fix the standpipe to the breeze block.
Hello Julie Street,
Did You read what I wrote about NOT installing a trapped Standpipe as you described ?
That was the `Expert Opinion` of a Professional Plumber.
I should have explained why not - here is the reason:
It is quite feasible that one or both of the waste pipes that are connected together could be discharging just as the Washing Machine is pumping out.
Although what You described would mean that the tee for the Standpipe would be very close to the Soil pipe boss there is a possibility that a `full bore` flow of water from the other 2 waste pipes could restrict the water flow from the Standpipe / Washing Machine coming into the vertical waste pipe causing it to build up within the Standpipe.
For the few seconds that the vertical existing waste pipe is full of water [imagine a full Kitchen Sink and perhaps a Basin emptying / water running] it is possible that the Washing Machine waste water could pump over the top of the Standpipe.
Even pumping over for just a few seconds this could be a significant amount of water overflowing the Standpipe.
Other Members might disagree with this but there are reasons why Professional Plumbers upsize Waste pipes or connect additional Waste pipes directly to a Soil stack / Soil pipe and this circumstance is one of those reasons.
Chris